Chapter 2

PM2.5	Challenges	and	Trends	in	the	San	Joaquin	

Valley	

2015 Plan for the 1997 PM2.5 Standard

SJVUAPCD	

 

 
San Joaquin Valley Unified Air Pollution Control District April 16, 2015

Chapter 2: PM2.5 Challenges and Trends in the San Joaquin Valley 

2015 Plan for the 1997 PM2.5 Standard

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Chapter 2: PM2.5 Challenges and Trends in the San Joaquin Valley 

Despite the unique geographical and meteorological challenges, the San Joaquin Valley 

(Valley) has made significant progress in reducing total emissions of directly emitted 

emissions of particulate matter that is 2.5 microns or less in diameter (PM2.5) and 

PM2.5 precursor emissions and in improving air quality for Valley residents. Through 

progressively more stringent regulations, improved control technologies, and innovative 

non-regulatory measures such as incentive programs, the annual average amount of 

directly emitted PM2.5 emissions has been steadily decreasing. Similarly, the overall 

amount of oxides of nitrogen (NOx) and oxides of sulfur (SOx) emissions continue to 

decrease. 

Achieving PM2.5 reductions has been challenging given frequent meteorological 

conditions conducive to PM2.5 formation that are characteristic of the Valley, and which 

are outside human (and regulatory) control. Annual fluctuations in weather patterns 

affect the Valley’s carrying capacity (the ability to disperse pollutants), which is reflected 

in long and short-term ambient air quality trends. Until the exceptional weather 

conditions experienced due to the recent drought, the Valley was on track to attain the 

1997 annual PM2.5 standard before the federally mandated deadline of December 

2014. 

2.1 CHALLENGES OF THE NATURAL ENVIRONMENT 

The Valley’s natural environment supports one of the most productive agricultural 

regions in the country: the Sierra Nevada provides the necessary water for growing the 

abundance of crops, and a temperate climate provides a long growing season. 

However, these same natural factors present significant challenges for air quality: the 

surrounding mountains trap pollution and block air flow, and the mild climate keeps 

pollutant-scouring winds at bay most of the year. Despite the challenges, the San 

Joaquin Valley Air Pollution Control District (District) and the Valley are making progress 

in attaining the national ambient air quality standards (NAAQS) and improving public 

health for Valley citizens. 

2.1.1 Unique Climate and Geography 

The challenge of PM2.5 NAAQS attainment in the Valley is grounded in the unique 

topographical and meteorological conditions found in the region. The Valley, as seen in 

Figure 2-1, is an inter-mountain valley encompassing nearly 25,000 square miles. 

Surrounded by mountain ranges to the west, east, and south, the air flow through the 

Valley can be blocked, leading to severely constrained dispersion. During the winter, 

high-pressure systems can cause the atmosphere to become stagnant for longer 

periods of time, where wind flow is calm and air movement is minimal. These stagnant 

weather systems can also cause severe nighttime temperature inversions, which 

exacerbate the build-up of PM2.5 and related precursors both beneath and above the 

evening inversion layer. 
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Figure 2-1 San Joaquin Valley Air Basin 

Under normal conditions, temperature decreases with increasing altitude, but during 

temperature inversions the normal temperature gradient is reversed, with temperatures 

increasing with altitude, causing warmer air to be above cooler air. Figure 2-2 shows 

that this reversal of the “normal” pattern impedes the upward flow of air, causes poor 

dispersion, and traps pollutants near the earth’s surface. Temperature inversions are 

common in the Valley throughout the year. Since the inversion is often lower than the 

height of the surrounding mountain ranges, the Valley effectively becomes a bowl 

capped with a lid that traps emissions near the surface. When horizontal dispersion 

(transport flow) and vertical dispersion (rising air) are minimized, PM2.5 concentrations 

can build quickly, especially in the winter. These naturally occurring meteorological 

conditions have the net effect of spatially concentrating direct PM2.5 concentrations 

near their sources; promoting the formation and regional buildup of secondary species, 

particularly ammonium nitrate; and chemically aged organic carbon species, resulting in 

an increase in their relative toxicity. Given these challenges, the Valley needs even 

more effective emissions reductions to attain the PM2.5 NAAQS; and the District 

continues to pursue these reductions through its numerous air quality attainment plans, 

prohibitory regulatory control strategy and innovative non-regulatory emission reduction 

strategy, which includes a robust incentive program, a comprehensive legislative 

platform, and rigorous outreach and education efforts. 
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Figure 2-2 Atmosphere with and without a Temperature Inversion 

Image source: http://ffden-2.phys.uaf.edu/212_spring2007.web.dir/Amber_Smith/Effects_of_Inversions.htm

2.1.2 The Valley’s Carrying Capacity 

In the context of air quality, carrying capacity refers to the density of emissions that an 

air basin can “absorb” or “carry” and still meet ambient air quality standards for a given 

pollutant. The key factors that shape variations in a regional carrying capacity include 

meteorology, climate, and the topography. Some air basins may have a high total 

pollutant emission rate (emissions per person or area), but if those emissions are easily 

dispersed or removed from the basin, that basin is much more likely to meet air quality 

standards despite the high emission rate. On the other hand, an air basin may have a 
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lower emission rate, but because of unfavorable environmental factors (low air flow, 

stagnant air, inversions) those pollutant concentrations typically accumulate (possibly 

above the standard) and remain in the air basin until weather patterns change. The 

latter scenario describes the Valley, and the first scenario is analogous to the Los 

Angeles (L.A.) air basin, especially for NOx emissions and the formation of ozone. 

