Appendix B: District and ARB Comment Letters on 

Proposed EPA Plan Disapproval Appendix B-1

Appendix B: District and ARB Comment Letters on 

Proposed EPA Plan Disapproval

On November 30, 2010 (75 Federal Register 74518-74543), The U.S. Environmental 

Protection Agency (EPA) proposed a limited approval and limited disapproval of the 

2008 PM2.5 Plan. The San Joaquin Valley Air District (District) and the California Air 

Resources Board (ARB) submitted extensive documentation and comments to EPA 

regarding the disapproval issues raised. The District believes that the collective 

comments from the District and the State of California fully address the issues EPA 

identified in its proposed disapproval. This Appendix presents the District’s and ARB’s 

written comments to EPA. 










Attachment 1 

Enforceable Commitment Level 

U.S. EPA’s proposed partial disapproval of the Plan is primarily based upon 

U.S. EPA’s position that the Plan’s attainment demonstration relies too heavily on 

enforceable commitments rather than on fully adopted measures. Specifically, 

U.S. EPA is proposing to disapprove the Plan because the percentage of emission 

reductions arising from enforceable commitments (i.e., 30%) is “too high and 

do[es] not represent a limited portion of the State’s current estimate of total 

emissions reductions needed . . . .” (75 Fed.Reg. 74536 (Nov. 30, 2010).) 

U.S. EPA does not identify the maximum percentage that would be acceptable, 

but implies that it might be 10% by stating: 

Historically, EPA has approved SIPs with enforceable commitments in the 

range of 10 percent or less of the total needed for attainment. (Id. at p. 

74535.) 

The Federal Register notice does not contain any real analysis to justify the 

conclusion that 30% is “too high.” Instead, U.S. EPA merely makes the above 

statement about its historical actions in approving other nonattainment area SIPs. 

There is in fact no firm rule dictating what percentage of emission reductions may 

be from enforceable commitments; the principal legal requirement regarding the 

allowable proportion of enforceable commitments is simply that it be “limited.” 

The language of the Clean Air Act (CAA) does not directly provide for the inclusion 

of enforceable commitments within a SIP. However, the CAA does state that a 

SIP may contain “means,” “techniques,” and/or “schedules and timetables for 

compliance.” (42 U.S.C. § 7410(a)(2)(A).) The U.S. EPA and courts have 

interpreted this language to allow U.S. EPA to approve a SIP that relies on 

enforceable commitments if the commitments meet a three part test: 

(1) whether the commitment addresses a limited portion of the 

statutorily-required implementation plan; (2) whether the state 

is capable of fulfilling its commitment; and (3) whether the 

commitment is for a reasonable and appropriate period of 

time. (BCCA Appeal Group v. U.S. EPA (2003) 355 F.3d 817, 

840.) 

Here, the San Joaquin Valley Plan relies on enforceable commitments for 

approximately 30% of its required reductions. (75 Fed.Reg. 74536.) In applying 

the test set out above, U.S. EPA found that the Plan satisfied the second prong, 

i.e. that the state would be capable of fulfilling its commitments, but did not meet 

the first and third prongs. (Id. at pp. 74535-74536.) 

Regarding the first prong, while 30% may be a larger portion of enforceable 

commitments than U.S. EPA has approved in the past, it is less than a third of the 
Attachment 1 

overall commitment. Moreover, U.S. EPA’s brief mention of what percentage has 

been approved in the past for different SIPs, under different circumstances, avoids 

any substantive analysis of why 30% is excessive in the context of this particular 

SIP revision. The failure to explain why 30% is “too high” is at odds with 

U.S. EPA’s past practice in analyzing other SIPS that contain enforceable 

commitments. 

Furthermore, we believe that there are unique circumstances that justify approving 

a higher percentage of enforceable commitments in this Plan. In this comment 

letter, we have explained the situation regarding ARB’s in-use off-road equipment 

rule and in-use truck rule, which together account for a significant proportion of the 

emission reductions needed to demonstrate attainment. These rules do not 

represent vague future commitments; they have already been adopted and are in 

effect today. The only reason ARB has not already submitted them to U.S. EPA is 

that we are in the process of amending the rules to address the unique 

circumstances caused by the most serious recession to hit this country since the 

Great Depression. We have explained why ARB’s proposed amendments to 

these rules continue to meet aggregate emission targets for these combined 

categories. Under these circumstances, we believe it is reasonable for U.S. EPA 

to accept a higher percentage of enforceable commitments than has been 

accepted in other SIPs where the circumstances were very different. 

