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March 16, 2015

MEMORANDUM

From: Adam Reff and Venkatesh Rao

U.S.E.P.A. Office of Air Quality Planning and Standards

Air Quality Assessment Division

To: Docket for Proposed PM2.5 SIP Requirements Rule

EPA-HQ-OAR-2013-0691

Subject: Background on atmospheric fine particulate matter composition and formation 

chemistry

I. Overview of PM and PM2.5.

Particulate matter (PM) refers to airborne particles of solid and liquid material in the ambient 

atmosphere. A major distinguishing feature of different particles is size, which is characterized 

by aerodynamic diameter. This refers to the diameter of a sphere with a settling velocity 

equivalent to that of a given particle. In the atmosphere, PM typically ranges in aerodynamic 

diameters from sub-micron to 30 m or larger. The behavior and chemical composition of PM 

varies considerably with size range. The method of origin is also strongly associated with size: 

larger particles (over about 10 m) are typically of "primary" origin, meaning they are directly 

emitted from their source; whereas smaller particles are often "secondary," meaning they form 

through gaseous chemical reactions followed by condensation in the atmosphere. Examples of 

coarse PM include soil dust and vegetative detritus; a major formation mechanism of fine PM is 

oxidation of SO2 and NH3 gases to form particulate ammonium sulfate. 

PM2.5 technically refers to PM with an aerodynamic diameter under 2.5 m, but filter-based 

sampling devices, including the Federal Reference Method (FRM), are not able to create a 

perfect cutoff between PM2.5 and the remaining ambient PM. Additionally, the chemical mix in 

the FRM sampler will not be the same as in ambient air due to processes such as volatilization of 

ammonium nitrate during sampling on warm days and changes in particle-bound water during 

gravimetric measurement. For regulatory purposes, PM2.5 is thus operationally defined as the 

PM collected by the FRM, and will tend to include some material from slightly larger (under

about 10 m) particles. PM2.5 can sometimes contain significant amounts of primary PM, 

although it is often secondary in origin. PM2.5 is the size fraction most strongly associated with 

human morbidity and mortality, and is also known to induce visibility degradation and climate 

change. This regulation is therefore focused on reducing ambient PM2.5 concentrations as 

measured by the FRM.

II. Composition, emissions, and spatial distribution of PM2.5 components.

PM2.5 is a complex chemical mixture, composed of potentially thousands of different individual 

compounds. The chemical makeup also varies considerably with time and location. The major 

compound groups typically present include 1) ions 2) organic matter (OM), 3) elemental carbon 

(EC), and 4) crustal material. Variable amounts of water are included in the reported mass, 
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which depend on the mixture of hygroscopic compounds. Trace elements are also included. 

Table 1 shows emissions from version 3 of the 2008 EPA National Emissions Inventory (NEI) 

for SO4, NO3, OM, EC, and the remaining (“Other” in the table) PM2.5. Some discussion of 

each of the four major components is given below.

Ions

The ionic fraction of PM is mostly composed of sulfate (SO4), nitrate (NO3), and ammonium 

(NH4). Example compounds include ammonium sulfate ((NH4)2SO4) and ammonium nitrate 

(NH4NO3). Sodium chloride (NaCl) is seen in marine and coastal environments. Less 

commonly measured compounds include ammonium bisulfate (NH4HSO4), sodium sulfate 

(Na2SO4), sulfuric acid (H2SO4), and other ions such as chloride (Cl), potassium (K) and 

carbonate (CO3). SO4, NO3, and NH4 originate through both primary and secondary 

mechanisms. Atmospheric chemistry converts SO2 to SO4, NO and NO2 to NO3, and NH3 to 

NH4 and is the main source of these ions; more details of this chemistry are provided in the 

below discussion about precursors. Additionally, the EPA NEI (Table 1) shows that the largest

primary emissions of SO4 and NO3 are produced by various industrial sources, boilers, and 

mobile sources, but the contribution of these emissions to ambient concentrations could be small 

compared to secondary atmospheric formation from gaseous precursors.