As an example, total NOx emissions for the L.A. basin were 754 tons per day (tpd) in 

2008. During that year, the L.A. basin recorded 80 days above the 1997 national 8-hour 

ozone standard. For the same year, the total NOx emissions for the Valley air basin 

were 409 tpd (over a much larger area), yet the Valley recorded 82 days above the 

standard. NOx dispersal is primarily dependent on summertime weather patterns. The 

L.A. basin experiences regular coastal winds through much of the summer that not only 

disperse pollutants from the air basin, but also moderates temperatures. Conversely, 

the Valley, surrounded by mountain ranges, routinely experiences stagnant weather 

patterns (less wind) and extended periods of high temperatures, both of which build and 

concentrate ozone to levels above the standard. In this real example, it is obvious that 

the Valley has a much lower carrying capacity than the L.A. basin for NOx, a precursor 

to ozone formation. 

While not as drastic as the NOx-ozone example above (in terms of emission rate), the 

Valley’s carrying capacity for PM2.5, when compared to the L.A. basin, is greatly 

affected by prevailing weather during the winter months and the region’s topography 

(surrounding mountains). For 2008, the annual average direct PM2.5 emission rate for 

the L.A. basin was 80 tpd; during that year, that basin recorded 19 days above the 

national PM2.5 24-hour standard. For the same year, the Valley’s annual average 

direct PM2.5 emission rate was 82 tpd; however, the Valley recorded 66 days above the 

24-hour standard. During this same time period, the NOx and SOx emissions, which 

are also precursors to PM2.5, were significantly lower in the Valley compared to the L.A. 

Basin (NOx—409 tpd and 754 tpd, respectively, as stated above; and SOx—13 tpd and 

54 tpd, respectively). As noted in Section 2.2.1, temperature inversions are common 

during the winter months in the Valley. During these sometimes lengthy stagnant air 

episodes, PM2.5 emissions from daily activities rapidly build up to levels above the 

standard. It is during these events (or anticipation of these events) that the District’s 

Check-Before-You-Burn program and Real-time Air Advisory Network (RAAN) system 

intervene to inform (or require) the public to limit activity that generates PM2.5 

emissions. 

The District uses quantitative carrying capacity analysis in its modeling of attainment 

demonstrations. Such analyses can determine which combinations of PM2.5 and 

PM2.5 precursor emissions reductions can contribute to future attainment given 

anticipated population and activity growth, potential regulations or control measures, 

and the unchanging natural physical constraints. 
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2.2 THE VALLEY’S UNIQUE CHALLENGES 

In addition to the climate and geography challenges discussed above, the Valley also 

has multiple other unique challenges that continue to impact the Valley’s progress 

toward attainment of air quality standards. The Valley has significant naturally occurring 

biogenic emissions. The California landscape also allows for air pollutant transport 

within the Valley, as well as between the Valley and other air basins. The Valley is also 

one of the fastest growing regions in the state. From 2010 to 2020, the Valley’s 

population is expected to increase by 18% (Table 2-1). In contrast, the total population 

for the State of California is projected to increase by only 9% over the same time period. 

Increasing population generally means increases in air pollutant emissions as a result of 

increased consumer product use and more automobile and truck travel. Between 2010 

and 2020, the Valley’s total vehicle miles traveled (VMT) will increase about 21%,1

consistent with the Valley’s population growth. The Valley is also home to the state’s 

major arteries for goods and people movement, which adds to the increase in vehicular 

traffic. 

Table 2-1 Estimated Valley Population by County (2010-2020)2

County Estimated 2010 Projected 2020 

Fresno 932,926 1,083,889 

Kern* 841,609 1,041,469 

Kings 152,996 179,722 

Madera 151,136 183,176 

Merced 256,345 301,449 

San Joaquin 686,651 795,631 

Stanislaus 515,229 582,746 

Tulare 443,567 536,429 

Total 3,980,459 4,704,511

*Kern County is separated into two air districts: San Joaquin Valley and 

Eastern Kern. This data is the Valley-portion of Kern only. 

Although reducing mobile source emissions is critical to the Valley’s attainment of air 

quality standards, the District does not have direct regulatory authority to reduce motor 

vehicle tailpipe emissions, which are regulated by the U.S. Environmental Protection 

Agency (EPA) and California Air Resources Board (ARB). The District must collaborate 

with interagency partners and use innovative approaches to reduce mobile source 

emissions. 

As Chapter 3 of this plan details, the formation and composition of PM2.5 can be 

complex, with some species impacting health more than others. Long-term trends show 

that PM2.5 concentrations throughout the Valley have declined since monitoring of this 

pollutant first began and are projected to continue on that trend. In addition to declining 

 

1

 California Air Resources Board: 2009 Almanac – Population and Vehicle Trends Tool. Retrieved July 2012 from 

http://www.arb.ca.gov/app/emsinv/trends/ems_trends.php

2

 California Department of Finance [DOF]: Interim Population Projects for California and its Counties 2010-2050. 

(May 2012). Retrieved from http://www.dof.ca.gov/research/demographic/reports/projections/interim/view.php
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PM2.5 concentrations, most emissions inventories of PM2.5 precursors are also 

projected to decrease despite future population growth. 

The District also assesses long-term trends of PM2.5 concentrations by looking at the 

number of days per year that a monitoring site measures concentrations over the 1997 

24-hour PM2.5 NAAQS limit of 65 μg/m3

. Figure 2-3 shows the trend in numbers of 

days that air monitoring sites recorded 24-hour PM2.5 averages over 65 μg/m3 at the 

Modesto (Stanislaus County), Fresno-First/Garland (Fresno County), and BakersfieldCalifornia (Kern County) air monitoring sites. An overall downward trend is apparent 

when comparing the early years of 1999 and 2000 to recent years. The current pattern 

shows generally that the northern Valley has the fewest days over the standard, that the 

southern Valley has the most days over the standard, and that the central Valley 

registers somewhere between the two. 

Figure 2-3 Trend in Days over the 24-hour PM2.5 Standard 

 
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2.3 PM2.5 EMISSIONS INVENTORY TRENDS 

The emissions inventory is the foundation for the attainment planning process. The 

District and ARB maintain an accounting of PM2.5 and precursor emissions for the 

Valley based on known sources within the Valley and those sources outside the Valley 

that influence Valley air quality (inter-region transport). The District requires detailed 

accounting of emissions from regulated sources throughout the Valley. ARB makes 

detailed estimations of emissions from mobile, area, and geologic sources using known 

emissions factors for each source or activity and accounting for relevant economic and 

population data. Together, these feed into the emissions inventory that represents an 

estimate of how much direct pollution is going into the Valley air basin as a result of the 

cumulative pollutant-generating activities and sources. 