Finally, it is illogical for U.S. EPA to determine that California is capable of fulfilling 

its commitments (i.e. satisfying the second prong), while simultaneously finding 

that the size of those commitments is “too high” to approve the Plan. It is also 

incongruous that U.S. EPA commends California’s “ambitious rule development, 

adoption, and implementation schedules” (id. at p. 74536) and states “we believe 

the State and District have provided reasonable and appropriate schedules for 

achieving their commitments” (ibid.) but then goes on to conclude that those 

schedules do not satisfy the third prong of the test. 

 

We believe that the Plan should be fully approved for the reasons stated above. 

 
Attachment 2 

Implementation of State 2007 SIP Commitments 

State Actions

• Smog Check Improvements (BAR) 

- Low Pressure Evaporative Test 

- Add Visible Smoke Test 

- More Stringent Cutpoints 

- Inspection of Light- and Medium-Duty Diesels 

• Expanded Light Duty Vehicle Retirement 

• Cleaner In-Use Heavy-Duty Trucks regulation, including Port Truck 

Modernization 

• Modifications to Reformulated Gasoline Program 

• Ocean Going Vessels Cleaner Main Ship Engines and Fuel Rule 

• Ship Auxiliary Engine Cold Ironing & Clean Technology 

• Harborcraft Engine Replacement Requirements 

• In-Use Off-Road Equipment Fleet Rule 

• Pesticides Regulation (DPR) 

• Portable Outboard Marine Tanks Standards 

• Vapor Recovery for Above Ground Storage Tanks 

• Consumer Products Regulation Revisions 

Ongoing Programs

• Accelerated Introduction of Cleaner Line-Haul Locomotive 

• Mobile Source Incentive Programs, including SB 118 and Moyer 
Attachment 3 

2007 State Strategy 

In-Use Heavy Duty Truck and Off-Road Equipment 

Emission Reduction Commitment 

THE ADOPTED 2007 STATE STRATEGY COMMITS TO ACHIEVE 

AGGREGATE EMISSIONS REDUCTIONS CONSISTENT WITH THE 

ATTAINMENT TARGET 

The 2007 State Strategy describes ARB’s aggregate emission reduction 

commitment as follows: 

“The total emission reductions from the new measures necessary to attain 

the federal standards are an enforceable State commitment in the SIP. 

While the State Strategy includes estimates of the emission reductions from 

each of the individual new measures, it is important to note that the 

commitment of the State Strategy is to achieve the aggregate emission 

reductions identified from the existing strategy and the adopted State 

Strategy. Therefore, if a particular measure does not get its expected 

emission reductions, the State still commits to achieving the total aggregate 

emission reductions, whether this is realized through additional reductions 

from the new measures, or from alternative control measures or incentive 

programs. If actual emission decreases occur that exceed the projections 

reflected in the emission inventories and the State Strategy, the actual 

emission decreases may be counted toward meeting ARB’s total emission 

reduction commitments.” 

April 2009 Revision to 2007 State Strategy, p.13 

The Board-adopted 2007 State Strategy identifies emissions targets on a regional 

basis by source categories, with each regional target equal to the emissions levels 

needed to demonstrate attainment. 

The South Coast Air Quality Management District applied the same emissions 

target approach (defined as “remaining emissions” or “carrying capacity”) to the 

1997/1999 Air Quality Management Plan for ozone which U.S. EPA approved as 

the air basin’s attainment demonstration. 
Attachment 3 

BOTH THE SOUTH COAST AND SAN JOAQUIN VALLEY PORTIONS OF THE 

STATE STRATEGY WILL MEET THE 2014 EMISSIONS TARGETS FOR IN-USE 

HEAVY DUTY TRUCKS AND OFF-ROAD EQUIPMENT 

In December 2010, the Board adopted amendments to ARB’s in-use heavy duty 

truck and off-road equipment rules which, taken together, achieve the expected 

emissions targets identified in the 2007 State Strategy for these categories. 

