A Carbon Monoxide Reevaluation:
Past and Future Trends and
Their Relationship to Conformity Hot Spot Policies
UCD- ITS- RR- 00- 13
December 2000
Prepared by
Douglas Eisinger, Program Manager, UC Davis- Caltrans Air Quality Project
Dr. Daniel Chang, Professor, Civil and Environmental Engineering
Kellie Dougherty, Graduate Student Research Assistant
Tom Kear, UC Davis- Caltrans Air Quality Project
University of California, Davis
Institute of Transportation Studies
One Shields Avenue
Davis, CA 95616
530- 752- 4909
http:// www. engr. ucdavis. edu/~ its/
Prepared for
Mike Brady, Air Quality Program Coordinator
Environmental Program, MS- 27
California Department of Transportation
1120 N Street
Sacramento, CA 94274
916- 653- 3738
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iii
ABSTRACT
Carbon monoxide ( CO) emission, concentration, and exposure trends in California and
the United States were analyzed. The study also included data analyses for northern and
southern California CO monitoring sites. Results demonstrate that CO concentrations are
decreasing at both the neighborhood and microscale levels. The microscale concentration
decreases are correlated with decreases in regional emissions and are projected to continue into
the future. Due to declining CO concentrations and the relationship between neighborhood and
microscale conditions, the study recommends that the U. S. Environmental Protection Agency
reevaluate the conformity requirements to conduct transportation project- level CO analyses. The
findings from this report suggest that conformity CO hot spot analysis requirements could be
appropriately limited to unusual circumstances identified through interagency consultation.
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v
TABLE OF CONTENTS
Section Page
ABSTRACT....................................................................................................................... ........... iii
LIST OF FIGURES....................................................................................................................... vii
LIST OF TABLES ......................................................................................................................... ix
EXECUTIVE SUMMARY....................................................................................................... ES- 1
1. INTRODUCTION.............................................................................................................. 1- 1
1.1 Motivation for This Study......................................................................................... 1- 1
1.2 Summary Approach and Findings ............................................................................ 1- 2
1.3 Report Organization.................................................................................................. 1- 2
2. EMISSION, CONCENTRATION, AND EXPOSURE TRENDS .................................... 2- 1
2.1 Emission Trends ....................................................................................................... 2- 1
2.1.1 California Emission Trends .......................................................................... 2- 1
2.1.2 National Emission Trends............................................................................. 2- 3
2.1.3 Relative Importance of On- Road Mobile Sources........................................ 2- 3
2.2 Concentration Trends................................................................................................ 2- 4
2.2.1 California Concentration Trends .................................................................. 2- 4
2.2.2 National Concentration Trends..................................................................... 2- 5
2.3 Exposure Trends ....................................................................................................... 2- 5
2.4 CO Emissions Projections ........................................................................................ 2- 6
2.4.1 California Air Quality Management Plans: Forecasts to the Year 2010 ..... 2- 6
2.4.2 California Transportation Plans: Forecasts to the Year 2020 and
Beyond.......................................................................................................... 2- 8
2.4.3 Differences Between Air Quality and Transportation Plans ...................... 2- 10
2.4.4 Forecasted National Trends ........................................................................ 2- 11
3. CONTROL PROGRAMS .................................................................................................. 3- 1
3.1 Control Programs to Date ......................................................................................... 3- 1
3.1.1 Exhaust Standards......................................................................................... 3- 1
3.1.2 Cleaner Burning Fuels .................................................................................. 3- 2
3.1.3 Motor Vehicle Inspection and Maintenance................................................. 3- 3
3.2 Expected Future Controls ......................................................................................... 3- 3
3.2.1 California Control Programs......................................................................... 3- 3
3.2.2 Federal Control Programs............................................................................. 3- 5
3.3 Control Program Summary....................................................................................... 3- 6
4. CASE STUDIES: SOUTHERN CALIFORNIA AND SACRAMENTO......................... 4- 1
4.1 Overview and Methodology ..................................................................................... 4- 1
4.2 Background Information on Monitoring Sites and Data Used ................................. 4- 1
4.2.1 Southern California Sites ( Los Angeles and Riverside) ............................... 4- 1
4.2.2 Sacramento Sites........................................................................................... 4- 2
4.2.3 Data Sources ................................................................................................. 4- 2
4.2.4 Analysis Methods ......................................................................................... 4- 2
vi
TABLE OF CONTENTS ( Concluded)
Section Page
4.3 Data Analysis: The Southern California Case Study............................................... 4- 3
4.3.1 Background: Projected Emission Trends to 2020 ....................................... 4- 3
4.3.2 Observations of Southern California Emission and Concentration
Trends by Monitoring Site............................................................................ 4- 6
4.3.3 Discussion of Observed Emission and Concentration Relationships ......... 4- 16
4.4 Sacramento Case Study .......................................................................................... 4- 18
4.4.1 Background: Projected Emission Trends to 2010 ..................................... 4- 18
4.4.2 Observations of Sacramento Emission and Concentration Trends by
Monitoring Site........................................................................................... 4- 19
4.4.3 Discussion of Observed Emission and Concentration Relationships ......... 4- 24
4.5 Discussion of Data Analyses .................................................................................. 4- 25
5. CONCLUSIONS ................................................................................................................ 5- 1
5.1 California and National Emission, Concentration, and Exposure Trends ................ 5- 1
5.2 Relationships Between Microscale and Regional CO Conditions ........................... 5- 1
6. POLICY IMPLICATIONS AND RECOMMENDATIONS ............................................. 6- 1
7. REFERENCES................................................................................................................... 7- 1
APPENDIX A: SACRAMENTO AND SOUTHERN CALIFORNIA PLOTS
ILLUSTRATING ROLLBACK ANALYSES USING ON- ROAD
EMISSIONS ................................................................................................ A- 1
APPENDIX B: LOS ANGELES EXAMPLE PLOTS FOR 10TH, 20TH, AND 100TH
HIGHEST CONCENTRATIONS............................................................... B- 1
APPENDIX C: RIVERSIDE EXAMPLE PLOTS FOR 10TH, 20TH, AND 100TH
HIGHEST CONCENTRATIONS............................................................... C- 1
vii
LIST OF FIGURES
Figure Page
2- 1. Decline in CO emissions by source type for the South Coast Air Basin, 1985- 1995...... 2- 3
2- 2. CO emission trends by source category, winter planning inventory................................ 2- 8
2- 3. Modeled decline in fleet average CO emission factors.................................................. 2- 10
4- 1. CO winter baseline year emissions ( tons per day) for the South Coast Air Basin........... 4- 4
4- 2. On- road mobile CO emissions as a fraction of total CO emissions for the
South Coast Air Basin, 1990- 2020................................................................................... 4- 5
4- 3. Lynwood ( microscale): total basin emissions and 2nd highest 1- hr concentrations ........ 4- 8
4- 4. Hawthorne ( neighborhood): total basin emissions and 2nd highest
1- hr concentrations........................................................................................................... 4- 8
4- 5. Lynwood ( microscale): on- road basin emissions and 2nd highest
1- hr concentrations........................................................................................................... 4- 9
4- 6. Hawthorne ( neighborhood): on- road basin emissions 2nd highest
1- hr concentrations........................................................................................................... 4- 9
4- 7. Lynwood ( microscale): total basin emissions and 2nd highest 8- hr concentrations ...... 4- 10
4- 8. Hawthorne ( neighborhood): total basin emissions and 2nd highest
8- hr concentrations......................................................................................................... 4- 10
4- 9. Lynwood ( microscale): on- road basin emissions and 2nd highest
8- hr concentrations......................................................................................................... 4- 11
4- 10. Hawthorne ( neighborhood): on- road basin emissions and 2nd highest
8- hr concentrations......................................................................................................... 4- 11
4- 11. Magnolia ( microscale): total basin emissions and 2nd highest 1- hr concentrations ...... 4- 12
4- 12. Rubidoux ( neighborhood): total basin emissions and 2nd highest
1- hr concentrations......................................................................................................... 4- 12
4- 13. Magnolia ( microscale): on- road basin emissions and 2nd highest
1- hr concentrations......................................................................................................... 4- 13
4- 14. Rubidoux ( neighborhood): on- road basin emissions and 2nd highest
1- hr concentrations......................................................................................................... 4- 13
viii
LIST OF FIGURES ( Concluded)
Figure Page
4- 15. Magnolia ( microscale): total basin emissions and 2nd highest 8- hr concentrations ...... 4- 14
4- 16. Rubidoux ( neighborhood): total basin emissions and 2nd highest
8- hr concentrations......................................................................................................... 4- 14
4- 17. Magnolia ( microscale): on- road basin emissions and 2nd highest 8- hr CO
concentrations................................................................................................................. 4- 15
4- 18. Rubidoux ( neighborhood): on- road basin emissions and 2nd highest
8- hr concentrations......................................................................................................... 4- 15
4- 19. 1990- 2010 CO emissions ( tons per day) for the Sacramento Valley Air Basin ............ 4- 18
4- 20. On- road emissions as fraction of total emissions for Sacramento Valley Air
Basin, 1990- 2010 ........................................................................................................... 4- 19
4- 21. El Camino ( microscale): total basin emissions and 2nd highest 1- hr concentrations .... 4- 20
4- 22. Del Paso ( neighborhood): total basin emissions and 2nd highest
1- hr concentrations......................................................................................................... 4- 20
4- 23. El Camino ( microscale): on- road basin emissions 2nd highest 1- hr concentrations...... 4- 21
4- 24. Del Paso ( neighborhood): on- road basin emissions and 2nd highest
1- hr concentrations......................................................................................................... 4- 21
4- 25. El Camino ( microscale): total basin emissions and 2nd highest 8- hr concentrations .... 4- 22
4- 26. Del Paso ( neighborhood): total basin emissions and 2nd highest
8- hr concentrations......................................................................................................... 4- 22
4- 27. El Camino ( microscale): on- road basin emissions and 2nd highest
8- hr concentrations......................................................................................................... 4- 23
4- 28. Del Paso ( neighborhood): on- road basin emissions and 2nd highest
8- hr concentrations......................................................................................................... 4- 23
ix
LIST OF TABLES
Table Page
2- 1. California CO emission trends, 1985- 1995...................................................................... 2- 1
2- 2. Carbon monoxide winter seasonal emission inventory trends ......................................... 2- 2
2- 3. Maximum 8- hr CO concentrations for selected California regions, 1980– 1997 ............. 2- 4
2- 4. Number of days California areas exceeded the federal 8- hr CO NAAQS....................... 2- 5
2- 5. Percent decline in annual average tons per day CO emissions, 2000 to 2010 ................. 2- 6
2- 6. CO emissions “ carrying capacity” of the South Coast Air Basin in tons per day............ 2- 7
2- 7. Days above the national CO 8- hr standards at Lynwood in the South Coast
Air Basin .......................................................................................................................... 2- 7
2- 8. CO winter- time emissions, in tons per day, for the South Coast Air Basin
as projected by SCAG...................................................................................................... 2- 9
2- 9. CO emissions in the Sacramento nonattainment area with implementation of
the 1999 Metropolitan Transportation Plan ..................................................................... 2- 9
2- 10. Base- year and future- year national on- road motor vehicle CO emissions
in tons per year ............................................................................................................... 2- 11
3- 1. Federal and California passenger vehicle 50,000 mile CO exhaust standards................. 3- 2
3- 2. Supplemental federal test procedure emission standards................................................. 3- 5
ES- 1
EXECUTIVE SUMMARY
Carbon monoxide ( CO) emissions and concentration data were evaluated to determine
whether hot spot, or microscale, CO analyses continue to be appropriate for transportation
projects. The control of CO air pollution is one of the major success stories in the air quality
management field. Monitoring data for the past 20 years show consistent declines in CO
concentrations under a wide array of conditions, for example, at regional- scale monitoring sites,
at microscale sites proximate to heavy traffic, and inside operating motor vehicles. Federal
conformity regulations require microscale CO modeling analyses for many proposed
transportation projects. The conformity microscale regulations “ apply at all times” and require
quantitative hot spot analyses to demonstrate that transportation projects eliminate or reduce the
severity and number of localized CO violations ( 40 CFR 93.116 and 93.123). As currently
written, the conformity requirements are static and are applied independently of the decline in
CO problems.
Given the decline in CO as an air quality problem and the continued conformity
requirement for quantitative CO hot spot analyses, there is interest within the transportation
planning community to evaluate the future of CO problems and to assess whether the conformity
regulations should be revised to provide additional flexibility. The California Department of
Transportation ( Caltrans) sponsored the University of California, Davis ( UCD) to review
CO emission and concentration trends and to assess whether those trends are likely to continue
into the future.
The UCD research team identified and summarized national and California CO emission
and concentration trends. The study team evaluated in greater detail CO emissions and
concentration data for Sacramento, Los Angeles, and Riverside. The Sacramento, Los Angeles,
and Riverside analyses contrasted CO data for a neighborhood site with similar data for a related
microscale site. The analyses helped elucidate the extent to which regional declines in CO
concentrations are observed at microscale sites, and to what extent microscale CO concentrations
are expected to decline in the future.