The District uses the emissions inventory to develop control strategies, to determine the 

effectiveness of permitting and control programs, to provide input into air quality 

modeling, to fulfill reasonable further progress requirements, and to screen regulated 

sources for compliance investigations. 

The following general list represents the major inventory categories for which emissions 

are recorded and tracked. Appendix B to this plan contains the detailed accounting of 

the emissions inventory with projected emissions based on anticipated growth of each 

source and the anticipated control (regulatory or non-regulatory) of each source, if 

applicable. 

 Mobile sources – motorized vehicles 

o On-road sources include automobiles, motorcycles, buses, and trucks 

o Other or off-road sources include farm and construction equipment, lawn 

and garden equipment, forklifts, locomotives, boats, aircraft, and 

recreational vehicles 

 Stationary sources – fixed sources of air pollution 

o Power plants, refineries, and manufacturing facilities 

o Aggregated point sources, i.e. facilities (such as gas stations and dry 

cleaners) that are not typically inventoried individually, but are estimated 

as a group and reported as a single source category 

 Area sources – human activity that takes place over a wide geographic area 

o Includes consumer products, residential wood burning, controlled burning, 

tilling, and unpaved road dust 

 Natural sources – naturally occurring emissions 

o Geologic sources, such as petroleum seeps 

o Biogenic sources, such as emissions from plants 

o Wildfire sources 

Figure 2-4 shows the PM2.5 emissions inventory trend for the mobile, stationary, and 

area source categories. 

 

 
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Figure 2-4 Valley PM2.5 Winter Emissions Inventory Trend 

Because NOx is a significant PM2.5 precursor, the District relies heavily on NOx 

emissions to also reduce PM2.5 emissions. Figure 2-5 summarizes the NOx emissions 

inventory trends for the mobile, stationary, and area source categories. District and 

ARB control strategies for NOx play a significant role in reducing both ozone and PM2.5 

emissions. 

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PM2.5 Emissions (tons per day)

TOTAL STATIONARY TOTAL MOBILE TOTAL AREAWIDE
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Figure 2-5 Valley Winter NOx Emissions Inventory Trend 

Emissions inventory trends show the progress made through progressive regulatory and 

non-regulatory activities, e.g. as rules are amended with tighter emission limits, or as 

reduction technologies improve, overall emissions decrease. Figure 2-6 shows how the 

overall tons of PM2.5 emissions per day have decreased in the past and are anticipated 

to continue decreasing in the future based on anticipated growth and controls. Figure 2-

6 also shows the comparative emissions inventory reduction of winter PM2.5. Winter 

PM2.5 emissions have decreased significantly, in large part due to the effectiveness of 

Rule 4901 (Wood Burning Fireplaces and Wood Burning Heaters). Continued 

emissions reductions are based on current control strategies that will continue to take 

effect into the future. In light of the Valley’s projected increase in population, the 

projected emissions reductions highlight the success of the control measures adopted 

and enforced by the District, ARB, and other regulatory agencies. 

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NOx Emissions (tons per day)

TOTAL STATIONARY TOTAL MOBILE TOTAL AREAWIDE
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Figure 2-6 Valley PM2.5 Annual and Winter Inventory Trends 

2.4 PM2.5 AIR QUALITY TRENDS 

As a public health agency charged with monitoring Valley air quality and ensuring 

progress toward meeting national air quality standards, the District has established an 

extensive air monitoring network that provides ongoing data for evaluating such 

progress. Information from this extensive monitoring network, which began measuring 

PM2.5 concentrations in 1999, allows the District to track air quality trends that show 

progress toward attainment and inform the planning process for reaching attainment. 

2.4.1 Air Monitoring Network 

Numerous pollutants and meteorological parameters are measured throughout the 

Valley on a daily basis using an extensive air monitoring network managed by the 

District, ARB, and other agencies. This network measures pollutant concentrations 

necessary to show progress toward compliance with the NAAQS. The network also 

provides real-time air quality measurements used for daily air quality forecasts, 

residential wood-burning declarations, Air Alerts, and RAAN. Air quality monitoring 

networks are designed to monitor areas with high population densities, areas with high 

pollutant concentrations, areas impacted by major pollutant sources, and areas 

representative of background concentrations. Together, the District and the ARB 

operate 33 air monitoring stations throughout the Valley; 20 of these sites measure 

PM2.5, either through the use of filter-based monitors that measure each 24-hour period 

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ANNUAL AVERAGE WINTER AVERAGE
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or hourly monitors that use light energy to provide near-continuous concentration levels. 

Figure 2-7 shows the Valley’s network of air monitoring sites. 

Figure 2-7 Air Monitoring Sites in the Valley 

PM2.5 is measured and expressed as the mass of particles contained in a cubic meter 

of air (micrograms per cubic meter, or μg/m3

). The data collected from the District’s 

network of PM2.5 monitors is used to calculate design values for the 24-hour and 

annual PM2.5 standards, as outlined in EPA guidance and regulations.3,4 

2.4.2 Air Quality Progress 

Air quality progress can be assessed in several ways. The calculation of design values

is the official method used to determine whether an area is in attainment of a standard; 

however, other indicators can reveal more about the progress being made toward 

attaining that standard. Comparing the days per year when each monitor exceeded the 

PM2.5 24-hour NAAQS threshold from year to year shows the progress in reducing the 

 

3

 Environmental Protection Agency [EPA]: Office of Air Quality Planning and Standards. (1999, April). Guideline on 

Data Handling Conventions for the PM NAAQS (EPA-454/R-99-008). Retrieved from 

http://www.epa.gov/ttn/oarpg/t1/memoranda/pmfinal.pdf 4

 Interpretation of the National Ambient Air Quality Standards for PM2.5, 40 C.F.R. Pt. 50 Appendix N (2012). 
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number of days with the highest concentrations, while quarterly averages can help to 

show progress with respect to seasonal peaks in concentration levels. Some of the 

conclusions from these analyses are included below, followed by a more detailed 

discussion in Appendix A, which also provides analysis results for a number of other air 

monitoring sites in the Valley. 