To illustrate how the emissions targets are being met for the on-road truck and offroad equipment categories, ARB staff developed a three-step approach. First, 

staff aggregated the categories to determine the combined emission reduction 

benefits; then a conversion factor was applied to normalize NOx and PM2.5 

emissions from these categories into a NOx equivalent emissions metric; in the 

third and final step, the SIP emissions target for these categories were compared 

with the emissions resulting from the regulations using the same NOx equivalent 

metric. 

Applying this approach to the 2007 South Coast and San Joaquin Valley SIPs, 

ARB staff first converted the NOx and PM2.5 inventories for the 2014 baseline for 

on-road trucks and off-road equipment into NOx equivalents. In the South Coast, 

this resulted in a NOx equivalent baseline of 329 tons per day (tpd). In the 

San Joaquin Valley, the NOx equivalent baseline was 246 tpd. 

Staff also converted into NOx equivalents the expected emission reductions in 

2014 for the South Coast and San Joaquin Valley portions of the State Strategy, 

and subtracted the results from their respective SIP baselines. This calculation 

provides the NOx equivalent emissions target for both categories to support the 

attainment demonstration: 187 tpd in the South Coast and 141 tpd in the San 

Joaquin Valley. 

Using the same NOx equivalent metric, staff then compared the amended truck 

and off-road equipment regulations (including inventory improvements) with their 

applicable emissions targets. In each case, the remaining emissions are lower 

than or identical to the aggregate emissions target: 182 tpd in the South Coast 

and 141 tpd in the San Joaquin Valley. 

The chart below illustrates this comparison, and shows that in the aggregate the 

proposed regulations will meet the SIP commitments.
Attachment 3 

Comparison of 2014 Remaining Emissions

On-Road Truck and Off-Road Equipment Source Categories

0

20

40

60

80

100

120

140

160

180

200

220

240

260

280

300

320

340

San Joaquin Valley South Coast

NOx Equivalent Emissions (tpd)

2007 SIP

Emissions

Target

2007 SIP

Emissions

Target

Adopted

Regs

(Dec 2010)

Adopted 

Regs

(Dec 2010)

2007 SIP

Baseline

Emissions

2007 SIP

Baseline

Emissions
Attachment 4 

1 

Air Resources Board comments on U.S. EPA’s November 30, 2010 proposal 

that VOC be considered a significant PM2.5 precursor for the San Joaquin 

Valley 2008 PM2.5 State Implementation Plan (SIP) 

In its November 30, 2010 Federal Register notice on the San Joaquin Valley’s 

2008 PM2.5 SIP, U.S. EPA has proposed to make a finding that volatile organic 

compounds (VOCs) are a significant PM2.5 precursor, which if made final, would 

impede California’s ability to implement the most cost-effective regulations for 

attaining PM2.5 standards in this region. The California Air Resources Board 

(ARB) is providing comments which document the scientific weight of evidence 

showing that VOC emission reductions are ineffective in reducing PM2.5 at 

current ambient concentrations in the San Joaquin Valley. A review of the full set 

of peer reviewed studies of particulate matter in the San Joaquin Valley, the 

current ambient air monitoring data, the emissions anticipated in the attainment 

year from enforceable SIP measures, and the complete set of SIP modeling 

results all support the conclusion that VOC should not be considered a significant 

PM2.5 precursor in this instance. 

U.S EPA’s PM2.5 Implementation Rule (Rule) establishes a presumption that 

VOC is not an attainment plan precursor (40 CFR 51.1002 (c )(3)). The federal 

Rule also provides that either a state or U.S. EPA can provide an appropriate 

technical demonstration for a specific area showing that VOC emissions 

significantly contribute to PM2.5 concentrations. In the case of the San Joaquin 

Valley PM2.5 SIP, U.S. EPA has proposed such a demonstration suggesting that 

it is supported by the administrative record. However, U.S. EPA’s proposal also 

indicates that the State of California should submit a demonstration to either 

support or reverse the presumption under the PM2.5 implementation rule that 

VOC is not an attainment precursor. 