The data analyses indicate that microscale concentrations correlate with regional
emission trends. CO concentrations measured at Sacramento, Los Angeles, and Riverside
microscale monitors declined at least as rapidly as CO concentrations measured at neighborhood
sites. Continued declines in CO emissions and concentrations are forecast based upon
California state implementation plans ( SIPs) for attainment and maintenance of the National
Ambient Air Quality Standards ( NAAQS). Virtually all areas of California are or soon will be in
attainment of the CO NAAQS. An exception is the border area of Calexico, which experiences
significant emission impacts from vehicles of Mexican registration. In addition, it is probable
that future state and federal regulations will result in further CO emissions reductions. Control
strategies that are undertaken to decrease regional concentrations should also lead to decreases at
the microscale level.
Caltrans asked UCD to answer four questions related to past and future trends and their
relationship to the conformity requirements. The research questions posed by Caltrans and the
answers obtained from this study are as follows:
ES- 2
Q. Are past declines in CO emissions and concentrations expected to continue into the future?
A. CO problems will continue to decline in the future. California Air Resources Board ( ARB)
data exemplify the project findings. ARB projects that from 1990 to the year 2010,
California CO emissions reductions will range from 29% in Modesto to as much as 58% in
Los Angeles; most major California metropolitan areas will experience emissions reductions
of at least 30% to 40% during this time period.
Q. Are microscale CO concentrations declining at a rate that is faster or slower than regional
scale CO concentrations?
A. Based on an analysis of past trends, the evidence obtained supports a hypothesis that
concentration reductions observed at microscale stations are greater than or equal to those
observed at neighborhood scale stations and are correlated with regional CO emissions
reductions.
Q. What are likely scenarios for future microscale CO concentrations?
A. Analysis results support the hypothesis that both neighborhood and microscale CO
concentrations are declining and will continue to decline. On- road emissions are declining,
and will become a less significant portion of total emissions in the future. The rate at which
microscale concentrations are reduced will probably be slower than past reduction rates,
given the reduced rate at which mobile emissions are declining.
Q. Given past trends and likely future conditions, does it seem appropriate to recommend to the
EPA reconsideration of the conformity requirements for microscale CO hot spot analyses?
A. The implications of these findings are significant for the transportation planning community
and for the need to conduct transportation project- level CO analyses. California data indicate
that in virtually all metropolitan areas, no existing transportation facility is expected to cause
a CO violation. Los Angeles has not yet attained the NAAQS but is on a path to do so in the
near future, and thus no existing transportation facilities would be expected to cause CO
violations in Los Angeles beginning within a few years. The one exception is the border area
of Calexico which is influenced by emissions from vehicles of Mexican registration. Thus,
for CO analysis purposes, a future transportation project can be reasonably compared to
existing facilities. If planned future transportation projects have similar sizes and
characteristics as existing facilities, and the existing facilities do not cause CO violations,
then it can be inferred that the planned projects should not cause violations either, accounting
for differences in regional background CO concentrations that might exist. This would allow
for the elimination of microscale modeling for most transportation projects. Modeling might
still be necessary for projects that are larger than existing facilities or those with
extraordinary characteristics, such as being located in Calexico. We recommend that EPA
reevaluate the continued need for the conformity CO hot spot analysis requirement, and
consider replacing the requirement for one that applies only under unusual circumstances,
such as those evident at the Calexico border site. The conformity consultation process could
then be used to identify potentially problematic projects and require CO analyses if needed.
1- 1
1. INTRODUCTION
1.1 MOTIVATION FOR THIS STUDY
Carbon monoxide ( CO) emissions and concentration data were evaluated to determine
whether “ hot spot,” or microscale, CO analyses continue to be appropriate for transportation
projects. The control of CO air pollution is one of the major success stories in the air quality
management field. Over the past 25 years, federal and California regulations have mandated the
introduction of cleaner operating motor vehicles, cleaner- burning automotive fuels, and motor
vehicle inspection and maintenance programs, all of which have significantly reduced
per- vehicle CO emissions. Reductions in motor vehicle CO emissions have resulted in
substantial declines in CO concentrations because in most urban areas motor vehicles are
responsible for up to 95% of CO emissions ( U. S. Environmental Protection Agency, 2000a;
p. 11). Monitoring data for the past 20 years show consistent declines in CO concentrations
under a wide array of conditions, for example, at regional- scale monitoring sites, at microscale
sites proximate to heavy traffic, and inside operating motor vehicles. The U. S. Environmental
Protection Agency ( EPA) recently reported that for the 10- year period 1989 through 1998,
exceedances of the federal 8- hr CO National Ambient Air Quality Standards ( NAAQS) have
declined 98% ( U. S. Environmental Protection Agency, 2000a; p. 11). EPA noted “… a
consistent decline in CO concentrations during the past 20 years. Nationally, the 1998 composite
average ambient concentration is 58% lower than 1979, and is the lowest level recorded during
the past 20 years of monitoring” ( U. S. Environmental Protection Agency, 2000a; p. 14).
Federal conformity regulations require microscale CO modeling analyses for many
proposed transportation projects. The conformity microscale regulations “ apply at all times” and
require quantitative hot spot analyses to demonstrate that transportation projects eliminate or
reduce the severity and number of localized CO violations ( 40 CFR 93.116 and 93.123). As
currently written, the conformity requirements are static and are applied independently of the
decline in CO problems.
Given the decline in CO as an air quality problem, and the continued conformity
requirement for quantitative CO hot spot analyses, there is interest within the transportation
planning community to evaluate the future of CO problems and to assess whether the conformity
regulations should be revised to provide additional flexibility. The California Department of
Transportation ( Caltrans) sponsored the University of California, Davis ( UCD) to review
CO emission and concentration trends and to assess whether those trends are likely to continue
into the future. Caltrans asked UCD to evaluate CO data and answer four questions:
1. Are past declines in CO emissions and concentrations expected to continue into the
future?
2. Are microscale CO concentrations declining at a rate that is faster or slower than regional
scale CO concentrations?
3. What are likely scenarios for future microscale CO concentrations?
1- 2
4. Given past trends and likely future conditions, does it seem appropriate to recommend to
EPA reconsideration of the conformity requirements for microscale CO hot spot
analyses?
1.2 SUMMARY APPROACH AND FINDINGS
The UCD research team identified and summarized national and California CO emission
and concentration trends. The study team evaluated in greater detail CO emissions and
concentration data for Sacramento and Southern California ( Los Angeles and Riverside). The
Sacramento and Southern California ( Los Angeles and Riverside) analyses contrasted CO data
for a neighborhood site with similar data for a nearby microscale site. The analyses helped us
understand to what extent regional declines in CO concentrations are observed at microscale
sites, and to what extent microscale CO concentrations are expected to decline in the future.
As the data analyses in this report indicate, microscale concentrations correlate with
regional emission trends. CO concentrations measured at Sacramento and Southern California
( Los Angeles and Riverside) microscale monitors declined at least as rapidly as CO
concentrations measured at neighborhood sites. Continued declines in CO emissions and
concentrations are forecast based upon California CO state implementation plans ( SIPs) for
attainment and maintenance of the NAAQS. Virtually all areas of California are or soon will be
in attainment of the CO NAAQS. An exception is the border area of Calexico which experiences
significant emissions impacts from vehicles of Mexican registration. In addition, it is probable
that future state and federal regulations will result in further CO emissions reductions. Control
strategies that are undertaken to decrease regional concentrations should also lead to decreases at
the microscale level. Given the expected continued decline in CO emissions, attainment of
federal CO health standards, and expected CO reductions at the microscale level, it appears
appropriate for EPA to provide greater flexibility for conformity hot spot analysis requirements
and to consider eliminating the CO hot spot analysis requirement entirely unless projects are in
problematic locations such as the Calexico border area. The conformity interagency consultation
process could be used to evaluate these unusual circumstances and require hot spot analyses if
needed.
1.3 REPORT ORGANIZATION
Section 2 includes a review of CO emission, concentration and exposure trends for both
California and the United States. Section 3 identifies past, present, and future control programs
that influence CO emissions, concentrations, and exposures. Section 4 presents the results of a
case study analysis of regional and microscale CO in the Los Angeles and Sacramento areas.
Section 5 presents conclusions, and Section 6 discusses policy implications. Appendices include
case study materials that graph examples of microscale and regional CO relationships not
included in the text.
2- 1
2. EMISSION, CONCENTRATION, AND EXPOSURE TRENDS
The overall trends in CO emissions, and resulting CO concentrations, show significant
decline over the past 20 years. More stringent tailpipe standards, enhanced motor vehicle
inspection and maintenance ( I/ M) in the worst polluted ozone ( O3) and CO nonattainment areas,
and the introduction of reformulated fuels have all contributed to the decline in mobile source
CO emissions. Forecasted emission inventories show that in future years, California mobile
source CO emissions are expected to continue to decrease, despite continued growth in
population, motor vehicles, and motor vehicle miles traveled.
This section documents California and national CO emission, concentration, and
exposure trends. The information presented draws extensively from recent publications, among
which are ( a) “ National Air Quality and Emissions Trends Report, 1998,” published by EPA in
March 2000 ( U. S. Environmental Agency, 2000a); ( b) “ Air Quality Criteria for Carbon
Monoxide,” published by EPA in June 2000 ( U. S. Environmental Protection Agency, 2000b);
and ( c) “ The 1999 California Almanac of Emissions & Air Quality,” published by the California
Air Resources Board ( California Air Resources Board, 1999a).
2.1 EMISSION TRENDS
2.1.1 California Emission Trends
On- road motor vehicle CO emissions have declined 20% in California from 1985 through
1997. The California Air Resources Board ( ARB) expects this trend to continue at least until
2010 ( California Air Resources Board, 1999a, p. 68). Table 2- 1 illustrates statewide emissions
trends for 1985 through 1995; the table documents the substantial decline in on- road motor
vehicle and total CO emissions. Similar trends are observed for individual California air basins.
Table 2- 1. California CO emission trends, 1985- 1995 ( annual average tons per day).
Emission Source 1985 1990 1995
All sources 27,538 26,088 21,162
Stationary sources 253 292 299
Area- wide sources 1,810 2,082 2,140
On- road mobile 22,856 20,787 15,444
Gasoline vehicles 22,634 20,455 15,134
Diesel vehicles 222 332 310
Other mobile sources 2,619 2,927 3,279
Source: California Air Resources Board, 1999a; Table 3- 4, p. 69.
During 1996, ARB submitted to EPA a SIP documenting attainment of the CO NAAQS
in all major metropolitan areas of the state except for Los Angeles. The SIP submittal forecasted
2- 2
continued declines in CO emissions throughout the state. In 1998, ARB updated the CO SIP
maintenance plan to reflect changes to the state’s oxygenated fuels program. Table 2- 2 includes
projected emissions for major California areas, as documented by ARB in its 1996 SIP and
reflected in SIP amendments adopted in 1998. ARB projects declining emissions through the
year 2010.
Table 2- 2. Carbon monoxide winter seasonal emission inventory trends.
CO Federal Maintenance Area 1990 E1m99i3ssions1 9E9s5timate2s0 ( 0T0ons/ D2a0y0) 5 2010
Bakersfield 423 356 348 346 318 298
Chico 229 189 183 173 160 157
Fresno 511 436 414 382 343 335
Lake Tahoe North Shore 32 28 26 23 20 19
Lake Tahoe South Shore 100 89 86 80 69 67
Modesto 311 282 270 251 225 220
Sacramento Area 1214 1026 971 873 727 665
San Diego 1927 1492 1345 1132 958 877
San Francisco- Oakland- San Jose 3731 3019 2786 2398 1988 1789
Stockton 463 400 380 351 310 296
Sources: California Air Resources Board 1996 ( 1990, 1993, and 1995 data); California Air Resources Board 1998, Table 3
( 2000, 2005, and 2010 data).
There are two important points related to the projected emissions in Table 2- 2. First, the
1995 emission estimates represent the attainment year emission inventory for these regions, and
thus approximate the allowable ceiling on CO emissions. In other words, emissions at or below
1995 levels will continue to result in attainment of the CO NAAQS. Second, ARB has stated
that the year 2000 emission projections and beyond actually overestimate emissions. ARB has
implemented several emission control programs that are not accounted for within the emissions
estimates included in Table 2- 2. Emissions reduction benefits due to improvements to “ basic-area”
inspection and maintenance programs, implementation of “ enhanced” inspection and
maintenance, and implementation of the second phase of the on- board- diagnostics program
( OBD- II) are not included in the Table 2- 2 projected emissions for year 2000 and beyond ( ARB,
1998; p. 5). Section 3 includes more detailed control program information.
The South Coast Air Basin has traditionally experienced the nation’s worst exceedances
of the CO NAAQS. ARB reports that on- road CO emissions in the South Coast Air Basin have
declined 35% between 1985 and 1995 ( California Air Resources Board, 1999a; p. 92). Figure 2-
1 shows the decline in annual average CO emissions in the South Coast Air Basin, from
approximately 9,900 tons per year in 1985 to approximately 7,200 tons per year in 1995.
2- 3
S o u th C o a s t A ir B a s in C O Em is s io n
T re n d s ( to n s / d a y , a n n u a l a v e ra g e )
0
2 0 0 0
4 0 0 0
6 0 0 0
8 0 0 0
1 0 0 0 0
1 9 8 5 1 9 9 0 1 9 9 5
S ta tio n a ry
A re a
O ff- R o a d M o b ile
O n - R o a d M o b ile
Figure 2- 1. Decline in CO emissions by source type for the South Coast Air Basin, 1985- 1995.
Note that stationary sources were less than 40 tons per day and are not visible in
the figure. Source: California Air Resources Board, 1999a; p. 92.