Rather than using yearly maximum concentrations for the PM2.5 standards, EPA 

requires the use of design values for the attainment metric. Design values represent a 

three-year average and help to smooth out outlier years with exceptional meteorology or 

exceptional events. Details on how PM2.5 design values are calculated are provided in 

Appendix A of this plan. As seen in Figure 2-8, the Valley maximum 24-hour and 

annual average PM2.5 design value trends show that although there is some year-toyear variation significant progress has been made in reducing long-term PM2.5 

concentrations. Valley 24-hour design value maximums have decreased by 40% over 

the 1999–2013 time period. 

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Figure 2-8 Historical PM2.5 24-hour and Annual Design Value Trends 

Since monitoring began, the Bakersfield-California and Bakersfield-Planz air monitoring 

sites in Kern County have consistently been among the highest PM2.5 design values in 

the Valley. Figure 2-9 shows the trend of the 24-hour average design value at 

Bakersfield-California through 2013, as demonstrated with the 2011-2013 design value 

(3-year average). Figure 2-10 shows the trend of the annual average design value at 
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Bakersfield Planz through 2013, as demonstrated with the 2011–2013 design value (3-

year average).

Figure 2-9 Trend of 24-Hour Average PM2.5 Design Values at BakersfieldCalifornia 

Figure 2-10 Trend of Annual Average PM2.5 Design Values at Bakersfield-Planz 

Overall decreasing PM2.5 concentrations at the Bakersfield-California and BakersfieldPlanz air monitoring sites are shown in the design value trends for those sites. The 

Bakersfield-California site now has a 24-hour design value at or below the 1997 24-hour 
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PM2.5 standard of 65 μg/m3 (see Figure 2-9). The annual average design value for 

2011–2013 continues to trend lower for Bakersfield-Planz at 17.3 μg/m3 (see Figure 2-

10). This downward trend will need to continue at all sites within the Valley as the 

Valley strives for attainment of increasingly stringent air quality standards. 

Since the Valley’s highest PM2.5 concentrations occur during the fall and winter 

months, the first (January through March) and fourth (October through December) 

quarters tend to have the highest average concentrations. Observing the trend in these 

quarterly averages can shed light on how the peak of the PM2.5 season is changing 

over time. 

Data from the Visalia monitoring site (Figure 2-11) is representative of fourth-quarter 

averages among the PM2.5 sites in the Valley. This data also shows a downward trend 

of 1.20 μg/m3 per year. The District anticipates continuation of this trend as the Valley 

gets closer to attaining the annual average PM2.5 standard. Refer to Appendix A for 

the detailed results of this analysis. 

Figure 2-11 Trend of Fourth-Quarter Average at Visalia 

2.4.3 Impact of Exceptional Drought-Related Weather Conditions on Valley 

PM2.5 Concentrations 

In 2012, the Bakersfield-Planz air monitoring site, which is the current peak PM2.5 site 

in the District, recorded an annual average value of 14.7 μg/m3

, below the standard of 

15.0 μg/m3

. This site, along with the rest of the District’s PM2.5 air monitoring sites, 

was making significant progress towards attaining the 1997 annual PM2.5 standard. 

However, due to the exceptional weather conditions experienced during the winter of 
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2013-2014, exceedingly high PM2.5 concentrations were experienced, causing a 2013 

annual average of 22.8 μg/m3 for the Bakersfield-Planz site, and an annual design value 

(2011-2013) of 17.3 μg/m3 (see Figure 2-10 above). 

Due to the extreme weather and high values already experienced at this site in the 1st

quarter of 2014, the averages for the 2nd, 3rd, and 4th quarters of 2014 would need to be 

zero for Bakersfield-Planz to reach attainment for the 2012-2014 period (see Chapter 

1). In addition, with the high values recorded in 2013 and 2014, attainment during the 

2013-2015 period is also impossible (see Chapter 4). The following discusses the 

magnitude of the weather conditions experienced during the winter of 2013-14, and its 

impact on the Valley’s ability to attain the 1997 annual PM2.5 standard. 

Meteorology during the Winter Season of 2013-2014 

This past winter, California Governor Jerry Brown declared a state of emergency due to 

extreme drought conditions in the state. This emergency declaration was based on 

record-low precipitation in 2013 and snow pack levels at only 20 percent of the normal 

amount of snow to provide water for the year. Specifically in the Valley, 2013 

represented the driest year since the start of recordkeeping in 1895. The Valley is 

currently experiencing an exceptional level of drought not seen in at least 119 years. 

Although the Valley has experienced reductions in PM2.5 concentrations over the last 

15 years since the pollutant first began to be measured, the winter months of November 

through February continue to record the peak levels of each year. The following figure 

displays the relative comparison between the lower concentrations in March through 

October, and the higher concentrations experienced during the winter. 

Figure 2-12 Average PM2.5 by Month in 2013 in Stockton, Fresno, Bakersfield 

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Stable meteorology during the winter season can increase PM2.5 concentrations to high 

levels by providing strong temperature inversions and low wind speeds. When this 

occurs, the PM2.5 concentrations during the winter months of November to February 

can climb to very high levels. As seen in Figure 2-13, the winter of 2013-2014 

experienced the strongest average atmospheric stability over the last 15 years (period 

during which PM2.5 concentrations have been recorded), creating conducive conditions 

for the formation and retention of high PM2.5 concentrations. This was a result of a 

persistent, strong high pressure ridge over the eastern Pacific that effectively blocked 

weather disturbances from entering California, which inhibited dispersion during 

November, December, and January. 