 ARB appreciates the opportunity to provide technical information which supports 

the presumption in the federal Rule that VOC is not a significant PM2.5 

precursor. While the 2008 SIP relies on a very comprehensive set of peer 

reviewed studies, modeling, and air monitoring data, ARB staff agrees that 

additional documentation of the scientific findings will better inform U.S. EPA and 

the public. 

On September 16, 2010, ARB provided U.S. EPA with information on the 

California Regional Particulate Matter Air Quality Study (CRPAQS) and the role it 

has played in the development of the 2008 PM2.5 SIP for the San Joaquin 

Valley. To date, over 60 papers have been published in peer-reviewed journals 

such as Atmospheric Environment, Environmental Science and Technology, and 

the Journal of Air and Waste Management. Over 50 presentations of CRPAQS 

results have been given at national and international conferences. Data 

collected at over 100 monitoring sites has resulted in a publically available 

database containing over 180,000,000 records and a comprehensive website 

provides study documentation and reports. A full study bibliography was 
Attachment 4 

2 

provided to U.S. EPA. CRPAQS study results have been previously been relied 

upon in U.S. EPA rulemakings for PM10, which in terms of the annual standard 

in the San Joaquin Valley is dominated by PM2.5. 

 

On January 28, 2011 ARB sent to U.S. EPA the full modeling documentation 

and results used in the 2008 PM2.5 SIP. All of this information is part of the 

record for U.S. EPA’s consideration of the San Joaquin Valley PM2.5 SIP. When 

the full weight of scientific evidence in the record is considered, U.S. EPA should 

reach the same conclusion about the efficacy of VOC controls as the Air 

Resources Board and San Joaquin Valley Air Pollution Control District. 

 

U.S. EPA itself found that the state of the science on secondary organic aerosols 

does not support a finding of significant contribution to PM2.5. ARB agrees with 

U.S. EPA’s finding on secondary organic aerosols. The other potential chemical 

mechanism by which VOC may contribute indirectly to PM2.5, is a series of 

reactions ultimately resulting in formation of ammonium nitrate. This is the 

chemical pathway that U.S. EPA cites as a basis for its proposal to overturn the 

negative presumption for VOC contained in its PM2.5 rule. 

While ARB agrees that this chemical pathway exists, the weight of scientific 

evidence shows that it is not significant for PM2.5 attainment in the San Joaquin 

Valley. U.S. EPA states that with respect to nitrate formation the monitoring 

studies are not conclusive, and that early box modeling differed on whether VOC 

controls would significantly affect PM2.5. This leaves the more sophisticated 

photochemical grid based modeling as U.S. EPA’s primary technical basis for its 

proposed finding that VOC controls are significant for reducing PM2.5 in the San 

Joaquin Valley. 

ARB scientists have consulted with U.S. EPA’s modeling staff in the Office of Air 

Quality Planning and Standards to better understand the basis for the agency’s 

interpretation of the published photochemical modeling studies with respect to 

the contribution of VOC to nitrate formation. ARB staff agrees with U.S. EPA that 

the cited PM2.5 modeling study (Kleeman et. al 2005), shows a benefit of 

50 percent VOC reductions at the higher emissions levels at the time of the 2000 

field study. However, that same modeling study shows a very different result 

when attainment year emission levels are taken into account. Using this data, 

the modeling study cited by U.S. EPA does not show a significant benefit of 

further VOC reductions. In fact, once the regional reductions in NOx (oxides of 

nitrogen) emissions are taken into consideration, VOC emission reductions 

produce essentially no benefit and in some instances, may actually lead to an 

increase in PM2.5 nitrate concentrations (see page 7 of Technical 

Demonstration). 

Additional PM2.5 modeling studies also reviewed by U.S. EPA show similar 

responsiveness to the 50 percent VOC reduction scenario done by Kleeman. 

However, the SIP submitted to U.S. EPA for approval reflects actual NOx and 
Attachment 4 

3 

VOC reductions in the San Joaquin Valley, rather than the theoretical scenario of 

an additional 50 percent reduction in VOC explored as part of the SIP 

development process. Such modeling sensitivity runs can be a preliminary step 

in the SIP process, but the actual emissions and reductions in the attainment 

year are the basis for the required attainment demonstration. U.S. EPA should 

reconsider its proposed VOC finding in light of the more refined analysis 

discussed below. 