2.1.2 National Emission Trends
National CO emissions trends mirror the decline observed in California. EPA annually
reports national emission and concentration trends throughout the United States For the 10- year
period 1989 through 1998, EPA documents a 16% nationwide decline in CO emissions. EPA
reports that on- road mobile source CO emissions have declined 24% during this time period,
despite a 23% rise in motor vehicle miles traveled ( U. S. Environmental Protection Agency,
2000a; p. 12).
2.1.3 Relative Importance of On- Road Mobile Sources
EPA documents that, over the past ten years, the fraction of total CO emissions
originating from on- road mobile sources has declined. For example, from 1988 to 1997, on- road
mobile sources declined from 61% to 57% of the national CO emission inventory ( U. S.
Environmental Protection Agency, 2000b; p. 3- 10). The national decline in the importance of
on- road mobile sources is consistent with California data, which show that on- road sources
accounted for 83% of total CO emissions in 1985, but only 73% of total CO emissions in 1995
( see Table 2- 1). The diminishing importance of on- road sources is expected to continue into the
future, as exemplified by the discussion in Section 2.4.
2- 4
2.2 CONCENTRATION TRENDS
2.2.1 California Concentration Trends
Consistent with the decline in emissions, observed CO concentrations have declined
throughout California during the past two decades. ARB notes that CO concentrations have
declined “… substantially in all areas of California… despite significant growth” ( California Air
Resources Board, 1999a; p. 70). Table 2- 3 documents the decline in maximum 8- hr CO
concentrations for several California air basins during the 1980 through 1997 time period. With
the exception of the South Coast Air Basin, all areas show consistently declining maximum
8- hr concentrations ( note that Calexico is not included in Table 2- 3). The South Coast Air Basin
experienced an increase in maximum CO concentrations in 1997; however ( as discussed below
and in Table 2- 4), the SCAB continues to show progress toward reducing the number of CO
NAAQS exceedances.
Table 2- 3. Maximum 8- hr CO concentrations ( in ppm) for selected California regions,
1980– 1997 ( does not include Calexico).
1980 1985 1990 1995 1997
Lake Tahoe Air Basin 19 16.3 10.1 6.3 3.8
Sacramento Valley Air Basin 14.3 13.3 14 7.4 7.2
San Diego Air Basin 10.1 13 9.1 6.3 5.3
San Francisco Bay Area Air Basin 16.4 16.1 11 5.8 6.1
San Joaquin Valley Air Basin 15.5 11 11.5 9.1 7.5
South Central Coast Air Basin 14 10.5 5.8 5.8 5.6
South Coast Air Basin 25.8 27.7 16.8 13.8 17.1
Source: California Air Resources Board 1999a; Table A- 14, p. 302.
The conformity regulations are focused on protecting against NAAQS exceedances. The
CO NAAQS include a 1- hr, 35 parts per million ( ppm) standard, and an 8- hr 9.0 ppm standard.
Both the 1- hr and 8- hr standards require areas not to exceed either standard more than one time
per year. In practice, the 8- hr requirement is the health standard targeted by air quality control
districts in their CO SIPs. As of 1999, the only areas in California that continue to exceed the 8-
hr CO NAAQS are the South Coast Air Basin portion of Los Angeles County and the city of
Calexico in Imperial County ( California Air Resources Board, 1999a; p. 70). Table 2- 4 includes
data documenting the substantial decline in the number of days California air basins exceed
federal 8- hr CO air quality standards.
2- 5
Table 2- 4. Number of days California areas exceeded the federal 8- hr CO NAAQS.
1980 1985 1990 1995 1997
Lake Tahoe Air Basin 26 27 5 0 0
Sacramento Valley Air Basin 9 11 12 0 0
San Diego Air Basin 1 3 0 0 0
San Francisco Bay Area Air Basin 12 17 2 0 0
San Joaquin Valley Air Basin 24 7 9 0 0
South Central Coast Air Basin 6 3 0 0 0
South Coast Air Basin 94 53 44 14 12
Source: California Air Resources Board 1999a; Table A- 16, p. 304.
Recent data for the South Coast Air Basin is consistent with overall trends in declining
CO exceedances. The South Coast Air Basin exceeded the federal 8- hr CO NAAQS on thirteen
days in 1998, and on eight days during 1999 ( U. S. Environmental Protection Agency, 2000c;
Table 4).
Calexico appears to be the sole exception to California’s consistent progress toward
reducing CO exceedances and concentrations. Calexico exceeded the federal 8- hr CO NAAQS
on nine days in 1996, twelve days in 1997, eight days in 1998, and thirteen days in 1999
( California Air Resources Board, 2000a; U. S. Environmental Protection Agency 2000c, Table 4).
2.2.2 National Concentration Trends
National concentration trends are consistent with the overall decline in CO emissions
nationwide. In 1991, following passage of the Clean Air Act Amendments of 1990, EPA
designated 42 metropolitan areas as CO NAAQS nonattainment. In 1998 and 1999, only six
metropolitan areas nationwide failed to meet the CO NAAQS. Two of these areas, Los Angeles
and Calexico, are in California. The remaining areas include Fairbanks, Alaska; Las Vegas,
Nevada; Des Moines, Iowa; and Weirton, West Virginia. Outside California, only Fairbanks
exceeded the CO NAAQS in 1999 ( Fairbanks exceeded the NAAQS on two days) ( U. S.
Environmental Protection Agency, 2000c; Table 4).
2.3 EXPOSURE TRENDS
Consistent with the decline in motor vehicle CO emissions and ambient CO
concentrations, motor vehicle occupants have experienced substantial reductions in exposures to
CO. EPA’s June 2000 CO Criteria Document ( U. S. Environmental Protection Agency, 2000b)
includes a comprehensive review of CO exposure. Based on 16 CO exposure studies conducted
in the United States, EPA’s Criteria Document estimates an approximate 90% reduction in
observed in- vehicle CO concentrations between 1965 and 1992 ( U. S. Environmental Protection
Agency, 2000b; p. 4- 31). The document cautions that exposure trends for the period following
1992 are affected by the increased use of sport utility vehicles ( SUVs). SUVs emit more CO
than light- duty passenger vehicles; however, in November 1999, California adopted more
stringent emission standards for SUVs that take effect beginning 2004 and require SUVs to
2- 6
reduce emissions to levels comparable to passenger vehicles. EPA established similar national
SUV controls in February 2000. Overall, EPA’s research concludes that “[ i] mplementation of
motor vehicle emission standards, catalytic converters, motor vehicle inspection and
maintenance programs, and cleaner burning fuels during the past three decades have reduced the
CO exposures of urban commuters” ( U. S. Environmental Protection Agency, 2000b; p. 4- 34).
Flachsbart ( 1999) reviewed CO exposure trends throughout the United States and other
nations as part of EPA’s effort to develop the June 2000 CO Criteria Document. Flachsbart
concluded, in part, that “[ i] mplementation of emission controls to satisfy motor vehicle emission
standards in the United States over the past three decades has significantly reduced the CO
exposures of US motorists, pedestrians and bicyclists on streets and highways. Moreover,
average CO concentrations in passenger cabins of motor vehicles are expected to drop further in
the near future” ( Flachsbart, 1999; p. 324).
2.4 CO EMISSIONS PROJECTIONS
2.4.1 California Air Quality Management Plans: Forecasts to the Year 2010
Table 2- 5 presents the projected decline in California CO emissions from the year 2000
to the year 2010. CO emissions are expected to decline from 9% to 25%, depending upon the
region.
Table 2- 5. Percent decline in annual average tons per day CO emissions, 2000 to 2010.
2000 2010 Decline
from 2000
Bakersfield 346 298 14%
Chico 173 157 9%
Fresno 382 335 12%
Lake Tahoe North Shore 23 19 17%
Lake Tahoe South Shore 80 67 16%
Modesto 251 220 12%
Sacramento Area 873 665 24%
San Diego 1132 877 23%
San Francisco- Oakland- San Jose 2398 1789 25%
Stockton 351 296 16%
Source: California Air Resources Board 1998, Table 3.
The South Coast Air Quality Management District ( SCAQMD) defines attainment of the
CO NAAQS as requiring no more than approximately 5,000 tons per day of CO emissions.
Table 2- 6 includes a breakdown of the SCAQMD’s anticipated attainment emissions inventory.
2- 7
Table 2- 6. CO emissions “ carrying capacity” of the South Coast Air Basin in tons per day.
Emissions Source Category
Maximum allowable CO emissions
in tons per day
Stationary and areas sources 294
On- road mobile sources 3,125
Off- road mobile sources 1,549
Total ( overall control strategy to meet the
CO NAAQS) 4,968
Source: South Coast Air Quality Management District, 1996; Table 5- 5.
The federal deadline to attain the CO NAAQS in Los Angeles is December 31, 2000.
The 1997 Air Quality Management Plan ( AQMP) projected attainment of the CO NAAQS in
Los Angeles by the year 2000 and projected declining CO emissions through the year 2010. As
shown in Figure 2- 2, the AQMP forecasts a 24% reduction in CO emissions between the years
2000 and 2010. [ The AQMP also includes projections to 2020; these are shown and discussed in
Section 4, Case Studies.] Recent monitoring data from the Los Angeles region indicate that the
South Coast Air Basin has not yet achieved attainment of the CO NAAQS but is continuing its
steady progress toward reduced days above the federal standards. Table 2- 7 indicates days
above the CO NAAQS in Los Angeles.
Table 2- 7. Days above the national CO 8- hr standards at Lynwood
in the South Coast Air Basin.
Calendar Year Days Above the Federal CO NAAQS
1996 20
1997 12
1998 11
1999 7
Source: California Air Resources Board, 2000b.
2- 8
0
2000
4000
6000
8000
10000
12000
1987 1990 1993 2000 2006 2010
Tons/ Day
Point Area On- Road Off- Road
9,409
7,573
9,276
4,260 3,893
5,142
Figure 2- 2. CO emission trends by source category, winter planning inventory. Source:
reproduced from South Coast Air Quality Management District, 1996;
Figure 2- 8B, page III- 2- 25.
2.4.2 California Transportation Plans: Forecasts to the Year 2020 and Beyond
CO emissions forecasts prepared by transportation planning agencies demonstrate
continued declining CO emissions over the next 20 years. Los Angeles and Sacramento data are
presented here to illustrate forecasted on- road motor vehicle CO emissions. Later sections of
this study use Los Angeles and Sacramento data to evaluate CO trends and the continued need
for quantitative CO “ hotspot” analyses.
The Southern California Association of Governments ( SCAG) is responsible for
developing transportation plans for the Los Angeles metropolitan area. SCAG forecasts on- road
vehicle activity and emissions over a 20- year period to comply with federal transportation
planning requirements. During September 2000, SCAG prepared a Regional Transportation
Improvement Program ( RTIP) conformity analysis that included wintertime CO emissions from
1990 through 2020. The SCAG findings are included in Table 2- 8 and project a nearly
50% decline in on- road motor vehicle CO emissions between the years 2000 and 2010, and an
approximate additional 10% decline between 2010 and 2020.
2- 9
Table 2- 8. CO wintertime emissions, in tons per day, for the South Coast Air Basin
( excluding Banning Pass), as projected by SCAG.
RTIP Scenario 1990 2000 2010 2020
“ Build” ( assumes implementation of various
new transportation improvements) 7,381 3,206 1,816 1,631
“ No- build” ( assumes implementation of
existing adopted transportation improvements) - - 1,835 1,688
Source: Southern California Association of Governments, 2000; p. 3.
The Sacramento Area Council of Governments ( SACOG) is responsible for developing
transportation plans for the Sacramento metropolitan area. As with SCAG, SACOG forecasts
on- road vehicle activity and emissions over at least a 20- year period to comply with federal
transportation planning requirements. In 1999, SACOG prepared a Metropolitan Transportation
Plan ( MTP) conformity analysis that included CO emissions from 1990 through 2022. The
SACOG findings are included in Table 2- 9 and project nearly a 50% decline in on- road CO
emissions between 1995 and 2005, and an approximate additional 24% decline in on- road CO
emissions between the years 2005 and 2022. SACOG forecasts a slight increase in on- road CO
emissions from the years 2015 to 2022.
Table 2- 9. CO emissions in the Sacramento nonattainment area with implementation
of the 1999 Metropolitan Transportation Plan. Note that the Sacramento area
CO SIP includes maximum allowable on- road motor vehicle CO emissions
( the “ emissions budget”) of 780 tons per day.
1990 1995 2005 2015 2022
On- road motor vehicle emissions
( tons per day) 589.4 421.1 215.33 163.45 163.53
Source: SACOG, 1999.
Air quality planners forecast substantial continued reductions in on- road motor vehicle
CO emissions. These forecasted emissions reductions are at the heart of forecasted declining CO
problems. Figure 2- 3 illustrates the projected decline in California fleet average on- road CO
emissions. Figure 2- 3 uses EMFAC 7F, the EPA- approved microscale modeling tool for
California project- level conformity analyses, to illustrate the decline in on- road motor vehicle
emissions in grams of CO per mile driven, from 1965 through the year 2020. The trend shown in
Figure 2- 3 holds for all vehicle speeds; the 20 mph values depicted were chosen because of their
correlation with the Federal Test Procedure ( FTP) average speed of 19.6 mph used for certifying
emissions compliance of new vehicles.