In addition to the historically strong atmospheric stability, the winter of 2013-2014 also 

experienced record low precipitation totals, with some locations breaking records over 

100 years old (see Table 2-2). These unprecedented dry conditions exacerbated the air 

quality challenge during the winter of 2013-2014. As a result of the extreme 

meteorology experienced in the Valley, PM2.5 concentrations reached peak levels that 

had not been recorded in over a decade, which in turn has increased the Valley’s PM2.5 

design values, making the journey to attainment of the PM2.5 standards even more 

challenging. 

Figure 2-13 Average Atmospheric Stability per Winter Season 

-1.5

-1

-0.5

0

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1

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1999-20002000-20012001-20022002-20032003-20042004-20052005-20062006-20072007-20082008-20092009-20102010-20112011-20122012-20132013-2014

T850MB Stability Parameter ( ̊C)
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Table 2-2 Calendar Year Rainfall Totals for Select California Cities 

City 1981-2010 

Average (inches) 

2013 Total 

(inches) 

Previous Record 

Low (inches) 

Previous 

Record Year 

Modesto 13.11 4.70 5.70 1929 

Merced 12.50 3.79 6.00 2007 

Fresno 11.50 3.01 3.55 1947 

Visalia 10.93 3.47 4.10 1910 

Bakersfield 6.47 3.43 1.87 1959 

Sacramento 18.52 5.81 6.67 1976 

San Francisco 23.65 5.59 9.00 1917 

San Jose 14.90 3.80 6.04 1929 

Los Angeles 12.82 3.65 4.08 1953 

San Diego 10.34 5.57 3.41 1953 

2.5 CONDENSABLE PARTICULATES 

Certain high-temperature processes emit gaseous pollutants that rapidly condense into 

particle form in the ambient air. After January 1, 2011, PM2.5 nonattainment areas are 

to consider these condensable particulates for purposes of establishing the emissions 

limits for Reasonable Further Progress (RFP), Reasonably Available Control 

Technology (RACT), and Reasonably Available Control Measures (RACM).5

 In 

December 2010, EPA revised its “Method 202” stationary source test method to 

measure condensable particulate matter.6 

While this issue may be new and more relevant to other regions, the District has 

historically included condensable particulate emissions in its definition of total 

particulate emissions, well ahead of federal and other states’ efforts to address this 

issue. This has included instituting permit requirements for various emissions sources 

that include condensable particulates as part of total particulate emissions limitations, 

and associated emissions testing requiring that condensable particulates be measured 

(including utilizing an EPA-approved modified test method ahead of EPA’s official test 

method, Method 202). Condensable particulates are thus a part of the total PM2.5 

inventory, and reductions in condensable particulate matter emissions were included in 

the District’s evaluation of various emission reduction opportunities for directly emitted 

PM2.5. 

2.6 INSIGNIFICANT PRECURSORS TO PM2.5 CONCENTRATIONS IN THE 

VALLEY 

The switch from CAA Subpart 1 to Subpart 4 (see Chapter 1) for PM2.5 implementation 

shifts the precursor presumption for planning purposes. Pursuant to Subpart 1, areas 

 

5

 40 CFR 51.1002(c) 

6

 http://www.epa.gov/ttn/emc/methods/method202.html 
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2-19 Chapter 2: PM2.5 Challenges and Trends in the San Joaquin Valley 

 2015 Plan for the 1997 PM2.5 Standard

were not required to address volatile organic compounds (VOCs) 7 and ammonia unless 

technical demonstration shows that VOC reductions or ammonia reductions contribute 

to PM2.5 attainment. Now, pursuant to Subpart 4 §189(e), areas must address 

potential precursors of PM2.5, including VOCs and ammonia, unless it is shown that 

they do not contribute to attainment. 

In the Valley, there is extensive scientific research and technical analysis demonstrating 

that VOC reductions and ammonia reductions do not contribute to PM2.5 attainment. 

As such, the Valley’s VOC emissions and ammonia emissions do not need to be 

reduced to address the federal PM2.5 standard. EPA concurs with the conclusion that 

VOC emissions do not contribute significantly to the formation of PM2.5 as stated in 

their proposed approval of the District’s plan to address the 2006 PM2.5 standard: 

“Based on a review of the information provided by the District and other information 

available to EPA, we propose to determine that at this time VOC emissions do not 

contribute significantly to ambient PM2.5 levels...”8 

Modeling shows that NOx controls are the most effective approach to reduce PM2.5 

nitrate concentrations, and once NOx controls are taken into consideration, VOC 

emissions reductions produce essentially no benefit. In fact, in some instances, VOC 

emissions reductions may actually lead to an increase in PM2.5 nitrate formation. 

Nitrogen-containing molecules 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.9

 

The extensive research mentioned above and summarized below demonstrates that 

there is a relative abundance of ammonia compared to nitric acid, and that the amount 

of nitric acid drives the ultimate formation of ammonium nitrate. Because of this 

regional surplus in ammonia, even substantial ammonia emissions reductions yield a 

relatively small reduction in nitrate. Reductions in nitrate concentrations of 30% to 50% 

were realized through a 50% reduction in NOx. Modeling a 50% reduction in ammonia, 

while unrealistic because it’s not technologically achievable, would only realize less than 

a 5% reduction in nitrate concentrations. Due to the extensive body of science that 

clearly shows the much greater efficacy of reducing NOx emissions relative to 

ammonia, ammonia reductions have not historically been considered a significant 

precursor to PM2.5 formation in the Valley. 