Organic Matter 

OM is the fraction of PM with the most diverse chemical composition, containing potentially 

thousands of different organic compounds, i.e. those composed of carbon, hydrogen, oxygen, 

nitrogen, and possibly some other elements. Bulk properties of the OM will depend on the 

chemical composition, for example more oxygen in the OM will make a more hygroscopic 

particle. Both primary sources and secondary chemistry contribute substantially to ambient OM 

concentrations, with combustion being the dominant type of emissions source (Table 1). 

Secondary OM formation involves oxidation of both anthropogenic and biogenic gaseous 

volatile organic compounds (VOCs) oxidizing to lower volatility compounds, and can involve 

numerous complex chemical reactions involving VOCs, atmospheric oxidants, and sunlight. 

Further details of the chemistry are described in the precursor discussion below. OM is often 

not directly measured - its concentration is usually estimated from a measurement of organic 

carbon (OC) by multiplying OM/OC ratios derived from case-study molecular compound 

measurements (Turpin and Lim (2001)). Recent statistical models that relate PM2.5 mass to its 

constituents have also demonstrated ability to calculate OM concentrations (Simon et al (2011), 

Malm et al (2011)).

Elemental Carbon

EC refers to particulate carbon that has a graphitic molecular structure and is relatively pure, 

with few bonds to other elements such as oxygen or hydrogen that occur in OM. However, in 

practice, it typically refers to the portion of measured total carbon that is not classified as 

organic. This fraction of carbonaceous aerosol also absorbs a large amount of visible light, and 

so is sometimes referred to as "Black Carbon" (BC); the exact term used depends on the method 

by which it is measured (EPA 2012). EC completely originates from primary emissions 

involving combustion, especially diesel-based vehicles as well as other processes involving 

burning fossil fuels. The latter includes anthropogenic sources such as boilers and waste 
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disposal (Table 1), as well as biomass combustion such as forest fires and residential wood 

combustion. Some research has indicated that EC is potentially useful as a tracer for 

distinguishing primary from secondary PM and OM (Turpin and Huntzicker (1995), Yu (2004), 

Chu et al (2004)). 

Crustal Material

The crustal portion is material from soil and similar materials. Elements such as silicon, 

aluminum, iron, calcium, titanium, magnesium, potassium, and others as well as oxygen are 

major components (Appel et al (2013), Sorooshian et al (2013)). Sources include dust from wind 

and mechanical resuspension (e.g. vehicles pushing dust into the air), and some combustion, 

especially of coal. Most of the mass of crustal PM resides in size ranges larger than that of 

PM2.5, but the tail end of the crustal size distribution as well as the imperfect cut point of the 

FRM can both yield a substantial crustal component in PM2.5 samples. Crustal elements and 

their oxides can also be emitted from various industrial sources and will be included in the 

estimate of PM2.5 crustal material, although receptor modeling techniques can attempt to resolve 

separate contributions of sources of these elements (Reff et al (2007)).

Reconstructing Ambient Mass

The sum of the bulk component measurements often does not sum to concurrently measured 

PM2.5 concentrations, which is due to: 1) loss of water and NO3 due to volatilization in the FRM 

analysis, 2) uncertainty in applying an OM/OC ratio, and 3) uncertain measurement of NH4. To 

address these issues, EPA developed the sulfate, adjusted nitrate, derived water, inferred 

carbonaceous material balance approach (SANDWICH) to reconcile measurements from the 

Chemical Speciation Network (CSN) with the PM2.5 FRM measurement (Frank, 2006). This 

involves first using a thermodynamic model to correct the measured NO3 concentration. NH4 is 

calculated by assuming all SO4 and NO3 are in the forms of ammonium sulfate and ammonium 

nitrate, respectively. Particulate water is then estimated from the ion concentrations using 

another chemical model. Crustal material is derived using a function of crustal elements (Si, Ca, 

Fe, Ti), and OM is assumed to comprise the remainder of the PM2.5 mass. It can thus be 

calculated as the difference between the sum of EC, crustal material, water and the ions, thus 

removing the need to assume any OM/OC ratio. In general, the calculated nitrate is less than the 

amount directly derived from chemical speciation measurements, and the quantity of estimated 

water resulting from the mixture of SO4, NO3 and NH4 can be a substantial portion of the 

measured PM2.5 mass. It can be reported as part of sulfate and nitrate mass which also include 

associated NH4. The derived OM from material balance correlates well with measured OC, 

especially at higher concentrations.