2- 10
0
10
20
30
40
50
60
70
80
1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020
Year
EF ( grams/ mile)
* Emission factors were obtained from EMFAC7F using FTP bag fractions for the 1993 SCAQMD vehicle fleet at 20 mph and 70° F
Figure 2- 3. Modeled decline in fleet average CO emission factors.*
2.4.3 Differences Between Air Quality and Transportation Plans
An important consideration in the projection of future emissions and concentrations is the
existing mismatch in planning requirements between air quality and transportation plans. Air
quality plans demonstrating attainment for a future date need only project emissions for the
attainment year. Air quality maintenance plans for areas that have already achieved the NAAQS
must demonstrate that over at least a 10- year period emissions will remain below the levels
required to continue to attain the NAAQS. In practice this means that in California the CO
maintenance plan revision prepared in 1998 ( California Air Resources Board, 1998a) projects
CO emissions out to the year 2010. Air quality planners adopt air quality control regulations in
response to air quality attainment and maintenance deadlines. Air quality plans, especially
maintenance plans, may extend no further than a 10- year planning horizon. Emission and
concentration projections beyond a 10- year horizon may or may not show continued emissions
reductions. California CO emissions projections beyond the year 2010 are conservative because
air quality planners have focused their control requirements to achieve reductions up to, but not
necessarily beyond, 2010. Future air quality planning revisions can be expected to address years
beyond 2010, but typically only within a 10 year planning horizon.
In contrast to the air quality community’s 10- year planning horizon, the transportation
planning community is required to prepare 20- year transportation plans. The implications of this
mismatch are that ( a) CO emission projections beyond 2010 are unlikely to fully reflect the
2- 11
benefits of future air quality controls; and ( b) project- level conformity analyses are less practical
for years beyond 2010 because of the lack of information concerning future- year air quality
control programs.
Given the mismatch between air quality and transportation planning horizons,
transportation plan conformity analyses for the “ out years” ( the years between the air quality
planning horizon and the transportation planning horizon; for example, years 11 through 20 of a
transportation plan) may indicate rising on- road emissions due to projected increases in
population and vehicle miles traveled ( VMT). An example of this is the CO emissions forecast
for Sacramento in the year 2022, which is marginally higher than the emissions forecast for the
year 2015 ( see Table 2- 9). When forecasts predict rising emissions, it is important to consider
two factors: ( a) whether the projected emissions are below attainment year emission inventories
( referred to in conformity terms as the “ emissions budget”); and ( b) whether emission
projections for years beyond the air quality planning horizon are conservative, given the likely
adoption of future air quality control programs. In the Sacramento example, year 2022 emissions
are marginally higher than year 2015 emissions but are still substantially below the allowable
emissions budget. In addition, ARB has already stated that future- year emissions forecasts fail to
include benefits from all control programs.
2.4.4 Forecasted National Trends
Nationally, an EPA- sponsored study projects an approximate 20% reduction in on- road
CO emissions between 1996 and the year 2007, followed by increasing on- road mobile source
CO emissions through the year 2030 ( Pechan & Associates, 2000). The EPA- sponsored analysis
was completed to support EPA rulemaking to reduce heavy- duty diesel vehicle emissions.
Table- 2- 10 includes national projections made by the EPA- sponsored study for on- road mobile
source CO emissions for 1996, 2007, 2020, and 2030. Forecasts show that although future- year
on- road motor vehicle CO emissions are projected to increase, they remain below base- year
( 1996) emissions until at least the year 2020.
Table 2- 10. Base- year and future- year national on- road motor vehicle CO emissions
in tons per year.
Control Scenario 1996 2007 2020 2030
Base case ( existing control
measures) 53,585,364 43,176,561 49,311,620 56,890,116
Control case ( EPA- proposed
controls for heavy- duty
vehicles and diesel fuel)
53,585,364 43,120,561 48,333,986 55,609,767
Source: Pechan & Associates, 2000.
3- 1
3. CONTROL PROGRAMS
3.1 CONTROL PROGRAMS TO DATE
Motor vehicles contribute the vast majority of CO emissions in urban areas, and the
dramatic reductions in CO emissions and concentrations over the past 20 years are due almost
entirely to motor vehicle controls. Statewide, 73% of 1995 California CO emissions were from
on- road mobile sources ( California Air Resources Board, 1999a; Table 3- 4). The on- road
fraction varied by metropolitan area; for example, 77% of 1995 CO emissions in the South Coast
( California Air Resources Board 1999a; Table 4- 6) and 66% in the Sacramento Valley
( California Air Resources Board, 1999a; Table 4- 38) were from on- road sources.
Dramatic per- vehicle reductions are evident when contrasted against the substantial rise
in on- road VMT over past decades. California VMT increased from approximately 100 billion
VMT in 1965 to approximately 315 billion VMT in the year 2000 ( Caltrans, 1997; Figure S3).
During the 10- year period from 1985 to 1995, statewide VMT increased from approximately
200 billion to 280 billion VMT, or a 40% increase. During this same 10- year time period,
statewide CO on- road emissions fell from 22,856 tons per day ( 1985) to 15,444 tons per day
( 1995), a 32% reduction ( California Air Resources Board, 1999a; Table 3- 4). On a per- mile
basis, CO emissions dropped from approximately 114 tons per billion miles driven ( 1985) to
55 tons per billion miles driven ( 1995), nearly a 50% reduction in per mile emissions over
10 years.
Three major control programs have contributed to reduced per- vehicle CO emissions:
exhaust standards, cleaner burning fuels, and motor vehicle inspection and maintenance
programs.
3.1.1 Exhaust Standards
California tailpipe CO emissions standards for new vehicles have dropped from 51 grams
per mile ( g/ mi) for the 1966 model year to 1.7 g/ mi for 1994 model year “ ultra low emitting
vehicles” ( ULEVs). Exhaust emissions standards fell by 85% from 1970 ( 23 g/ mi) to 1993
( 3.4 g/ mi ). Table 3- 1 illustrates the reduction in tailpipe CO emissions standards for federal and
California vehicles.
3- 2
Table 3- 1. Federal and California passenger vehicle 50,000 mile CO exhaust
standards ( g/ mi) ( see note 1).
Federal Standards California Standards
Model Year CO standard in
g/ mi
Reduction from
last standard
CO standard in
g/ mi
Reduction
from last
standard
Pre- control era 84 84
1966 ( see note 2) ( 51) 39%
1968 ( 51) 39%
1970 23 55% 23 55%
1972 39 ( 70%)
1975 15 62% 9 61%
1980 7 53%
1981 3.4 51% 7 22%
1993 3.4 51%
1994 ( see note 3) 3.4 TLEV, LEV
1.7 ULEV
0 LEV
50% ( ULEV)
Notes: 1. EPA Tier 2 standards and ARB LEV- II standards are discussed in Section 3.2.
2. Standards in parentheses are standards adjusted to current test procedures.
3. There is no set phase- in requirement for ULEV vehicles; generally, as automakers meet more stringent “ fleet
averaging” requirements, an increasing percentage of vehicles will be ULEVs from the 1994 through 2010 model
years. The ZEV mandate ( 10% of vehicle sales) begins 2003, with demonstration program requirements for the
1998 – 2002 model years.
Source: Chrysler, 1998; pp. 18, 33.
3.1.2 Cleaner Burning Fuels
Beginning in the winter of 1992- 1993, California implemented an oxygenated gasoline
program to reduce motor vehicle CO emissions. The program resulted in an approximate 5% to
10% reduction in ambient CO concentrations ( Dolislager, 1997; p. 783). Methyl tertiary butyl
ether ( MTBE) accounted for approximately 95% of the oxygenate used ( Dolislager, 1997;
p. 776).
CO emissions reduction benefits from oxygenated fuels are generally considered to be
greater in the near term while there are still significant numbers of older vehicles on the road.
The addition of oxygenates helps reduce CO emissions by helping older cars operate with leaner
fuel- air mixtures ( Calvert et al., 1993; p. 42). Various studies have shown that CO emissions
from older vehicles ( e. g., 1982 or older model year vehicles) are reduced at greater rates than
newer vehicles with computer- controlled oxygen sensors ( Wilkes and Anderson, 1997).
Due to concerns about MTBE contamination in various water supplies, ARB rescinded
the oxygenated fuels requirement for much of California beginning with the 1998- 1999 winter
season. However, state and federal requirements call for continued oxygenated fuels use in a
number of California metropolitan areas. ARB retained the oxygenate requirement in areas
3- 3
where CO concentrations were still problematic; for example, several southern California areas,
including Los Angeles, are still required to include oxygenates during the winter season. Some
mountain county areas, including Lake Tahoe, are required to continue wintertime oxygenates
until 2001. Federal rules include an oxygenate requirement for ozone nonattainment areas,
requiring both the San Diego and Sacramento areas to continue a year- round oxygenate program.
ARB estimates that rescission of the wintertime oxygenate program resulted in an approximate
9% increase in motor vehicle CO emissions for the affected areas ( California Air Resources
Board, 1998a). ARB estimated that the 9% emissions increase was a “ worst case” scenario, and
forecasted that following the elimination of the fuels requirement, CO emissions would
“… remain well below levels required to maintain the carbon monoxide standard” ( California Air
Resources Board, 1998A; p. 4).
3.1.3 Motor Vehicle Inspection and Maintenance
California implemented a motor vehicle inspection and maintenance ( IM) program,
called Smog Check, beginning in 1984. Following passage of the 1990 Clean Air Act
Amendments, EPA required the worst- polluted areas to implement “ enhanced” IM programs.
California began implementation of its enhanced program, called Smog Check II, in 1998. The
original program initiated in 1984 reduced motor vehicle CO emissions by approximately 15%
( IMRC, 1993). During the summer of 2000, both ARB and the California IM Review
Committee ( IMRC) evaluated the newly implemented enhanced IM program. ARB and IMRC
estimated that, in 1999, Smog Check II reduced CO emissions by 13% to 28% ( California Air
Resources Board 2000c, p. ES- 8; IMRC 2000, p. ES- 1).
3.2 EXPECTED FUTURE CONTROLS
3.2.1 California Control Programs
Further motor vehicle CO emissions reductions will occur over time, beyond those
already committed to in California’s air quality management plans. Emissions projections
included in the California CO SIP ( see Table 2- 2) do not take credit for a number of control
programs that are adopted or planned ( California Air Resources Board, 1998a; p. 5). Examples
include
1. Oxygenated fuels use— The California CO SIP does not take credit for oxygenated fuels
use, despite ongoing state and federal oxygenated fuels program requirements in several
California areas such as Los Angeles and Sacramento.
2. IM program improvements— The CO SIP does not take credit for 1998 and later
improvements to basic IM, or for enhanced IM. In addition, in August 2000 ARB
committed to further improve the Smog Check II program to reduce emissions of
hydrocarbons ( HC) and oxides of nitrogen ( NOx). These program improvements will
likely include subsidiary CO benefits as more high- polluting vehicles are identified and
repaired.
3- 4
3. On Board Diagnostics ( OBD) — Beginning with the 1996 model year, ARB required full
phase- in of the OBD- II program. OBD- II triggers illumination of a dashboard
malfunction indicator light ( MIL) when an onboard computer senses that an emission
control system component has malfunctioned. The onboard computer stores the
malfunction data for later retrieval by qualified repair technicians. Although ARB
established OBD- II program requirements to achieve HC and NOx reductions, CO
benefits will also occur. A recent American Petroleum Institute ( API) study of high
emitting vehicles showed that approximately half of the excess emissions from fuel
injected vehicles are due to electrical component failures related to the emission control
system ( API, 1996; pp. ES- 4 and ES- 5). API found that for fuel- injected vehicles,
virtually all HC repairs, and approximately half of the NOx repairs, resulted in CO
emissions reductions ( API, 1996; Table ES- 1).
4. LEV- II— ARB amended its low- emitting vehicle ( LEV) program in late 1998 and
established LEV- II regulations that take effect with the 2004 model year. LEV- II
includes at least three major actions that will reduce CO emissions. First, LEV- II extends
passenger car exhaust standards to most sport utility vehicles ( SUVs), thus reducing CO
tailpipe standards for SUVs. For most SUVs ( approximately 90%), this means that rather
than meeting 50,000- mile exhaust standards of 4.4 or 5.0 gms of CO per mile, the
standard will be 3.4 gms/ mi. Second, LEV- II increases emission control durability
standards from 100,000 to 120,000 miles for passenger cars and light trucks. Third,
LEV- II tightens fleet- average emission standards during the 2004 to 2010 time period,
including creating a “ super- ultra low emission vehicle” category with CO emissions less
than half those of ultra low emission vehicles ( ARB, 2000d).
5. Federal Test Procedure ( FTP) improvements— EPA and the State of California have
established additions to the FTP to examine CO and other emissions under more realistic
driving conditions. The Supplemental Federal Test Procedure ( SFTP) contains the
following segments: the “ US06” test cycle to represent high speed, high acceleration,
and rapid speed fluctuations; the A/ C Test to measure emissions while air conditioning is
in use; and a test to measure emissions during intermediate periods when the engine is
turned off. The SFTP also includes the “ SC01” test cycle to simulate start driving
behavior and rapid speed changes. SC01 is to be conducted after a 60- minute soak with
full air conditioning simulation. EPA estimates that the SFTP, along with additional
emission standards changes, will result in an 11% reduction in CO emissions in the
United States ( U. S. Environmental Protection Agency, 1996). Under California
regulations, model year 2001 and beyond passenger cars and light- duty trucks must meet
the US06 and A/ C Test CO emission standards listed in Table 3.2. The standards for
medium- duty trucks are also listed. The vehicles must meet these standards after
approximately 4,000 miles of travel ( California Air Resources Board, 1999b).