 

7

 EPA defines VOCs as any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic 

carbides or carbonates, and ammonium carbonate, that participates in atmospheric photochemical reactions to form 

ozone or particulates. A subset of non-reactive VOCs does not contribute to ozone or particulates and are exempt 

from regulatory controls. Many VOCs are human-made chemicals used and produced in the manufacture of paints, 

adhesives, petroleum products, pharmaceuticals. The full EPA definition is available at 

http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&rgn=div8&view=text&node=40:2.0.1.1.2.3.8.1&idno=40 8

 Approval and Promulgation of Implementation Plans; Designatio of Areas for Air Quality PlanningPurposes; 

California; San Joaquin Valley Moderate Area Plan and Reclassification as Serious Nonattaiemtn for the 2006 PM2.5 

NAAQS; Proposed Rule, 80 Fed. Reg. 8. pp. 1816-1846. (p. 1826) (2015, January 13). 9

 Meng, Z., Dabdub, D., and Seinfeld, J.H., 1997, Chemical Coupling Between Atmospheric Ozone and Particulate 

Matter, Science, 277, 116-119. 
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 2015 Plan for the 1997 PM2.5 Standard

2.6.1 VOC Contribution to PM2.5 Concentrations 

VOC emissions have the potential to contribute to the formation of two different PM2.5 

components: secondary organic aerosols (SOAs) and ammonium nitrate (nitrate). 

While these components contribute to observed PM2.5 concentrations in the Valley, 

their contribution is minimal. The anthropogenic VOC contribution (those not from 

biogenic sources) to both components is so minimal, that invoking a VOC-centric control 

strategy is much less effective than primary PM2.5 controls or NOx controls, as shown 

through the recent research and modeling. 

2.6.1.1 VOC Contribution to SOA Formation 

Secondary organic aerosols form when intermediate molecular weight VOCs emitted by 

anthropogenic and biogenic sources react and condense in the atmosphere to become 

aerosols. Lighter VOCs also participate in the formation of atmospheric oxidants, which 

then participate in the formation of SOA. SOAs derived from anthropogenic VOC 

emissions account for only 1% to 2% of the annual total PM2.5 concentrations 

throughout the Valley. 

As part of the attainment demonstration for the District’s 2008 PM2.5 Plan, ARB used 

the Community Multi-scale Air Quality (CMAQ) model to show that primary PM2.5 

emissions are the main contributor to organic aerosols, with SOAs being a small fraction 

of the total organic aerosol concentration. Furthermore, SOAs are mostly formed during 

the summer and from predominantly biogenic sources, when total PM2.5 concentrations 

are low. As such, SOAs derived from anthropogenic VOC emissions make up only 3% 

to 5% of the annual average organic aerosol concentrations. 

Related to this finding, the California Regional Particulate Air Quality Study (CRPAQS) 

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

50% reduction in anthropogenic VOC emissions has limited effect on the modeled 

PM2.5 organic matter.10 Together, these study results show that for SOAs, further VOC

reductions would have very limited effectiveness in reducing PM2.5 concentrations. 

2.6.1.2 VOC Contribution to Nitrate Formation 

Nitrate forms by means of two primary chemical pathways: during the day, NO2 is 

oxidized to nitric acid, some of which then reacts with ammonia to form nitrate through 

interactions with sunlight, VOCs, and background ozone; and during the night, when 

nitric acid is formed through oxidation of NO2 (via N2O5) by background ozone, which 

then reacts with ammonia to form nitrate. Several modeling studies11,12,13,14 have 

 

10 Pun, B.K., Balmori R.T.F, & Seigneur, C. (1998). Modeling Wintertime Particulate Matter Formation in Central 

California, Atmospheric Environment, 43, 402-409. 11 Pun, B.K., & Seigneur, C. (1998) Conceptual Model of Particulate Matter Pollution in the California San Joaquin 

Valley. Prepared for Pacific Gas & Electric, Document CP045-1-98. 12 Pun, B.K. (2004). CRPAQS Task 2.7 when and where does high O3 correspond to high PM2.5? How much PM2.5 

corresponds to photochemical end products? Prepared for the San Joaquin Valleywide Air Pollution Study Agency. 13 Lurmann, F.W., Brown, S.G., McCarthy, M.C., & Roberts, P.T. (2006). Processes Influencing Secondary Aerosol 

Formation in the San Joaquin Valley during Winter. Journal of Air and Waste Management Association, 56, 1679-

1693. 
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2-21 Chapter 2: PM2.5 Challenges and Trends in the San Joaquin Valley 

 2015 Plan for the 1997 PM2.5 Standard

investigated the relative veracity of these two mechanisms within the Valley and 

attempted to determine the specific role and contribution of VOCs on Valley nitrate 

concentrations. While the specific conclusions were mixed, there was general 

agreement that the nighttime formation of nitrate in the Valley would not be sensitive to 

VOC reductions. 

Further modeling studies15,16,17,18,19,20 evaluated the significance of VOC controls in 

reducing nitrate concentrations in the Valley. ARB evaluated each of these studies in 

the context of two key considerations: whether further VOC reduction would provide 

significant benefits to expedite attainment beyond the District’s existing NOx control 

program, and what would be the feasible magnitude of any potential VOC reductions 

beyond the existing and already rigorous VOC control program. Nitrate was only 

responsive to a 50% reduction in VOCs at very high PM2.5 concentrations, 

concentrations that are no longer reached in the Valley. In contrast, a 50% reduction in 

NOx can reduce significantly more nitrate at current PM2.5 concentrations, one study21

reporting a 38% reduction in nitrate. 

Despite the insignificance of VOC emissions with regard to PM2.5 concentrations in the 

Valley, VOC emissions have been reduced and will continue to be reduced through 

implementation of the 2007 Ozone Plan and the 2013 Plan for the Revoked 1-Hour 

Ozone Standard. A more detailed discussion of the VOC influences on PM2.5 

concentrations can be found in the 2012 PM2.5 Plan.