Measurements and spatial distributions

Table 2 shows regionally-averaged concentrations of PM2.5 and bulk components measured by 

the EPA Chemical Speciation Network (CSN) at non-attainment sites determined by the 2009-

2011 design values (DVs) (http://www.epa.gov/airtrends/values.html). These data have been 

processed with the SANDWICH method, and illustrate broad observed spatial patterns in PM2.5

and its bulk composition. For example, PM2.5 concentrations are highest in the West and South 

East regions. Sulfate mass (the sum of SO4, NH4 and water) comprises a larger fraction of 

PM2.5 than nitrate in the North Eastern U.S., whereas nitrate has an almost equal contribution to 
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that of sulfate in the West. OM is the dominant component in all regions except the Central, and 

has the highest concentrations in the West, North West and South East. The concentrations, 

contributions, and spatial gradients of EC and crustal material are relatively low compared to the 

other components.

The "Urban Increment" (UI) is an ambient concentration derived as the difference between an 

urban sampling site and the surrounding rural values. UI values for PM2.5 and bulk components 

are also shown in Table 2, and represent an estimate of the contribution of an urban area's 

sources to its own concentrations after accounting for background concentrations. The data 

show that PM2.5 UIs are highest in the North West and South West. The composition of the UI is 

dominated by OM throughout the U.S. The contribution of EC to PM2.5 is relatively small; 

however, the urban contribution to EC is the strongest of all species, which is consistent with 

previous measurements (Hand et al (2011)). Although OM can result from both primary and 

secondary formation, primary OM emission contributions are related to the EC portion of the 

increment. UI values are generally a small fraction (< 50%) of the PM2.5 concentrations in most 

regions, which suggests that transport of PM2.5 from rural areas is the dominant source of 

ambient PM2.5. A substantial fraction of this rural PM2.5 is estimated to be secondary in origin, 

as discussed in the next section.

III. Secondary formation of PM2.5 from Gaseous Precursors

Overview of Atmospheric Chemistry

As noted above, PM2.5 composition is complex and highly variable. This is due in part to the 

strong contribution of secondary PM2.5 to the total atmospheric burden, and the complexity of the 

secondary formation processes. A very large number of possible chemical reactions, often nonlinear in structure, can convert SO2, NOx, NH3, and VOC precursors to PM2.5. Given the 

complexity of PM formation processes, new information from the scientific community 

continues to emerge to improve our understanding of the relationship between sources of PM 

precursors and secondary particle formation. Much of this new understanding in PM formation 

mechanisms, important parameters in aerosol chemistry, and emission sources can be found in 

EPA’s most recent Integrated Science Assessment for Particulate Matter [ref 3] and in the 

NARSTO fine particle assessment (ref 4). A brief discussion of sulfate, nitrate and SOA 

formation follows based on the background provided in references 3 and 4, with some added new 

references on the subject.

SO2 to Sulfate Processes

SO2 is emitted mostly from the combustion of fossil fuels in boilers operated by electric utilities 

and other industry. Based on the 2008 NEI (ref 5), less than 10% of SO2 emissions nationwide 

are from other sources, mainly from other industrial processes including oil refining and pulp 

and paper production.

The formation of sulfuric acid from the oxidation of SO2 is an important process for most areas 

in North America. There are three different pathways for this transformation. First, gaseous 

SO2 can be oxidized by the hydroxyl radical (OH) to create sulfuric acid. This gaseous SO2 

oxidation reaction occurs slowly and only in the daytime. The hydroxl radical is an important 
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product of the atmospheric chemistry process that forms ozone through the oxidation of NOx to 

form nitric acid. It is also involved in the formation of secondary organics. 