3- 5
Table 3- 2. Supplemental federal test procedure emission standards.
Vehicle type
Gross Vehicle
Weight ( lbs.)
US06 CO emission
standard ( g/ mi)
A/ C Test CO emission
standard ( g/ mi)
Passenger cars All 8.0 2.7
Light- duty trucks 0 - 3750 8.0 2.7
Light- duty trucks 3751- 5750 10.5 3.5
Medium- duty trucks 5751 – 8500 11.8 4.0
Gross Vehicle Weight is the adjusted loaded vehicle weight. Standards apply uniformly to
LEVs ( low- emission vehicles), ULEVs ( ultra- low- emission vehicles), and SULEVs ( super-ultra-
low- emission vehicles).
The light duty vehicle and truck emission standards are for vehicles certified at 4,000± 250
miles and applicable for MY 2001 vehicles and beyond.
The medium- duty vehicle emission standards are for vehicles certified at 4,000± 250 miles and
are applicable for MY 2003 vehicles and beyond.
Source: California Air Resources Board, 1999b.
6. Low- sulfur fuel— Sulfur can adsorb to catalytic converters, diminishing their ability to
remove CO and other pollutants. An ARB analysis of a sample vehicle fleet found that a
10 ppm reduction in sulfur content could lead to an almost 1% reduction in CO emissions
( California Air Resources Board, 1999c). California has mandated the use of reduced-sulfur
gasoline as part of the Reformulated Gasoline Phase 3 standards. Beginning
December 31, 2002, average gasoline sulfur content will be lowered to 15 ppm from
30 ppm, with a cap of 60 ppm. The cap will again be lowered to 30 ppm, starting in
2005 ( California Air Resources Board, 2000e; Table 1).
3.2.2 Federal Control Programs
EPA, through its Tier 2 motor vehicle standards, has also mandated that light- duty trucks
must meet passenger vehicle emission standards, with phase- in beginning in 2004. A portion of
the overall VMT in California is driven by vehicles purchased outside California, so the new
federal emission standards will help to reduce CO emissions in California ( U. S. Environmental
Protection Agency, 2000d; p. 6718). EPA has also mandated lowering the sulfur content of
gasoline sold outside California. Currently, gasoline outside California is allowed to have a
sulfur content of about 300 ppm. The new standards limit gasoline sulfur content to,
approximately, an average of 120 ppm and a cap of 300 ppm in 2004. The Tier 2 standards
reduce the average sulfur content produced by most refiners to 30 ppm, with a cap of 80 ppm, by
2006 ( U. S. Environmental Protection Agency, 2000d; p. 6702). California implemented similar
standards in 1996. The availability of low- sulfur gasoline throughout the country should ensure
that California vehicles using out- of- state gasoline, and out- of- state vehicles that travel in
California, will have more effective catalysts and thus lower CO emissions.
3- 6
3.3 CONTROL PROGRAM SUMMARY
Past reductions in motor vehicle CO emissions can be attributed to tailpipe exhaust
standards, motor vehicle inspection and maintenance, and reformulated gasoline. Combined,
these control programs have dramatically reduced per- vehicle and fleet- total CO emissions,
resulting in substantial declines in ambient CO concentrations. California and federal agencies
continue to implement more stringent emission controls, and future CO emissions can be
expected to continue to drop, even beyond the reductions currently accounted for in existing
SIPs.
Based on emissions and concentrations trends ( Section 2), and past and future control
program efforts ( Section 3), we are able to answer the first of the four questions posed at the
beginning of this report ( see Section 1.1): “ Are past declines in CO emissions and
concentrations expected to continue into the future?”
Past declines in CO emissions and concentrations are documented to continue at least
through the next 10 to 15 years and will likely continue for many more years. Some state and
federal data indicate increased CO emissions 15 to 20 years or more beyond the year 2000.
However, as discussed in Section 2.4.3, emissions projections greater than 10 to 15 years into the
future tend to be conservative because they do not incorporate the benefit of air quality control
programs that will likely be adopted to address these future- year emissions. Despite the fact that
some projections show increased CO emissions in the distant future, these future- year CO
emission totals fall substantially below the thresholds allowed. A good example of this involves
the Sacramento area where year 2022 motor vehicle CO emissions are estimated to be about 164
tons per day— above prior years but below the 780 tons- per- day allowable level ( see Table 2- 9).
4- 1
4. CASE STUDIES: SOUTHERN CALIFORNIA AND SACRAMENTO
4.1 OVERVIEW AND METHODOLOGY
Regional emissions and concentration trends show substantial declines over time, with
reductions expected to continue into the future. Key questions for this study focused on the
relationship between regional and microscale CO concentrations. The study team used data from
northern and southern California to explore the regional- to- microscale relationship. Our
analyses involved three steps:
1. Determine how well microscale concentrations track the overall decline in regional CO
concentrations by analyzing data from microscale monitors located near high- density
traffic activity centers, and data from nearby neighborhood monitors that provided
regional CO concentration values.
2. Examine how robust the regional versus microscale relationship is across several CO
concentration metrics including highest- observed CO concentrations, 2nd highest values,
and other observations.
3. Establish microscale- to- regional relationships based on monitored concentration data,
regression- based emission and concentration trend analyses, and a “ rollback” analysis
that projected future CO concentrations.
4.2 BACKGROUND INFORMATION ON MONITORING SITES AND DATA USED
4.2.1 Southern California Sites ( Los Angeles and Riverside)
Four CO monitoring sites were chosen within the South Coast Air Basin. Two sites were
selected from the Los Angeles area to represent worst- case urban area CO concentrations. We
also selected two sites to represent Riverside, a suburban area outside the urban core but
generally on the downwind side of the South Coast Air Basin. Each pair of monitors consisted
of a microscale monitor and the nearest neighborhood scale monitor. The Los Angeles area sites
included Lynwood and Hawthorne. The Lynwood monitor is designated as a microscale monitor
and generally has experienced the highest CO concentrations and the concentrations most
persistently above the CO NAAQS of any monitor in California ( and perhaps the United States).
The Lynwood site is located near the intersection of two busy arterials, Imperial Boulevard and
Long Beach Boulevard, in a mixed- use area of south central Los Angeles. The Hawthorne site
is the neighborhood- scale monitor nearest to the Lynwood station. The Hawthorne monitoring
station is approximately 10 miles to the west ( and generally upwind) of the Lynwood monitor. It
is located near the 405 freeway but is otherwise in a “ neighborhood” setting with various
buildings and trees to the north and south of the monitor ( Cassmassi, 2000). The Riverside sites
include monitors at Magnolia and Rubidoux. The Magnolia station is designated as a microscale
monitor. The Magnolia site is located near the intersection of Magnolia and Arlington
Boulevards, both heavily trafficked routes, in a mixed- use residential and store- front business
area. The Rubidoux site is a neighborhood scale monitor. The Rubidoux site is approximately
4.5 miles to the north of the Magnolia station, in a relatively rural area. In the immediate vicinity
4- 2
are a vacant lot, a senior citizens apartment complex, a residential neighborhood that includes
homes and livestock ( horses and other animals) boarding areas, and a shopping center beyond
the vacant lot ( Bermudez, 2000).
4.2.2 Sacramento Sites
Two sites, El Camino and Del Paso, were selected for the Sacramento Valley Air Basin.
The El Camino site is near an intersection and is a designated microscale monitor. The Del Paso
station is designated as a neighborhood- scale monitor. The Del Paso monitor is approximately
1 mile east northeast from the El Camino monitor. The Del Paso site is in a suburban, residential
area and is located adjacent to an elementary school and a small neighborhood park. The area
has relatively low traffic density and is just north and east of the downtown Sacramento area.
The El Camino site is located near the intersection of El Camino Avenue and Watt Avenue. The
monitor is approximately ten feet from El Camino Avenue on a median strip with the road on
one side and the north end of the Country Club Plaza Shopping Center parking lot on the other
side. The area experiences relatively high traffic density, particularly since Watt Avenue is one
of the main Sacramento arterials ( Ching, 2000).
4.2.3 Data Sources
CO monitoring data from 1988 through 1998 were obtained for the four southern
California monitors from the Meteorology Section of the SCAQMD. The total basin and on-road
CO emissions estimates for the South Coast Air Basin used in the data analysis were taken
from the South Coast Air Basin’s 1997 Air Quality Management Plan ( South Coast Air Quality
Management District, 1996). Sacramento monitoring data were obtained from ARB ( California
Air Resources Board, 1998b). The Sacramento emissions data were taken from the Proposed
Carbon Monoxide Redesignation Request and Maintenance Plan for Ten Federal Planning
Areas ( California Air Resources Board, 1996).
4.2.4 Analysis Methods
The first analysis objective was to establish observable trends for each monitoring site,
and then compare the trend lines for the neighborhood and microscale site pairs. The analysis
proceeded by plotting, for each site, CO concentration and emissions data from approximately
1990 to 1998. For each year of monitoring data, the highest, 2nd highest, 10th highest,
20th highest, and 100th highest 1- hr and 8- hr average concentrations were calculated and plotted
for each site. To facilitate site- to- site comparisons, the emissions and concentration data were
both normalized by dividing by their respective 1990 values, and then plotted. The 1990 base
year was chosen because it was the first year for which there were both concentration and
emissions data. In addition, a fitted trend line through the concentration and emissions data was
added to each plot.
A rollback analysis was also performed on the 2nd highest 1- hr and 8- hr concentration
curves. The goal of the rollback analysis was to project forward in time the expected CO
4- 3
concentrations. Analyses were projected to 2020 for southern California ( the worst- polluted
region), and to 2010 for Sacramento. The rollback analysis steps are as follow:
1. The projected CO total basin emissions ( 1999 and beyond) were divided by a base- year
value ( the 1997 total basin emission estimate for the southern California sites; the
1995 total basin emission estimate for the Sacramento sites). Thus, for each future year
we created a ratio value ( Fr) of future total basin emissions to base- year emissions.
2. The base- year ( 1997 for southern California sites; 1995 for Sacramento sites)
concentration value was then multiplied by Fr for each future year.
3. To “ smooth out” meteorological variability, Fr was applied to the base- year concentration
values located on the fitted observed concentration trend line, not on the actual
concentration data points.
We also performed the rollback analysis using just on- road emissions to create the value
of Fr; the results are included as Appendix A. As discussed previously, on- road emissions
decline in relative importance over time. Correspondingly, future- year CO concentrations
predicted by rollback analyses that use only the on- road emissions are substantially less than the
CO concentrations predicted using total basin emissions. Comparing the plots in the body of the
text to those in Appendix A will help readers visualize the increasing importance of stationary,
area, and off- road CO emissions.
Included for presentation in the main body of this report are plots that portray the
2nd highest 1- hr and 8- hr average concentrations, emissions estimates for past and future years,
trend lines fitted through observed concentrations and emissions for past years, and rollback
analysis results. The values for the 2nd highest are presented since they are typically of greatest
interest ( an area is determined to have violated the NAAQS if it exceeds the standards more than
once in a given calendar year; thus the importance of the 2nd highest value). Included as
Appendices B and C are southern California plots of the other thresholds examined ( 10th highest,
20th highest, 100th highest).
4.3 DATA ANALYSIS: THE SOUTHERN CALIFORNIA CASE STUDY
4.3.1 Background: Projected Emission Trends to 2020
The SCAQMD’s 1997 AQMP included emission projections by source category to the
year 2020. These projections document expected continued emissions reductions overall,
including a projected reduction in the relative importance of on- road mobile source CO
emissions. A brief summary of these trends is provided here to establish the overall context for
the microscale versus regional analyses.
As discussed earlier ( see Figure 2- 2) total and on- road CO emissions have been
decreasing in the South Coast Air Basin since 1990. Emissions are projected to continue to
decrease through the year 2010. By 2010, total emissions are forecast to be about 42% of their
1990 levels. On- road emissions are forecast to be about 26% of their 1990 levels. Total
emissions are expected to remain essentially constant between 2010 and 2020, while on- road
4- 4
emissions will decrease slightly. Figure 4- 1 presents the SCAQMD’s winter planning baseline
year emissions inventory projections for 1990 through 2020. Note that the SCAQMD’s AQMP
included control measures that reduce the baseline forecasted year 2020 emissions by 37%
( South Coast Air Quality Management District, 1996; Appendix III, Attachment D, Table D- 4).
Approximately 18% of these reductions ( 18% of 37%, or about 7% of the total emissions
reduction benefit) were forecast to come from on- road mobile source control measures.
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
1990 1995 2000 2005 2010 2015 2020
Year
Total Basin Emissions
On- Road Emissions
Non- Road & Stationary Source Emissions
This plot presents emissions estimates for the South Coast Air Basin. The estimates were obtained from the 1997 Air Quality Management
Plan ( SCAQMD, 1996; Appendix III, Attachment B).
Figure 4.1. CO winter baseline year emissions ( tons per day) for the South Coast Air Basin.