U.S. EPA’s PM2.5 implementation rule appropriately directs SIP planning efforts 

and regulation to those pollutants generally known to significantly contribute to 

PM2.5 concentrations. In the implementation rule preamble U.S. EPA defines 

“significantly contribute” to mean a significant emission reduction in precursors 

which would result in a significant change in the PM2.5 concentration. The 

CRPAQS modeling shows that in the attainment year even a 30 to 50 percent 

further reduction in VOC would not significantly affect PM2.5 concentrations. 

Therefore it is essential that California has the ability to focus on the pollutants of 

most significance to attainment of air quality standards. States need the 

discretion to develop the best approach for a meeting an air quality standard in 

each nonattainment area taking into account the supporting science, technical 

feasibility of regulations, and the costs. 

TECHNICAL DEMONSTRATION: 

VOC emissions have the potential to contribute to the formation of two different 

components of PM2.5, secondary organic aerosols (SOA) and ammonium 

nitrate. While these components contribute to observed PM2.5 concentrations in 

the San Joaquin Valley to a small degree, the weight of evidence indicates that 

anthropogenic VOC is not a significant contributor to PM2.5. 

Secondary Organic Aerosols (SOA) 

On an annual average basis, secondary organic aerosols derived from 

anthropogenic VOC emissions account for only 1 percent to 2 percent of the 

annual total PM2.5 concentrations throughout the Valley. SOA form when 

intermediate molecular weight VOCs emitted by anthropogenic and biogenic 

sources react and condense in the atmosphere to become aerosols. In addition, 

lighter VOCs participate in the formation of atmospheric oxidants which then 

participate in the formation of SOA. As noted by U.S. EPA in the November 30, 

2010 proposal, the processes of SOA formation are complex and have not been 

fully characterized. 

ARB air quality modeling exercises conducted as part of the San Joaquin Valley 

2008 PM2.5 SIP attainment demonstration analysis using the Community 

Multiscale Air Quality (CMAQ) model showed that primary PM2.5 emissions are 

the main contributor to organic aerosols and SOA contribute to only a small 

extent. Furthermore, as illustrated in Figure 1, SOA are mostly formed during the 
Attachment 4 

4 

summertime, when total PM2.5 concentrations are low, and are mainly derived 

from biogenic emission sources. On an annual average basis, SOA derived 

from anthropogenic VOC emissions are a small part of the organic aerosol 

concentrations (3 percent to 5 percent). 

BAC

0

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4

6

8

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163146617691

106121136151166181196211226241256271286301316331346361

SB

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Figure 1. Daily contributions to organic aerosol concentrations in Bakersfield in 2000 modeled 

with CMAQ: Primary organic aerosols (PA), secondary aerosols formed from biogenic VOC 

emissions (SB) and secondary aerosols formed from anthropogenic source VOC emissions (SA). 

Units are μg/m3. 

As part of the CRPAQS study, simulations of a wintertime episode conducted 

using CMAQ-Madrid, a model with an enhanced secondary organic aerosol 

formation mechanism, also found that organic aerosol concentrations were 

dominated by directly emitted (“primary”) emissions. The study found that, 

because of the dominance of primary PM2.5 organic matter, overall, a 

50 percent reduction in anthropogenic VOC emissions has limited effects on the 

modeled PM2.5 organic matter (Pun, et al., 2009). 

These study results show that for secondary organic aerosols, further VOC 

reductions would have very limited effectiveness in reducing PM2.5 

concentrations. U.S. EPA’s proposal acknowledges the state of the science on 

secondary organic aerosols, and indicates that the overall considerations 

presented in the 2008 PM2.5 SIP are not enough to find VOC as a significant 

PM2.5 SIP precursor for SOA in the San Joaquin Valley. 