Second, SO2 can dissolve in cloud water (or fog or rain water), and there it can be oxidized to 

sulfuric acid by a variety of oxidants, or through catalysis by transition metals such as 

manganese or iron. If ammonia is present and taken up by the water droplet, then ammonium 

sulfate will form as a precipitant in the water droplet. After the cloud changes and the droplet 

evaporates, the sulfuric acid or ammonium sulfate remains in the atmosphere as a particle. This 

aqueous phase production process involving oxidants can be very fast; in some cases all the 

available SO2 can be oxidized in less than an hour. 

Third, SO2 can be oxidized in reactions in the particle-bound water in the aerosol particles 

themselves. This process takes place continuously, but only produces appreciable sulfate in 

alkaline (dust, sea salt) coarse particles (ref 6). Oxidation of SO2 has been also observed on the 

surfaces of black carbon and metal oxide particles. During the last twenty years, much progress 

has been made in understanding the first two major pathways, but some important questions still 

remain about the smaller third pathway. Models indicate that more than half of the sulfuric acid 

in the eastern United States and in the overall atmosphere is produced in clouds (ref 7). A recent 

doctoral dissertation by El-Zanan updates much of the scientific evidence in the area of SO2 

oxidation mechanisms and cloud chemistry (ref 8).

The sulfuric acid formed from the above pathways reacts readily with ammonia to form 

ammonium sulfate, (NH4)2SO4. If there is not enough ammonia present to fully neutralize the 

produced sulfuric acid (one molecule of sulfuric acid requires two molecules of ammonia), part 

of it exists as ammonium bisulfate, NH4HSO4 (one molecule of sulfuric acid and one molecule 

of ammonia) and the particles are more acidic than ammonium sulfate. In extreme cases (in the 

absence of sufficient ammonia for neutralization), sulfate can exist in particles as sulfuric acid, 

H2SO4. Sulfuric acid often exists in the plumes of stacks where SO2, SO3, and water vapor are 

in much higher concentrations than in the ambient atmosphere, but these concentrations become 

quite small as the plume is cooled and diluted by mixing.

Finally, there is a large amount of emerging scientific evidence that SO2 may contribute to 

formation of biogenic emissions (see section below on biogenic VOCs). It’s likely that the 

additional SOA formed via this route is not included in most current atmospheric models (ref 9, 

ref 10, ref 11).

NOx to Nitrate Processes

The main sources of NOx are combustion of fossil fuel in boilers and mobile sources. Together 

they account for more about 83% of national anthropogenic NOx emissions (based on the 2008 

NEI), with boilers and EGUs contributing about 33% and onroad/nonroad the remaining 67%. 

Nitrates are formed from the oxidation of oxides of nitrogen into nitric acid either during the 

daytime (reaction with OH) or during the night (reactions with ozone and water) (ref 12).

Nitric acid continuously transfers between the gas and the condensed phases through 

condensation and evaporation processes in the atmosphere. However, unless it reacts with other 

species (such as ammonia, sea salt, or dust) to form a neutralized salt, it will volatize in the 

atmosphere and not be measured using standard PM2.5 chemical speciation measurement 
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techniques (ref 13). As mentioned earlier, further volatilization of ammonium nitrate can occur 

during the sampling for PM2.5 mass. The formation of aerosol ammonium nitrate is favored by 

the availability of ammonia, low temperatures, and high relative humidity. Because ammonium 

nitrate is not stable in higher temperatures, nitrate levels are typically lower in the summer 

months and higher in the winter months. The resulting ammonium nitrate is usually in the sub 

micrometer particle size range. 

Secondary Organic Aerosol (SOA) Formation from VOCs

The organic component of ambient particles is a complex mixture of hundreds or even thousands 

of anthropogenic and biogenic organic compounds (ie VOCs). These organic compounds are 

either emitted directly from sources (i.e. primary organic aerosol) or can be formed by reactions 

in the ambient air (i.e. secondary organic aerosol, or SOA). 