4- 5
Figure 4- 2 illustrates the relative change in importance over time for the on- road mobile
source fraction of the CO emission inventory. From the CO emission projections shown in
Figure 4- 2, it can be seen that on- road emissions are expected to become a less significant source
in the future. In 1990, on- road emissions accounted for approximately 80% of total CO
emissions in the South Coast Air Basin. By the year 2020, on- road emissions are forecast to
account for approximately 45% of total baseline year emissions and approximately 57% of total
emissions under the “ controlled” scenario included in the SCAQMD AQMP. As on- road mobile
sources decline in importance, stationary, area, and off- road emissions are forecast to increase
both in magnitude and as a fraction of total emissions by the year 2020. In the base case ( before
additional controls) the total of all emissions other than on- road mobile are predicted to be
2,116 tons/ day by the year 2020, which is slightly above their 1990 level ( 1,896 tons/ day). In
1990, stationary, area, and off- road sources accounted for approximately 20% of total emissions.
They are forecast to account for approximately 55% of total baseline emissions by the year 2020,
and 43% of the emissions in the SCAQMD’s controlled scenario ( South Coast Air Quality
Management District, 1996; Appendix III, Attachments B and D).
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1990 1995 2000 2005 2010 2015 2020
Year
Fraction
The information in this plot was obtained by dividing on- road emissions estimates by the total emissions for each year. The emissions data was taken from
1997 Air Quality Management Plan( SCAQMD, 1996; Appendix III, Attachment B).
Note to Figure: projections illustrated use baseline emissions data; “ controlled” scenario forecasts show the fraction of on- road
mobile source contribution to be 57% instead of 45% by the year 2020, however the relative contribution of on- road mobile
sources declines over time in either scenario.
Figure 4- 2. On- road mobile CO emissions as a fraction of total CO emissions for the
South Coast Air Basin, 1990- 2020.
4- 6
4.3.2 Observations of Southern California Emission and Concentration Trends by
Monitoring Site
We analyzed the relationship between microscale and regional CO trends for each of the
Los Angeles and Riverside area monitoring locations. Figures 4- 3 through 4- 10 show the
2nd highest 1- hr and 8- hr concentrations for each of the Los Angeles area sites: Lynwood
( microscale) and Hawthorne ( neighborhood). Figures 4- 11 through 4- 18 show the 2nd highest
1- hr and 8- hr concentrations for each of the Riverside sites: Magnolia ( microscale) and
Rubidoux ( neighborhood). The figures also include emissions data, trend lines, and rollback
analysis results. Similar figures are presented in Section 4.4 for the Sacramento case study.
The southern California and Sacramento figures included in the body of the text
incorporate the information essential for understanding the results of the case studies and are
worth explaining in some detail. The plots portray seven important pieces of information:
1. Historical emissions data. These data points extend from 1990 to 1995 ( in Sacramento)
and from 1990 to 1997 ( in southern California).
2. Emissions projections. These data points represent air district- forecasted CO emissions
for future years ( 2000- 2010 for Sacramento; 1998- 2020 for southern California).
3. Concentration data. These are observed CO concentration measurements, as reported by
ARB, for the years 1988 through 1998.
4. Trend lines fitted by regression through the historical emissions data. The trend line
through the historical emissions data is carried forward in time to the last analysis year
( 2010 in Sacramento; 2020 in southern California). By comparing the trend line to the
emissions projections, the reader can visually compare the rate at which CO emissions
declined in the past, versus the rate at which CO is expected to decline in the future. The
trend lines include the algebraic descriptions of the fitted lines. The algebraic
descriptions identify the shape of the fitted curves and provide a numerical comparison of
the rate at which emissions have declined with the rate at which concentrations have
declined. In the algebraic description, the exponential term is the key determinant to the
rate at which the lines trend downward. The more negative the exponential value ( i. e.,
the larger the absolute value of the exponent), the greater rate at which the trend line
declines.
5. Trend lines fitted by regression through the concentration data. These trend lines end at
the last observed CO concentration value. The trend lines serve two purposes. First, the
fitted lines help smooth out year- to- year variability that may be due to changing
meteorological conditions. Second, the lines help the reader visually separate past
concentration observations from the future- projected concentrations predicted by the
rollback analysis. Algebraic descriptions of the trend lines are included to facilitate
comparisons to the emissions trends.
6. Rollback analysis results. Future CO concentrations as predicted by the rollback analysis
are plotted, extending from the trend lines fitted through the concentration data.
7. CO NAAQS. The Y axis of the plots is scaled to allow comparisons to 1990. Both the
concentration and emission data have been normalized to a 1990 value of 1.0. To
facilitate understanding how the normalized concentrations compare to real- world values,
4- 7
the plots include a line to indicate where a concentration equal to the NAAQS would fall
on the plot.
The plots facilitate comparisons between emissions and concentrations, between past and
forecasted trends, and between microscale and neighborhood conditions. To be conservative, the
emission projections shown for southern California are from the “ baseline” projections included
in the SCAQMD AQMP, rather than the “ controlled scenario” projections. Also, it is important
to restate the point made in Section 3 that for both Sacramento and southern California the
forecasted emissions do not fully reflect ARB and EPA control programs.
From the plots in Figures 4- 3 through 4- 18, it can be seen that measured concentrations
showed a decreasing trend at all sites between 1988 and 1998. In general, the concentrations at
the peak events ( maximum and 2nd highest values) decreased more quickly than the
concentrations at the less extreme events ( see Appendices B and C). Measured concentrations
at microscale monitors are decreasing at a greater rate than concentrations at the
neighborhood scale monitors. The various 1- hr concentrations tended to decrease at a faster
rate than the corresponding 8- hr concentrations ( i. e., the maximum 1- hr concentrations curve for
a particular monitor has a steeper slope than the maximum 8- hr concentration curve).
For both the Los Angeles and Riverside pairs of monitors, the trends of the emissions and
concentrations tended to match better ( i. e., have better correlation in their slopes) at the extreme
concentration events ( maximum, 2nd highest) than at the less extreme events ( e. g., 20th highest,
100th highest). In the Riverside set of monitors, the microscale monitor ( Magnolia) tracks both
the total and on- road emissions trends better than the neighborhood monitor ( Rubidoux). This
relationship is not as strong in the Los Angeles monitors. For the most part, the microscale
monitor ( Lynwood) tracks the emissions trends better, but the neighborhood scale monitor
( Hawthorne) tracks emissions better in a few circumstances.
4- 8
y = 1.285e- 0.056x
y = 1.142e- 0.063x
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
0 4 8 12 16 20 24 28 32
Year
2nd high 1- hr concentrations
Total basin emissions
Projected total basin emissions
1- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 1- hr concentrations
Fitted regression line based on total basin emissions
1988 1992 1996 2000 2004 2008 2012 2016 2020
Federal 1- hr standard = 35 ppm
Figure 4- 3. Lynwood ( microscale): total basin emissions and 2nd highest 1- hr concentrations
( with concentration rollback based on total basin emissions).
y = 1.212e- 0.064x
y = 1.142e- 0.063x
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
0 4 8 12 16 20 24 28 32
Year
2nd high 1- hr concentrations
Total basin emissions
Projected total basin emissions
1- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 1- hr concentrations
Fitted regression line based on total basin emissions
1988 1992 1996 2000 2004 2008 2012 2016 2020
Federal 1- hr standard = 35 ppm
Figure 4- 4. Hawthorne ( neighborhood): total basin emissions and 2nd highest
1- hr concentrations ( with concentration rollback based on total basin emissions).
4- 9
y = 1.285e- 0.056x
y = 1.200e- 0.081x
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
0 4 8 12 16 20 24 28 32
Year
2nd high 1- hr concentrations
On- road basin emissions
Projected on- road basin emissions
1- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 1- hr concentrations
Fitted regression line based on on- road basin emissions
1988 1992 1996 2000 2004 2008 2012 2016 2020
Federal 1- hr standard = 35 ppm
Figure 4- 5. Lynwood ( microscale): on- road basin emissions and 2nd highest
1- hr concentrations ( with concentration rollback based on total basin emissions).
y = 1.212e- 0.064x
y = 1.200e- 0.081x
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
0 4 8 12 16 20 24 28 32
Year
2nd high 1- hr concentrations
On- road basin emissions
Projected on- road basin emissions
1- hr NAAQS
Concentration rollback based on total emissions
Fitted regression line based on 2nd high 1- hr concentrations
Fitted regression line based on on- road basin emissions
1988 1992 1996 2000 2004 2008 2012 2016 2020
Federal 1- hr standard = 35 ppm
Figure 4- 6. Hawthorne ( neighborhood): on- road basin emissions 2nd highest
1- hr concentrations ( with concentration rollback based on total basin emissions).
4- 10
y = 1.336e- 0.054x
y = 1.142e- 0.063x
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 4 8 12 16 20 24 28 32
Year
2nd high 8- hr concentrations
Total basin emissions
Projected total basin emissions
8- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 8- hr concentrations
Fitted regression line based on total basin emissions
1988 1992 1996 2000 2004 2008 2012 2016 2020
Federal 8- hr standard = 9 ppm
Figure 4- 7. Lynwood ( microscale): total basin emissions and 2nd highest 8- hr concentrations
( with concentration rollback based on total basin emissions).
y = 1.162e- 0.047x
y = 1.142e- 0.063x
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 4 8 12 16 20 24 28 32
Year
2nd high 8- hr concentrations
Total basin emissions
Projected total basin emissions
8- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 8- hr
concentrations
Fitted regression line based on total basin emissions
1988 1992 1996 2000 2004 2008 2012 2016 2020
Federal 8- hr standard = 9 ppm
Figure 4- 8. Hawthorne ( neighborhood): total basin emissions and 2nd highest
8- hr concentrations ( with concentration rollback based on total basin emissions).
4- 11
y = 1.336e- 0.054x
y = 1.200e- 0.081x
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 4 8 12 16 20 24 28 32
Year
2nd high 8- hr concentrations
On- road basin emissions
Projected on- road basin emissions
8- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 8- hr concentrations
Fitted regression line based on on- road basin emissions
1988 1992 1996 2000 2004 2008 2012 2016 2020
Federal 8- hr standard = 9 ppm
Figure 4- 9. Lynwood ( microscale): on- road basin emissions and 2nd highest
8- hr concentrations ( with concentration rollback based on total basin emissions).
y = 1.162e - 0.047x
y = 1.200e - 0.081x
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 4 8 12 16 20 24 28 32
Year
2nd high 8- hr concentrations
On- road basin emissions
Projected on- road basin emissions
8- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 8- hr concentrations
Fitted regression line based on on- road basin emissions
1988 1992 1996 2000 2004 2008 2012 2016 2020
Federal 8- hr standard = 9 ppm
Figure 4- 10. Hawthorne ( neighborhood): on- road basin emissions and 2nd highest
8- hr concentrations ( with concentration rollback based on total basin
emissions).
4- 12
y = 0.926e- 0.056x
y = 1.142e- 0.063x
0
0.5
1
1.5
2
2.5
3
3.5
4
0 4 8 12 16 20 24 28 32
Year
2nd high 1- hr concentrations
Total basin emissions
Projected total basin emissions
1- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 1- hr concentrations
Fitted regression line based on total basin emissions
1988 1992 1996 2000 2004 2008 2012 2016 2020
Federal 1- hr standard = 35 ppm
Figure 4- 11. Magnolia ( microscale): total basin emissions and 2nd highest
1- hr concentrations ( with concentration rollback based on total basin
emissions).
y = 1.101e- 0.047x
y = 1.142e- 0.063x
0
0.5
1
1.5
2
2.5
3
3.5
4
0 4 8 12 16 20 24 28 32
Year
2nd high 1- hr concentrations
Total basin emissions
Projected total basin emissions
1- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 1- hr concentrations
Fitted regression line based on total basin emissions
1988 1992 1996 2000 2004 2008 2012 2016 2020
Federal 1- hr standard = 35 ppm
Figure 4- 12. Rubidoux ( neighborhood): total basin emissions and 2nd highest
1- hr concentrations ( with concentration rollback based on total basin emissions).
4- 13
y = 0.926e - 0.056x
y = 1.200e - 0.081x
0
0.5
1
1.5
2
2.5
3
3.5
4
0 4 8 12 16 20 24 28 32
Year
2nd high 1- hr concentrations
On- road basin emissions
Projected on- road basin emissions
1- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 1- hr concentrations
Fitted regression line based on on- road basin emissions
1988 1992 1996 2000 2004 2008 2012 2016 2020
Federal 1- hr standard = 35 ppm
Figure 4- 13. Magnolia ( microscale): on- road basin emissions and 2nd highest
1- hr concentrations ( with concentration rollback based on total basin emissions).
y = 1.101e- 0.047x
y = 1.200e- 0.081x
0
0.5
1
1.5
2
2.5
3
3.5
4
0 4 8 12 16 20 24 28 32
Year
2nd high 1- hr concentrations
On- road basin emissions
Projected on- road basin emissions
1- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 1- hr concentrations
Fitted regression line based on on- road basin emissions
1988 1992 1996 2000 2004 2008 2012 2016 2020
Federal 1- hr standard = 35 ppm
Figure 4- 14. Rubidoux ( neighborhood): on- road basin emissions and 2nd highest
1- hr concentrations ( with concentration rollback based on total basin
emissions).