22

2.6.2 Ammonia Contribution to PM2.5 Concentrations 

Early air quality research in the Valley identified ammonium nitrate (nitrate) as a 

predominant secondary PM2.5 species in the region, with high concentrations forming 

during the winter months.23 Studies have continued to show that ammonium nitrate is a 

primary component of wintertime PM2.5 in the Valley, followed by other species, such 

 14 Ying, Q., Lu, J., & Kleeman, M. (2009). Modeling Air Quality during the California Regional PM10/PM2.5 Air Quality 

Study (CRPAQS) Using the UCD/CIT Source-Oriented Air Quality Model – Part III Regional Source Apportionment of 

Secondary and Total Airborne Particulate Matter. Atmospheric Environment, 43, 419-430. 15 Stockwell, W.R., Watson, J.G., Robinson, N.F., Steiner, W., & Sylte, W.W. (2000). The Ammonium Nitrate Particle 

Equivalent of NOx Emissions for Wintertime Conditions in Central California’s San Joaquin Valley, Atmospheric 

Environment, 34, 4711-4717. 16 Pun, B.K., & Seigneur, C. (2001). Sensitivity of Particulate Matter Nitrate Formation to Precursor Emissions in the 

California San Joaquin Valley. Environmental Science and Technology, 35, 2979-2987. 17 Kleeman, M.J., Ying, Q., & Kaduwela, A. (2005). Control Strategies for the Reduction of Airborne Particulate Nitrate 

in California's San Joaquin Valley. Atmospheric Environment, 39, 5325-5341. 18 Meng, Z., Dabdub, D., & Seinfeld, J.H. (1997) Chemical Coupling Between Atmospheric Ozone and Particulate 

Matter. Science, 277, 116-119. DOI:10.1126/science.277.5322.116 19 Livingstone, P.L., Magliano, K., Gürer, K., Allen, P.D., Zhang, K.M., Ying, Q., ... Byun, D. (2009). Simulating PM 

Concentrations during a Winter Episode in a Subtropical Valley: Sensitivity Simulations and Evaluation Methods. 

Atmospheric Environment, 43, 5971-5977. 20 Pun, B.K., Balmori R.T.F, & Seigneur, C. (2009). Modeling Wintertime Particulate Matter Formation in Central 

California. Atmospheric Environment, 43, 402-409. 21 Ibid. 25 22 SJVAPCD. 2012 PM2.5 Plan (2013) http://www.valleyair.org/Air_Quality_Plans/PM25Plans2012.htm 23 Smith, T.B.; Lehrman, D.E.; Reible, D.D.; and Shair, F.H. (1981). The origin and fate of airborne pollutants within 

the San Joaquin Valley: Extended summary and special analysis topics. Report No. 2. Prepared for the California Air 

Resources Board, and by the California Institute of Technology, Pasadena, CA. 
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2-22 Chapter 2: PM2.5 Challenges and Trends in the San Joaquin Valley 

 2015 Plan for the 1997 PM2.5 Standard

as organic carbon, ammonium sulfate, and geologic material.24 In addition, PM2.5 

speciation data, collected for many years at four Valley urban monitoring locations, also 

shows nitrate’s substantial contribution to the Valley’s total PM2.5 concentrations, 

especially on days when peak 24-hour average concentrations are experienced. 

2.6.2.1 Ammonium Nitrate Formation and Precursors 

Formation of ammonium nitrate is described by Kleeman et al. (2005, pp. 5326-7):25

Particulate ammonium nitrate (NH4NO3) forms when the concentration 

product of gas-phase ammonia (NH3) and nitric acid (HNO3) exceeds a 

saturation point dependent on temperature, relative humidity, and the 

composition of the pre-existing particles that act as condensation substrate 

(Wexler and Seinfeld, 1991).26 Ammonia is a relatively stable compound 

directly emitted to the atmosphere that does not undergo significant chemical 

reaction on the time scale of interest to regional air quality problems. Nitric 

acid is an end product of the photochemical transformation of NOx (NO + 

NO2). The majority of the NOx in the SJV is emitted as NO that is then 

transformed into various species including NO2, NO3, HNO2, HNO3, HNO4, 

Peroxy Acetyl Nitrate (PAN), Particulate Protein Nitrogen (PPN), particulate 

nitrate, etc. The sum of NOx and the entire family of NOx reaction products is 

called “reactive nitrogen” (NOy). The fraction of reactive nitrogen that forms 

HNO3 and/or nitrate depends on the concentration of NOx and VOC as well 

on meteorological conditions such as temperature, relative humidity, and 

solar intensity (Aw and Kleeman, 2003; Nguyen and Dabdub, 2002).

27 28 

Measurements taken at the remote Kern Wildlife Station in the San Joaquin 

Valley show that approximately 22% of the reactive nitrogen exists as 

particulate (ammonium) nitrate during typical winter conditions (Chow and 

Egami, 1997).29 

Nitrate buildup is a signature outcome of multi-day stagnation periods during the winter 

(similar buildup is not observed during warmer seasons). The modeled regional 

variation of nitrate concentrations is shown in Figure 2-14. Higher concentrations of 

nitrate occur in the southernmost Valley as a result of slower wind speeds and higher 

levels of reactive nitrogen and ammonia. 

 

24 Ying, Q. & Kleeman, M.J., (2009). Regional Contributions to Airborne Particulate Matter in Central California during 

a Severe Pollution Episode. Atmospheric Environment, 43, 1218–1228. 25 Kleeman, M.J., Ying, Q., & Kaduwela, A. (2005). Control Strategies for the Reduction of Airborne Particulate Nitrate 

in California's San Joaquin Valley. Atmospheric Environment, 39, 5325–5341. 26 Wexler, A.S., Seinfeld, J.H. (1991). 2nd-Generation inorganic aerosol model. Atmospheric Environment Part aGeneral Topics 25 (12), 2731–2748. 27 Aw, J., Kleeman, M.J. (2003). Evaluating the First-Order Effect of Intra-Annual Temperature Variability on Urban 

Air Pollution. Journal of Geophysical Research-Atmospheres 108 (D12). 28 Nguyen, K. & Dabdub, D. (2002). NOx and VOC Control and Its Effects on the Formation of Aerosols. Aerosol 

Science and Technology 36 (5), 560–572. 29 Chow, J.C. & Egami, R.T. (1997). San Joaquin Valley Integrated Monitoring Study: Documentation, Evaluation, and 