VOCs (both anthropogenic and biogenic) are key precursors in both the SOA and ozone 

formation processes. The lightest organic molecules (i.e., molecules with six or fewer carbon 

atoms) occur in the atmosphere mainly as vapors and typically do not directly form organic 

particles at ambient temperatures due to the high vapor pressure of their products. However, 

they participate in atmospheric chemistry processes resulting in the formation of ozone and 

certain free radical compounds (such as the hydroxyl radical [OH]) which in turn participate in 

the oxidation of semivolatile organic compounds (SVOCs) to form secondary organic aerosols, 

sulfates and nitrates. These VOCs include all alkanes with up to six carbon atoms (from 

methane to hexane isomers), all alkenes with up to six carbon atoms (from ethene to hexene 

isomers), benzene and many low molecular weight carbonyls, chlorinated compounds, and 

oxygenated solvents. The relative importance of organic compounds in the formation of organic 

particles varies from area to area, depending upon local emissions sources, atmospheric 

chemistry, and season of the year. A recent publication (ref 14) has reviewed which organic 

compounds contribute most to SOA in the atmosphere, and which sources contribute most to 

those sources. The authors have developed a “Secondary Organic Aerosol Potential (SOAP)” for 

the most prevalent anthropogenic VOCs. The SOAP indicator shows xylenes, toulene, and 

ethylbenzene to be some of the most important VOCs for SOA production. 

Intermediate weight organic molecules (i.e., compounds with 7 to 24 carbon atoms, and usually 

termed “IVOCs”) often exhibit a range of volatilities and can exist in both the gas and aerosol 

phase. For this reason they are also referred to as “semivolatile” compounds. Semivolatile 

compounds react in the atmosphere to form secondary organic aerosols. These chemical 

reactions are accelerated in warmer temperatures, and studies show that SOA typically comprises 

a higher percentage of carbonaceous PM in the summer as opposed to the winter. Research in 

this area has accelerated in recent years, since models are not able to produce estimated amount 

of SOA from ambient data in the summertime. A recent article by Pye et al. (ref 15) shows 

many of these references as part of their review as well as plans to include SOA from two classes 

of IVOCs (long-chained alkanes and PAHs) into EPA’s CMAQ (Community Multiscale of Air 

Quality) photochemical air quality model.

The production of SOA from the atmospheric oxidation of a specific VOC depends on four 

factors: its atmospheric abundance, its chemical reactivity, the availability of oxidants (O3, OH, 

HNO3), and the volatility of its products. In addition, recent work by Jang and others suggests

that the presence of acidic aerosols may lead to an increased rate of SOA formation (ref 16). 
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Aromatic compounds such as toluene, benzene, xylene, and trimethyl benzene are considered to 

be the most significant anthropogenic SOA precursors and have been estimated to be responsible 

for 50 to 70% of total SOA in some airsheds (ref 17). In fact, current CMAQ considers SOA 

from toluene and benzene (and other high-yield aromatics) as well as xylene (and other low-yield 

aromatics). (ref 18)

As organic gases such as aromatics are oxidized in the gas phase by species such as the hydroxyl 

radical (OH), ozone (O3), and the nitrate radical (NO3) their oxidation products accumulate. 

Some of these products have low volatility and condense on available particles in an effort to 

establish equilibrium between the gas and condensed phases. Man-made sources of aromatics 

gases are mobile sources, petrochemical manufacturing, oil and gas emissions, and solvents. The 

early experimental work of Odum and others (ref 18) showed that the secondary organic aerosol 

formation potential of gasoline could be accounted for solely in terms of its aromatic fraction. 

More recent work (ref 19) has shown that diesel-based mobile sources can also contribute to 

atmospheric SOA under some atmospheric conditions.