4- 14
y = 1.164e- 0.046x
y = 1.142e- 0.063x
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 4 8 12 16 20 24 28 32
Year
2nd high 8- hr concentrations
Total basin emissions
Projected total basin emissions
8- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 8- hr concentrations
Fitted regression line based on total basin emissions
1988 1992 1996 2000 2004 2008 2012 2016 2020
Federal 8- hr standard = 9 ppm
Figure 4- 15. Magnolia ( microscale): total basin emissions and 2nd highest
8- hr concentrations ( with concentration rollback based on total basin
emissions).
y = 1.322e- 0.043x
y = 1.142e- 0.063x
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 4 8 12 16 20 24 28 32
Year
2nd high 8- hr concentrations
Total basin emissions
Projected total basin emissions
8- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 8- hr concentrations
Fitted regression line based on total basin emissions
1988 1992 1996 2000 2004 2008 2012 2016 2020
Federal 8- hr standard = 9 ppm
Figure 4- 16. Rubidoux ( neighborhood): total basin emissions and 2nd highest
8- hr concentrations ( with concentration rollback based on total basin emissions).
4- 15
y = 1.164e- 0.046x
y = 1.200e- 0.081x
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 4 8 12 16 20 24 28 32
Year
2nd high 8- hr concentrations
On- road basin emissions
Projected on- road basin emissions
8- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 8- hr concentrations
Fitted regression line based on on- road basin emissions
1988 1992 1996 2000 2004 2008 2012 2016 2020
Federal 8- hr standard = 9 ppm
Figure 4- 17. Magnolia ( microscale): on- road basin emissions and 2nd highest 8- hr CO
concentrations ( with concentration rollback based on total basin emissions).
y = 1.322e - 0.043x
y = 1.200e - 0.081x
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
0 4 8 12 16 20 24 28 32
Year
2nd high 8- hr concentrations
On- road basin emissions
Projected on- road basin emissions
8- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 8- hr concentrations
Fitted regression line based on on- road basin emissions
1988 1992 1996 2000 2004 2008 2012 2016 2020
Federal 8- hr standard = 9 ppm
Figure 4- 18. Rubidoux ( neighborhood): on- road basin emissions and 2nd highest
8- hr concentrations ( with concentration rollback based on total basin
emissions).
4- 16
4.3.3 Discussion of Observed Emission and Concentration Relationships
Emissions
As Figures 4- 3 through 4- 18 show, the trend lines through both the estimated total and
on- road emissions data ( 1990- 1997) decrease more quickly than the forecasts provided by
SCAQMD for years 1990 through 2020. The SCAQMD forecasts show the majority of the
emissions reductions between 1990 and 2020 occur in the first ten years. Thus, the regression
through the actual emissions data has a steeper downward slope than the SCAQMD projections.
SCAQMD forecasts that total emissions will essentially remain constant and on- road emissions
will decrease slightly between 2010 and 2020. The SCAQMD projections include control
measures approved as of September 1996. As discussed in Section 3, a number of additional
control programs that were not included in the SCAQMD report have been adopted and will
likely continue to be adopted in the future. Thus, the SCAQMD projections may be considered a
conservative forecast. The actual emissions trend will probably be steeper than the decline
predicted by the SCAQMD estimates, but probably less steep than the trend line through the
historical emissions estimates because the rate of emissions reduction will likely be slower in the
future than it was between 1990 and 1997.
Concentrations
The concentrations at the extreme events tended to decrease more rapidly than the
concentrations at the less extreme events for the southern California monitors. This observation
can be understood in the sense that peak events represent the most unfavorable meteorological
conditions. ( In the case of CO, these conditions consist of low wind speed for sustained periods
and strong surface- based inversions.) Therefore, comparison of " peak events" for CO actually
reduces meteorological variability ( i. e., the worst local meteorology cannot get much worse in
the sense that the wind speed cannot decrease nor stability increase significantly) and are a better
reflection of the impact of emissions reductions.
The measured concentrations at microscale monitors generally decreased at a greater rate
than concentrations at neighborhood scale monitors in southern California. Figures 4- 7 and 4- 8
help illustrate this finding. Figures 4- 7 and 4- 8 compare 2nd highest 8- hr concentrations for
Lynwood and Hawthorne. The algebraic descriptions of the concentration trend lines in each
figure show that concentrations at the microscale site, Lynwood, dropped at a greater rate than
concentrations at Hawthorne ( i. e., the exponent term in the algebraic equation is a larger negative
number in the Lynwood example). In fact, with only one exception, all of the Sacramento and
southern California microscale 2nd highest concentrations decreased at a faster rate than their
neighborhood counterparts. In general, microscale monitors are located near heavily trafficked
areas and thus reflect a greater contribution of emissions from motor vehicles. In contrast,
neighborhood scale monitors represent motor vehicle contributions as well as area- wide source
contributions ( stationary, area, and off- road emissions). As the emissions data from ARB and
the SCAQMD illustrate, area- wide contributions are not decreasing and are becoming a larger
fraction of total CO emissions over time ( see Figures 4- 1 and 4- 2). Thus, the fact that microscale
concentrations are decreasing more rapidly for the peak events suggests that an area- wide vehicle
control strategy will produce a greater rate of emissions reductions for peak concentrations at
microscale monitors than at neighborhood monitors.
4- 17
Relationship Between Emissions and Concentrations
For the southern California locations, measured concentrations do not track total
emissions as well as would be expected during the period for which there are measured data
( 1988- 1998). Emissions estimates are less accurate than the measured concentrations, and it is
possible that some portion of the CO emissions ( either on- road, off- road, stationary, or area) was
not accounted for in the SCAQMD inventory. Various tunnel studies and other emission
inventory evaluations have indicated that the emissions modeling tools used over the past five to
ten years have generally under- predicted on- road emissions. ARB improved its mobile source
emissions modeling tools during the late 1990s to address under- prediction problems, but an
older model version was used to construct the SCAQMD 1997 AQMP.
An example of how modeled emission inventories change over time with improved
modeling tools is provided by comparing EMFAC7G and EMFAC 2000 motor vehicle emission
forecasts. The SCAQMD emissions projections for southern California included in this report’s
figures are based on the EMFAC7G model ( South Coast Air Quality Management District, 1996;
Appendix III, p. III- 1- 10). Following the release of the SCAQMD’s 1997 AQMP, ARB
approved a new version of its mobile source emissions modeling tool, called EMFAC 2000.
EMFAC 2000 estimates CO emissions in the South Coast Air Basin that are 30% higher for the
year 2000, and 32% higher for the year 2010 than the CO estimates produced by EMFAC7G
( California Air Resources Board, 2000f; pp. 17- 18). Mobile source emission inventory under-prediction
could help explain the differences observed between the emissions and concentration
trend lines on Figures 4- 5, 4- 6, 4- 9, 4- 10, 4- 13, 4- 14, 4- 17, and 4- 18, where the on- road
emissions curves drop more substantially than the concentration curves.
The trend lines through the measured concentrations at the microscale and neighborhood
scale stations in southern California generally match the total emissions projections better than
the on- road emissions projections. This observation is consistent with a hypothesis that there
may be some inaccuracy in the on- road portion of the inventory, as demonstrated by the change
in projected on- road CO emissions when comparing EMFAC7G to EMFAC 2000.
[ Interestingly, as discussed in the Sacramento case study, the situation was reversed for the
Sacramento monitors.]
From the southern California plots, it can be seen that CO emissions estimates are
dropping more rapidly than measured concentrations over the range of measured data. One
explanation for this is that as concentrations drop, a greater percentage of the measured CO is
due to “ background” CO concentration levels. As CO concentrations approach background
conditions, further emissions reductions have a reduced impact on ambient concentrations.
Another important observation is that on- road emissions estimates are decreasing more
rapidly than total emissions. This observation is consistent with the substantial reduction in
on- road motor vehicle emissions documented in Sections 2 and 3. Given that microscale
measurements should represent a greater fractional contribution from on- road emissions than
what is observed at neighborhood monitors, we would expect to observe that microscale CO
concentrations decrease at least as quickly, if not more so, than neighborhood concentrations. As
noted above, and illustrated in Figures 4- 5 through 4- 18, the measured concentrations at
4- 18
microscale monitors generally decreased at a greater rate than concentrations at neighborhood
scale monitors in southern California.
4.4 SACRAMENTO CASE STUDY
Generally, the data analysis results for the two Sacramento sites are consistent with the
results for the four southern California sites. Given the similarities between the Sacramento and
southern California results, this discussion briefly highlights the Sacramento findings and notes
differences between the two study regions.
4.4.1 Background: Projected Emission Trends to 2010
Both total and on- road emissions in the Sacramento Valley Air Basin ( Sacramento,
Placer, and Yolo counties) have been declining since 1990. ARB forecasts that emissions will
continue to decrease at least through 2010. ARB forecasts that the 1990 emissions of 1,214 tons
per day will drop to 635 tons per day by the year 2010, or to approximately 52% of their
1990 levels ( California Air Resources Board, 1996; Staff Report Table 7). Year 2010 on- road
emissions are forecast to be about 33% of their 1990 levels. Emissions estimates for 1990 to
2010 for the Sacramento Valley Air Basin are presented in Figure 4- 19.
0
200
400
600
800
1000
1200
1400
1990 1995 2000 2005 2010
Year
Emissions ( tons/ day)
Total Basin Emissions
On- Road Emissions
Non- Road & Stationary Source
Emissions
This plot presents emissions estimates for the Sacramento Valley Air Basin. The estimates were obtained from the Proposed
Carbon Monoxide Redesignation Request and Maintenacne Plan for Ten Federal Planning Areas ( CARB, 1996).
Figure 4- 19. 1990- 2010 CO emissions ( tons per day) for the Sacramento Valley Air Basin.
4- 19
As indicated in Figure 4- 19, on- road emissions are forecast to become a less significant
portion of total emissions. In 1990, on- road emissions constituted approximately 81% of total
emissions. By the year 2010, on- road emissions are expected to account for about 51% of total
emissions. Figure 4- 20 illustrates the declining contribution of on- road mobile source emissions
to the Sacramento CO inventory.
0
0 .1
0 .2
0 .3
0 .4
0 .5
0 .6
0 .7
0 .8
0 .9
1990 1995 2000 2005 2010
Year
Fraction
The info rm a tion in this p lo t was o b ta ine d b y d iv id in g o n - roa d em is sion s e s tim a tes b y th e total em issio ns for e ach yea r. T h e
em is s io n s d a ta w a s ta k e n from th e Pro pos e d C arb o n Mon oxid e R ede sign a tio n R equ est and M ainten anc e P la n for T e n Fed eral
Plan n in g A reas ( CA RB , 1 9 9 6 ).
Figure 4- 20. On- road emissions as fraction of total emissions for Sacramento Valley
Air Basin, 1990- 2010.
By the year 2010, stationary, area, and off- road sources are expected to increase in both
magnitude and as a fraction of total emissions. The emissions from stationary, area, and off- road
sources are expected to reach approximately 309 tons per day by the year 2010, which is more
than their 1990 level of 235 tons per day. Stationary, area, and off- road sources accounted for
about 19% of total emissions in 1990. By the year 2010, they are forecast to contribute
approximately 49% of total CO emissions.
4.4.2 Observations of Sacramento Emission and Concentration Trends by Monitoring
Site
We performed a similar analysis for Sacramento as for southern California. This
discussion summarizes the emissions, concentrations, and microscale versus regional monitoring
data analyses. Figures 4- 21 through 4- 28 portray the various 2nd highest 1- hr and
8- hr concentrations, emissions trends, and rollback analyses for the Sacramento sites. The
concentrations at the microscale monitor ( El Camino) decreased at a greater rate than the
4- 20
concentrations at the neighborhood scale monitor ( Del Paso) for the 2nd highest events. The
1- hr thresholds tended to decrease more quickly than the corresponding 8- hr concentrations.
y = 1.089e- 0.081x
y = 1.086e- 0.046x
0
0.5
1
1.5
2
2.5
3
0 2 4 6 8 10 12 14 16 18 20 22
Year
2nd high 1- hr concentrations
Total basin emissions
Projected total basin emissions
1- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 1- hr concentrations
Fitted regression line based on total basin emissions
1988 1992 1996 2000 2004 2008
Federal 1- hr standard = 35 ppm
Figure 4- 21. El Camino ( microscale): total basin emissions and 2nd highest 1- hr concentrations
( with concentration rollback based on total basin emissions).
y = 1.079e- 0.076x
y = 1.086e- 0.046x
0
0.5
1
1.5
2
2.5
3
0 2 4 6 8 10 12 14 16 18 20 22
Year
2nd high 1- hr concentrations
Total basin emissions
Projected total basin emissions
1- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 1- hr concentrations
Fitted regression line based on total basin emissions
1988 1992 1996 2000 2004 2008
Federal 1- hr standard = 35 ppm
Figure 4- 22. Del Paso ( neighborhood): total basin emissions and 2nd highest
1- hr concentrations ( with concentration rollback based on total basin emissions).
4- 21
y = 1.089e- 0.081x
y = 1.124e- 0.064x
0
0.5
1
1.5
2
2.5
3
0 2 4 6 8 10 12 14 16 18 20 22
Year
2nd high 1- hr concentrations
On- road basin emissions
Projected on- road basin emissions
1- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 1- hr concentrations
Fitted regression line based on on- road basin emissions
1988 1992 1996 2000 2004 2008
Federal 1- hr standard = 35 ppm
Figure 4- 23. El Camino ( microscale): on- road basin emissions 2nd highest 1- hr concentrations
( with concentration rollback based on total basin emissions).
y = 1.079e- 0.076x
y = 1.124e- 0.064x
0
0.5
1
1.5
2
2.5
3
0 2 4 6 8 10 12 14 16 18 20 22
Year
2nd high 1- hr concentrations
On- road basin emissions
Projected on- road basin emissions
1- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 1- hr concentrations
Fitted regression line based on on- road basin emissions
1988 1992 1996 2000 2004 2008
Federal 1- hr standard = 35 ppm
Figure 4- 24. Del Paso ( neighborhood): on- road basin emissions and 2nd highest
1- hr concentrations ( with concentration rollback based on total basin
emissions).