Ammonium Nitrate 

While VOCs can influence the formation of ammonium nitrate, in the San Joaquin 

Valley this effect is minor. U.S. EPA cited ten references as part of their 

technical demonstration. Four were monitoring studies conducted as part of 

CRPAQS that suggested that VOCs could under some conditions be important in 
Attachment 4 

5 

the formation of secondary ammonium nitrate. However, as U.S. EPA noted, the 

studies differed in their assessment of the overall role of VOCs, and they cannot 

be used to evaluate whether VOC controls would reduce ammonium nitrate 

concentrations. The six remaining studies used differing types of air quality 

modeling to quantitatively assess the expected change in ammonium nitrate to 

hypothetical VOC reductions. The two earliest studies used photochemical box 

modeling. This simplified type of modeling approach does not fully capture 

atmospheric dynamics and reflect older chemical mechanisms and levels of 

emissions. Thus these studies provide limited usefulness in assessing VOC 

responsiveness under current conditions. ARB focused the SIP evaluation on 

the more recent modeling studies which used more robust photochemical grid 

modeling. These studies provide a more appropriate basis for evaluating the 

complex NOx and VOC interactions that occur in the atmosphere under realworld emissions control scenarios. 

There are two primary pathways through which ammonium nitrate can form. 

During the day, NO2 is oxidized to nitric acid. Nitric acid then reacts with 

ammonia to form ammonium nitrate. This daytime pathway involves sunlight, 

VOCs, and background ozone. During the night, nitric acid is formed through 

oxidation of NO2 (via N2O5) by background ozone, which then also reacts with 

ammonia to form ammonium nitrate. Studies by Pun et al. (1998, 2004) 

suggested that the daytime pathway may be important and therefore the 

formation of ammonium nitrate would be sensitive to changes in VOC emissions. 

However, other studies (Lurmann et al., 2006), suggest that on average, daytime 

production of nitric acid in the San Joaquin Valley is relatively slow and that 

nighttime production of ammonium nitrate aloft, which then mixes to the surface 

after sunrise could explain the observed homogeneous patterns of ammonium 

nitrate in the Valley. Ying et al. (2009) also theorized that the ozone 

concentration aloft in the San Joaquin Valley is predominantly due to the regional 

background and does not vary significantly with surface-level VOC emissions. 

Therefore, nighttime ammonium nitrate formation in the San Joaquin Valley 

would not be sensitive to VOC reductions. 

ARB agrees with U.S. EPA that while the monitoring studies cited above provide 

evidence that the VOC pathway may be important at times, these studies do not 

provide quantitative information about the overall role and cannot be used to 

evaluate the benefits of VOC controls. Rather, modeling studies are more 

appropriate to assess the overall impact of precursor controls. 

Staff reviewed the results of seven modeling studies containing information on 

the significance of VOC controls in reducing ammonium nitrate in the SJV. While 

the results of the earliest studies were mixed, later studies provide generally 

consistent results regarding the role of VOCs. In assessing the potential benefits 

of VOC controls it is important that significance be interpreted in the context of 

California’s overall control program with its strong focus on NOx control to 

achieve benefits for both PM2.5 and ozone. ARB staff’s review of whether VOCs 
Attachment 4 

6 

should be considered a significant precursor was examined in the context of two 

key considerations: 

1) Whether further VOC reductions would provide significant benefits to 

expedite attainment beyond the existing NOx control program; and 

2) The feasible magnitude of any potential VOC reductions beyond ARB’s 

already rigorous VOC control program. 

Two early studies used simplified box modeling to explore the sensitivity of 

ammonium nitrate to VOC and NOx reductions. One of the two studies 

simulated a typical winter episode (Stockwell et al., 2000) and found that 

decreases in VOC emissions had little effect. The second study (Pun and 

Seigneur, 2001) simulated winter conditions during the 1996 CRPAQS pilot study 

around the Fresno area. The study found that ammonium nitrate formation 

decreased with VOC emission reductions, but increased with NOx reductions. 

Pun and Seigneur (2001) theorized that reducing NOx could lead to higher 

concentrations of OH and increase the overall rate of nitrate production, despite 

the reductions in NOx. However, the box modeling approach has a number of 

limitations, including lack of transport into/out of the box, robust vertical transport, 

and use of an older chemical mechanism. In addition, the VOC emissions were 

increased by a factor of two to improve model performance. As such, the box 

modeling did not fully represent the complete scope of atmospheric variations 

and has limited usefulness in assessing the responsiveness to VOC controls. 