Descriptive Analysis of PM10 and PM2.5, and Precursor Gas Measurements. Technical support studies No. 4 and 

No. 8. Final Report prepared for the California Air Resources Board, Sacramento, CA. Desert Research Institute, 

Reno, NV. 
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2-23 Chapter 2: PM2.5 Challenges and Trends in the San Joaquin Valley 

 2015 Plan for the 1997 PM2.5 Standard

Figure 2-14 Modeled Regional Distribution of Ammonium Nitrate30

Both nitric acid and ammonia are needed to form ammonium nitrate. The extensive 

research conducted through California Regional Particulate Air Quality Study 

(CRPAQS) and subsequent studies, as well as ongoing evaluation and modeling 

demonstrates that there is a relative abundance of ammonia (NH3) compared to nitric 

acid (HNO3), and that the amount of nitric acid (resulting from NOx emissions) drives 

the ultimate formation of ammonium nitrate. Figure 2-15 illustrates this ammonia 

abundance at the rural Angiola (Fresno County) air monitoring site in the Valley during 

the CRPAQS field study. Ammonia concentrations are considerably higher than nitric 

acid concentrations throughout the Valley, including urban areas with concentrated NOx 

emissions.31 See Appendix G to the 2012 PM2.5 Plan for more information. 

 

30 Chow, J.C., Chen, L.-W.A., Lowenthal, D.H., Doraiswamy, P., Park, K., Kohl, S., Trimble, D.L., & Watson, J.G. 

(2005). California Regional PM10/PM2.5 Air Quality Study (CRPAQS) – Initial Data Analysis of Field Program 

Measurements. Report No. 2497. Prepared for California Air Resources Board, Sacramento, CA, by Desert Research 

Institute, Reno, NV. 

31 Magliano, K. L. (2009) Science-Based Policies for Particulate Matter Air Quality Management in California. 

International Aerosol Modeling Algorithms Conference. Davis CA. 
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2-24 Chapter 2: PM2.5 Challenges and Trends in the San Joaquin Valley 

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Figure 2-15 Ammonia versus Nitric Acid Measurements at Angiola32

2.6.2.2 Reducing Ammonium Nitrate 

Because of the regional surplus in ammonia, even substantial ammonia emissions 

reductions yield a relatively small reduction in nitrate. Figures 2-16 and 2-17 provide a 

simplified illustration of this situation. As seen in Figure 2-18, a comparable modeling 

analysis based on CRPAQS observational data found a higher disparity between the 

efficiency of NOx versus ammonia controls. Reductions in nitrate concentrations of 

30% to 50% were realized through a 50% reduction in NOx. Modeling a 50% reduction 

in ammonia, while unrealistic because it is not technologically achievable, would only 

realize less than a 5% reduction in nitrate concentrations. Finally, Figure 2-19 provides 

clear correlative evidence from observed data that NOx controls are effectively reducing 

ammonium nitrate, despite an increase in the regional ammonia inventory over the 

same time period. 

 

32 McCarthy, M. (2005) The Role of Nighttime Chemistry in Winter Ammonium Nitrate Formation in the San Joaquin 

Valley. American Association for Aerosol Research (AAAR), Supersites Conference, February 2005, Atlanta, GA. 
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2-25 Chapter 2: PM2.5 Challenges and Trends in the San Joaquin Valley 

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Figure 2-16 Abundance of Ammonia in the San Joaquin Valley33 

Figure 2-17 NOx Control Reduces Ammonium Nitrate Most Efficiently 

 

33 Stockwell, W.R., Watson, J.G., Robinson, N.F., Steiner, W., & Sylte, W.W. (2000). The Ammonium Nitrate Particle 

Equivalent of NOx Emissions for Wintertime Conditions in Central California’s San Joaquin Valley, Atmospheric 

Environment, 34, 4711-4717. 
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2-26 Chapter 2: PM2.5 Challenges and Trends in the San Joaquin Valley 

 2015 Plan for the 1997 PM2.5 Standard

Figure 2-18 Modeled Ammonium Nitrate Response to Ammonia vs. NOx 

Controls34

Figure 2-19 Correlation between NOx Reductions and Observed Ammonium 

Nitrate in Fresno35

 

34 Stockwell, W.R., Watson, J.G., Robinson, N.F., Steiner, W., & Sylte, W.W. (2000). The Ammonium Nitrate Particle 

Equivalent of NOx Emissions for Wintertime Conditions in Central California’s San Joaquin Valley, Atmospheric 

Environment, 34, 4711-4717. 35 Stockwell, W.R., Watson, J.G., Robinson, N.F., Steiner, W., & Sylte, W.W. (2000). The Ammonium Nitrate Particle 

Equivalent of NOx Emissions for Wintertime Conditions in Central California’s San Joaquin Valley, Atmospheric 

Environment, 34, 4711-4717. 

-60%

-50%

-40%

-30%

-20%

-10%

0%

50% NOx Reduction 50% Ammonia Reduction
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2-27 Chapter 2: PM2.5 Challenges and Trends in the San Joaquin Valley 

 2015 Plan for the 1997 PM2.5 Standard

Due to this extensive body of science that clearly shows the much greater efficacy of 

reducing NOx emissions relative to ammonia, ammonia reductions have not historically 

been considered a significant precursor to PM2.5 formation in the Valley. However, the 

District and ARB have continued to examine the potential role of ammonia with regard 

to PM2.5 formation (see Appendices F and G of the 2012 PM2.5 Plan). 

The modeling sensitivity analysis shows that reductions in ammonia emissions achieve 

insignificant reductions in the PM2.5 design values compared to reductions of direct 

PM2.5 and NOx emissions. Relative to the other pollutants, ammonia reductions at the 

Bakersfield-California site are only 2.3% as effective as direct PM2.5 reductions, and 

only 10% as effective as NOx reductions. Ammonia is not a significant precursor to 

PM2.5 values in the Valley. 

 
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