4- 22
y = 1.022e - 0.086x
y = 1.0856e- 0.0456x
0
0.2
0.4
0.6
0.8
1
1.2
0 2 4 6 8 10 12 14 16 18 20 22
Year
2nd high 8- hr concentrations
Total basin emissions
Projected total basin emissions
8- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 8- hr concentrations
Fitted regression line based on total basin emissions
1988 1992 1996 2000 2004 2008
Federal 8- hr standard = 9 ppm
Figure 4- 25. El Camino ( microscale): total basin emissions and 2nd highest
8- hr concentrations ( with concentration rollback based on total basin emissions).
y = 1.045e - 0.081x
y = 1.0856e - 0.0456x
0
0.2
0.4
0.6
0.8
1
1.2
0 2 4 6 8 10 12 14 16 18 20 22
Year
2nd high 8- hr concentrations
Total basin emissions
Projected total basin emissions
8- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 8- hr concentrations
Fitted regression line based on total basin emissions
1988 1992 1996 2000 2004 2008
Federal 8- hr standard = 9 ppm
Figure 4- 26. Del Paso ( neighborhood): total basin emissions and 2nd highest
8- hr concentrations ( with concentration rollback based on total basin emissions).
4- 23
y = 1.022e- 0.086x
y = 1.1239e- 0.0636x
0
0.2
0.4
0.6
0.8
1
1.2
0 2 4 6 8 10 12 14 16 18 20 22
Year
2nd high 8- hr concentrations
On- road basin emissions
Projected on- road basin emissions
8- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 8- hr concentrations
Fitted regression line based on on- road basin emissions
1988 1992 1996 2000 2004 2008
Federal 8- hr standard = 9 ppm
Figure 4- 27. El Camino ( microscale): on- road basin emissions and 2nd highest
8- hr concentrations ( with concentration rollback based on total basin emissions).
y = 1.045e - 0.081x
y = 1.1239e - 0.0636x
0
0.2
0.4
0.6
0.8
1
1.2
0 2 4 6 8 10 12 14 16 18 20 22
Year
2nd high 8- hr concentrations
On- road basin emissions
Projected on- road basin emissions
8- hr NAAQS
Concentration rollback based on total basin emissions
Fitted regression line based on 2nd high 8- hr concentrations
Fitted regression line based on on- road basin emissions
1988 1992 1996 2000 2004 2008
Federal 8- hr standard = 9 ppm
Figure 4- 28. Del Paso ( neighborhood): on- road basin emissions and 2nd highest
8- hr concentrations ( with concentration rollback based on total basin
emissions).
4- 24
4.4.3 Discussion of Observed Emission and Concentration Relationships
Emissions
As with the southern California case study, the trend lines through the estimated total and
on- road basin emissions values ( 1990- 1995) for Sacramento decrease more rapidly than the
projections from ARB for the years 1990 through 2010. As noted above, ARB emissions
estimates are conservative; they do not include all future control measures ( see Section 3). It is
also unlikely that emissions will continue to decline at the rate observed between 1990 and 1995.
The actual emissions trend will likely lie between the trend line through the observed data and
the ARB projections.
Concentrations
As with the southern California monitors, the concentrations at the Sacramento
microscale monitor tended to decrease more rapidly than the concentrations at the neighborhood
monitor. As noted above, microscale monitors reflect the contributions of on- road vehicles,
while neighborhood scale monitors are more likely to be influenced by stationary and area
sources, which are increasing in magnitude in both the SCAQMD and the Sacramento areas.
Relationship Between Emissions and Concentrations
Both the microscale and neighborhood scale trends matched the on- road emissions
projections better than the total emissions projections. This was in contrast to southern
California, where the trend lines through the measured concentrations generally matched the
total emissions projections better than the on- road emissions projections.
The Sacramento data analysis showed that, unlike the southern California sites,
concentrations dropped more rapidly than emissions. A possible explanation for this observation
is related to regional growth patterns. It is possible that VMT growth is occurring more on the
fringe of the Sacramento Valley Air Basin while the monitors in this study are located towards
the center of the basin. Therefore, the changes in vehicle emissions over time would not be
expected to influence the measured concentrations as much as they would if the VMT growth
was occurring near the stations. In the South Coast Air Basin, the VMT growth may be
distributed more evenly throughout the basin, so that any change in vehicle emissions is more
likely to affect concentrations at existing monitoring locations.
As noted earlier, the concentrations and emissions tended to match better at the extreme
events than at the less extreme events. As described in the Los Angeles case study, two
explanations may be ( a) the meteorology of peak events that contribute consistency across peak
events, and ( b) the growing importance of background CO concentrations for non- peak events
such as the 20th highest and especially the 100th highest events.
4- 25
4.5 DISCUSSION OF DATA ANALYSES
Four questions guided this work ( see Section 1.1). The control program analysis in
Section 3 addressed the first question. The Sacramento and southern California case studies
addressed questions two and three:
• Are microscale concentrations declining at a rate faster or slower than regional
concentrations?
• What are likely scenarios for future microscale CO concentrations?
The case studies illustrate that microscale concentrations have declined at a rate faster
than what has been observed at the regional level. The sharper declines at the microscale are
consistent with the greater importance of on- road motor vehicle emissions at the microscale and
the substantial declines achieved in on- road emissions. Analyses show that future ( 2000 to 2010
and 2020) reductions at both the microscale and the regional scale are likely to occur but at rates
slower than experienced during the 1990 to 2000 time period.
Finally, the analyses suggest that the relationship between emissions and concentrations
trends can differ by air basin. During the 1990 to 1997 period, Sacramento concentrations
dropped more rapidly than emissions. During the same time period in Los Angeles, emissions
dropped more rapidly than concentrations. These differences do not alter the observed
relationship between microscale and regional trends. The differences point out interesting
possibilities related to the accuracy of emission inventories and perhaps the spatial importance of
where emissions occur in relation to regional and microscale monitors.
5- 1
5. CONCLUSIONS
5.1 CALIFORNIA AND NATIONAL EMISSION, CONCENTRATION, AND
EXPOSURE TRENDS
Nationally and in California, regional CO problems have lessened dramatically over the
past two decades, due in large measure to the introduction of cleaner vehicles, the use of
reformulated fuels, and implementation of vehicle IM programs. ARB projects that from 1990 to
the year 2010, California CO emissions reductions will range from 29% in Modesto to as much
as 58% in Los Angeles; most major California metropolitan areas will experience emissions
reductions of at least 30% to 40% during this time period ( Table 2- 2 and Figure 2- 2). National
trends mirror those in California. As an example, national on- road motor vehicle CO emissions
are projected to decrease 20% between 1996 and 2007. Although national data indicate
increased on- road emissions are possible in the future, especially during the 2020 to 2030 time
period, these projections for “ out years” lie beyond the current planning horizon of traditional air
quality management programs ( Table 2- 10; Section 2.4.3). California has experienced
substantial drops in observed CO concentrations that are consistent with the emissions reductions
achieved ( Table 2- 4). In California, lack of violations of the CO NAAQS have resulted in
re- designation of all air basins except Los Angeles as either attainment or maintenance areas.
Los Angeles continues to demonstrate steady progress toward achieving the CO NAAQS. An
exception to California’s progress is Calexico, a US- Mexico border area influenced by emissions
from motor vehicles of Mexican registration. Nationally, as of 1999 only a handful of areas
remained in violation of the CO NAAQS, compared to more than 40 areas in 1991. Studies also
document significant reductions in human CO exposure based upon in- vehicle and personal
exposure monitoring and modeling.
5.2 RELATIONSHIPS BETWEEN MICROSCALE AND REGIONAL
CO CONDITIONS
This report examined the hypothesis that regional CO emissions reductions have led to
decreasing regional and microscale CO concentrations. Based on an analysis of past trends, the
evidence obtained supports a hypothesis that concentration reductions observed at microscale
stations are greater than or equal to those observed at neighborhood scale stations and correlate
with regional CO emissions reductions. This is consistent with common sense, given that at the
microscale, on- road motor vehicles are an even more dominant contributor than at the
neighborhood scale, and on- road mobile emissions have dropped significantly while stationary
and area source emissions have stayed the same or even increased.
This report also examined what is likely to occur at the microscale in the future. Analysis
results support the hypothesis that both neighborhood and microscale CO concentrations are
declining and will continue to decline. Although this analysis did not evaluate statistical
significance because the number of case studies explored was limited, trends from one air basin
to another appear compelling. Measured data from the worst nonattainment area in the state, the
South Coast Air Basin, and the Sacramento Valley Air Basin were analyzed to determine long-term
CO trends. On- road emissions are declining and will become a less significant portion of
5- 2
total emissions in the future. In addition, it appears that the microscale data correlate with the
regional scale emissions estimates, which are projected to decrease in the future. Future
reductions in regional emissions should lead to continued reductions in concentrations at the
microscale level. The rate at which microscale concentrations are reduced will probably be at a
slower rate than past reductions, given the reduced rate at which mobile emissions are declining.
A strategy of regional emissions reductions, however, appears to be an effective means of
preventing microscale CO exceedances, particularly given that future mobile source emissions
are likely to continue to decline. The above analysis suggests that a linear rollback methodology
for CO is an effective tool for projecting microscale concentrations.
6- 1
6. POLICY IMPLICATIONS AND RECOMMENDATIONS
Concentrations at microscale stations appear to be decreasing at least as rapidly as
neighborhood stations, if not more so. Since emissions are expected to continue to decrease and
regional attainment is expected to continue, microscale analysis will not be as important as it was
in the past because concentrations at microscale monitors will be increasingly influenced by
regional emissions.
The emissions estimates from the South Coast Air Basin and the Sacramento Valley Air
Basin indicate that on- road emissions will decline and constitute a smaller portion of total
emissions over time. This trend, combined with the projected increase of stationary, area, and
off- road source emissions, suggests that more consideration should be given to control measures
for sources other than on- road vehicles.
California’s air quality management plans project that the entire state will be in
attainment of the federal CO standards by the year 2000 and will remain in attainment until at
least 2010 ( South Coast Air Quality Management District, 1996; California Air Resources
Board, 1996). Recent data illustrate that Los Angeles will not achieve the CO NAAQS by 2000,
but that the area is continuing to demonstrate steady progress toward reaching attainment in the
future. Additional data have identified Calexico, a border area influenced by emissions from
vehicles of Mexican registration, as an exception to the overall progress made.
The implications of these findings are significant for the transportation planning
community and for the need to conduct transportation project- level CO analyses. California data
indicate that in virtually all metropolitan areas, no existing transportation facility is expected to
cause a CO violation. Los Angeles has not yet attained the NAAQS but is on a path to do so in
the near future, and thus no existing transportation facilities would be expected to cause CO
violations in Los Angeles beginning within a few years. The one exception is the border area of
Calexico which is influenced by emissions from vehicles that do not meet California's stringent
emission standards. Thus, for CO analysis purposes, any future transportation project can be
reasonably compared to existing facilities in the vast majority of the state. If future
transportation projects have similar sizes and characteristics as existing facilities, and the
existing facilities do not cause a CO violation, then it can be inferred that the planned projects,
accounting for changes in background concentration, should not cause violations either. This
would allow for the elimination of microscale modeling for most transportation projects.
Modeling might still be necessary for projects that are larger than existing facilities or those with
extraordinary characteristics, such as projects located in Calexico.
The motivation for this report was to evaluate CO trends throughout the state and to
determine the appropriateness of continued microscale modeling for transportation projects. The
last of the four questions posed at the beginning of this study can now be addressed. Caltrans, as
the project sponsor, asked, “ Given past trends and likely future conditions, does it seem
appropriate to recommend to the EPA reconsideration of the conformity requirements for
microscale CO hot spot analyses?”
6- 2
The analysis of emissions and concentration trends at sites throughout California indicate
that both emissions and concentrations are decreasing. Furthermore, the trends at both the
regional and microscale level can be correlated with emissions trends. As emissions continue to
decrease ( as they are projected to do), concentrations at both the neighborhood- scale and
microscale levels should also decrease and remain below the federal standards ( with the
exception of Calexico). Given these trends, it appears that the contribution from individual
projects should not be a concern. We recommend that EPA reevaluate the continued need for the
conformity CO hot spot analysis requirement and consider replacing the requirement for one that
applies only under unusual circumstances, such as those evident at the Calexico border site. We
recommend using the conformity interagency consultation process to evaluate these unusual
circumstances and require hot spot analyses on a case- by- case basis.
7- 1
7. REFERENCES
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UC Davis- Caltrans Air Quality Project, December 15.
California Air Resources Board ( 1996). Proposed Carbon Monoxide Redesignation Request and
Maintenance Plan for Ten Federal Planning Areas.”
California Air Resources Board ( 1998a). Staff report. Public meeting to consider revision to
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California Air Resources Board ( 1998b). Ambient air quality data: 1980- 1997. Planning &
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California Air Resources Board ( 1999a). The 1999 California Almanac of Emissions & Air
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California Air Resources Board ( 1999d). Proposed regulation order: the California
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