Subsequent modeling sensitivity studies for the same winter episode were 

conducted with the UCD/CIT model, an advanced research grade modeling 

system (Kleeman et al., 2005). The authors concluded that NOx emission 

controls are more effective in reducing PM2.5 nitrate concentrations in the 

San Joaquin Valley. Summary study results indicate that on average, large 

reductions in VOC emissions (on the order of 50 percent) reduced PM2.5 nitrate 

concentrations by approximately 17 percent. However, to evaluate the 

significance and effectiveness of VOC controls in the context of control strategy 

design, the study’s isopleths of PM2.5 nitrate response to combined NOx/VOC 

emission reductions provide more in-depth information. 

Figures 2(a) and 3(a) show that, based on the shapes of the graphs, NOx 

controls are the most effective approach to reduce PM2.5 nitrate concentrations 

at Fresno and at the location with the highest modeled PM2.5 nitrate 

concentration (grid location -85 km Northing, 90 km Easting) respectively. Once 

NOx controls are taken into consideration, VOC emission reductions produce 

essentially no benefit and in some instances, may actually lead to an increase in 

PM2.5 nitrate concentrations. For example, as illustrated in Figure 2(a) for 

Fresno, after considering an approximately 60 percent reduction in NOx 

emissions resulting from existing and proposed controls, reductions in VOC 

emissions to any level would not decrease PM2.5 nitrate concentrations. 

Furthermore, at grid location -85 km Northing, 90 km Easting, any level of VOC 
Attachment 4 

7 

emission reductions would actually cause an increase in nitrate concentrations. 

Nitrogen-containing molecules such as PAN can act as temporary sinks for NO2. 

When VOCs are controlled, the reduced availability of certain radicals which are 

generated from VOCs reduces the amount of NO2 that is sequestered, thereby 

increasing the availability of NO2 and enhancing ammonium nitrate formation 

(Meng et al., 1997) 

. 

Figure 2. 24-h average NOx/VOC particulate nitrate isopleths at Fresno for (a) all sources, 

(b) diesel engines, (c) catalyst equipped gasoline engines, and (d) upwind sources of nitrate. 

Units are μg/m3. (Source: Kleeman et al., 2005, Figure 3 pg 5333). 
Attachment 4 

8 

Figure 3. 24-h average NOx/VOC particulate nitrate isopleths at grid location -85 km Northing, 

90 km Easting for (a) all sources, (b) diesel engines, (c) catalyst equipped gasoline engines, and 

(d) upwind sources of nitrate. Units are μg/m3. (Source: Kleeman et al., 2005, Figure 5 pg 5335). 

Four additional modeling studies investigated the more recent two-week winter 

episode of 2000-2001 that occurred during the CRPAQS field study. 

In the first study, preliminary data from modeling of this CRPAQS winter episode 

conducted using the Lagrangian form of the UCD/CIT model qualitatively confirm 

that NOx control is the most efficient method to reduce nitrate concentrations 

(Kleeman, M.J., personal communication, May 2008). Figure 4 illustrates the 

response of PM2.5 nitrate concentrations to NOx and VOC emission reductions 

at a rural (Angiola) and an urban (Fresno) site on December 31, 2000. Again, 

based on their shapes, these graphs show that NOx controls are the most 

effective approach to reduce PM2.5 nitrate concentrations. Once NOx controls 

are taken into consideration (approximately 60 percent reduction in NOx 

emissions), reductions in VOCs of up to 30 percent produce basically no benefit 
Attachment 4 

9 

(Fresno). Futhermore, at some locations (Angiola) any VOC emission reductions 

may actually lead to an increase in PM2.5 nitrate concentrations. 

Figure 4. The isopleths plot of PM2.5 nitrate with emission control of NOx and VOC at Angiola 

(ANG) and Fresno (FEI) after a 5-day back trajectory simulation for December 31, 2000. Units 

are in μg/m3

. (Source: Kleeman, M.J., personal communication, May 2008) 

A second study conducted simulations of the two-week CRPAQS episode with 

the CMAQ photochemical model (Livingston, et al., 2009). The study consisted 

of two simulations. The first was a baseline scenario using a preliminary 

emissions inventory. This simulation showed that 50 percent reductions in 

anthropogenic VOC and NOx emissions had similar effects in reducing 

ammonium nitrate (about 20 percent each). A second simulation was conducted 

using an updated emission inventory representing a more accurate spatial 

distribution of total ammonia emissions (referred to as “Vehicle NH3” scenario, 

per Livingston, P., personal communication, January 19, 2011). This second 

50 percent VOC reduction simulation showed a much lower response to VOC 

controls. The response was lowered to a 12 percent reduction in ammonium 

nitrate, with a corresponding increase in responsiveness to NOx control of 

38 percent. These results are consistent with those found by Kleeman et al., 

2005. 

In a third analysis, the San Joaquin Valley Air District (District) in a PM2.5 SIP 

appendix included a bounding sensitivity analysis on the responsiveness to VOC 

controls. Based on this hypothetical scenario the appendix included a finding 

that VOC reduction is effective for the annual standard and the winter episode for 

reduction of total carbon secondary particulates. This analysis used preliminary 

results from CMAQ modeling, namely that a 50 percent reduction in VOC 

emissions led to a 10 percent reduction in particulate nitrate. This is basically 

the same level of responsiveness reported from the studies by Livingston et al., 
Attachment 4 

10 

2009 and Pun et al., 2009. However, as previously discussed, a hypothetical 

scenario of 50 percent reduction should not replace an analysis using actual 

emissions and SIP reductions. Overall 2008 PM2.5 SIP concluded that VOC is 

not a significant precursor to PM2.5. 

A fourth study modeled one-week of the CRPAQS episode using a version of 

CMAQ with a more advanced chemical mechanism (CMAQ-Madrid) 

(Pun et al, 2009). In contrast to the earlier Pun study using a simplified box 

modeling approach, this later work found that on average, nitrate was most 

sensitive to reductions in NOx emissions. While isopleths were not provided, the 

time evolution of nitrate and PM2.5 mass to VOC response illustrated in Figure 5 

provides further detail regarding the efficacy of VOC control. The response of 

nitrate to a 50 percent reduction in VOC emissions increased as PM2.5 levels 

rose during the episode. In urban areas, a 50 percent reduction in anthropogenic 

VOC emissions caused small reductions in nitrate, on the order of 10 percent on 

the modeled days when 24-hour PM2.5 concentrations measured over 

100 μg/m3

 at urban sites and above 65 μg/m3 in rural areas. 

The difference in the VOC response on the days with the higher PM2.5 

concentrations as compared to those days with lower concentrations may be due 

to a difference in the chemical formation regime for nitrate. In general, there is 

sufficient background ozone to generate enough free radicals to initiate and 

propagate the chemistry of nitrate formation (Ying et. al, 2009). However, on 

days with high PM2.5 concentrations, the daytime photochemistry may have 

contributed to a rapid increase in nitrate, resulting in higher VOC and NOx 

sensitivity. It does not appear that VOCs contributed significantly to the free 

radical budget on the simulated days mainly because rapid increases in ozone 

were not observed. The effect of VOC levels on nitrate formation may also have 

a diurnal pattern since the hydroxyl and hydroperoxyl radical levels are high 

during the daytime and negligible at night. In addition, more reactive VOCs react 

quickly during the day and there is a minimal carry over to the next day. 

Therefore, it is reasonable to assume that the higher response to VOC and NOx 

at higher PM2.5 concentrations may be due the nitrate formation mechanism 

rather than to PM2.5 accumulation due to the length of the episode. 

Overall, nitrate was only responsive to a 50 percent reduction in VOCs at PM2.5 

concentration levels that are no longer reached in the San Joaquin Valley. 

Currently, the 24-hour PM2.5 design value in the Valley is 70 μg/m3

 recorded at 

Bakersfield and the rest of the Valley records 24-hour design values between 

50 μg/m3 and 60 μg/m3. Given the current levels of PM2.5, we believe the Valley 

is now in a nitrate chemical formation regime that is less responsive to VOC 

controls. 
Attachment 4 

11 

Figure 5. Time series with daily observations, base case simulation results and results from the 

sensitivity cases of (a) nitrate and (b) PM2.5 at Angiola (left) and Bakersfield (right). (Source: 

Pun et al., 2009, excerpt from Figure 2, pg 406) 
Attachment 4 

12 

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