Some biogenic VOCs, emitted by vegetation/trees, can also contribute significantly to SOA, 

especially in heavily forested areas, such as the southeastern United States. Terpenes (α and β 

pinene, limonene, carene, etc.) and the sesquiterpenes are expected to be major contributors to 

SOA in areas with significant vegetation cover; in fact, reference 15 shows that the current 

CMAQ model considers SOA from isoprene, monoterpenes, and Sesquiterpenes. Unlike 

thought previously, recent research has shown that isoprene is also a significant contributor to 

SOA in the atmosphere (ref 20) 

Recent research has also probed the question of “to what extent can biogenic SOA can be 

controlled for PM2.5 reduction purposes?” The traditional notion that biogenic SOA cannot be 

controlled for PM2.5 reductions has been questioned due to the interactions that biogenic SOA 

can have with anthropogenic SOA precursors (see previous discussion on sulfate formation) (ref 

21, ref 22).

Table 1 summarizes this entire discussion on the role of organic gases (anthropogenic and 

biogenic VOCs) in SOA formation. 

Table 1. Role of Organic Gases in Secondary Organic Aerosol Formation. 

SOA-Forming Organic Gases Non SOA-Forming Organic Gases

Anthropogenic - Aromatics (esp. toluene, 

xylenes, trimethyl-benzenes, 

benzene)

- Higher alkanes (>6 C atoms)

- Lower alkanes <6 C atoms 

(ethane to hexane isomers)

- Lower MW carbonyls, 

chlorinated compounds and 

oxygenated solvents

Biogenic - Terpenes (esp. α- and 

β-pinene, limonene, carene)

- Sesquiterpenes

- Isoprene
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The contribution of the primary and secondary components of organic aerosol to the measured 

organic aerosol concentrations remains a controversial issue. Early studies suggested that the 

majority of the observed organic particulate matter was secondary in nature. Later investigators 

focusing on the emissions of primary organic material proposed that about 80% of the organic 

aerosol in Southern California on a monthly basis resulted from direct organic particle emissions 

(ref 23). More recent studies suggest that the primary and secondary contributions are highly 

variable even during the same day. Studies of pollution episodes indicated that the contribution 

of SOA to the organic particulate matter varied from 20 to 80% during the same day (ref 24). 

Despite significant progress that has been made in understanding the origins and properties of 

SOA, it remains the least understood component of PM2.5. The reactions forming secondary 

organics are complex and the number of intermediate and final compounds formed is 

voluminous. Some of the best efforts to unravel the chemical composition of ambient organic 

aerosol matter have been able to quantify the concentrations of hundreds of organic compounds 

representing only 10 to 20% of the total organic aerosol mass. For this reason, SOA continues to 

be a significant topic of research and investigation.

The role of Ammonia in sulfate, nitrate, and SOA formation

Ammonia (NH3) is a gaseous pollutant that is emitted by natural and anthropogenic sources. 

Emissions inventories for ammonia are considered to be among the most uncertain of any species 

related to PM. One recent estimate shows, however, that livestock (73 %) and fertilizer 

application (17 %) are the two primary sources of emissions (ref 25). EPA’s 2008 NEI shows 

that the fertilizer application and livestock sector together (nationally) contribute about 83% of 

total ammonia emissions. It should be noted that the 2008 NEI also shows mobile sources to 

contribute about 3% of nationwide ammonia emissions. It is expected that this percentage will 

be much higher (up to 50%) in urban areas (ref 26). These estimates do not include natural 

(biogenic) emissions from soil, which can be significant in some locations during certain times 

of year.

Ammonia serves an important role in neutralizing acids in clouds, precipitation and particles. In 

particular, ammonia neutralizes sulfuric acid and nitric acid, the two key contributors to acid 

deposition (acid rain). Deposited ammonia also can be an important nutrient, contributing to 

problems of eutrophication in water bodies (ref 27). Ammonia would not exist in particles, if not 

for the presence of acidic species with which it can combine to form a particle. In the eastern 

US, sulfate, nitrate, and the ammonium associated with them can together account for between 

roughly 30% and 75% of the PM2.5 mass. The ammonium itself roughly accounts for between 

5% and 20% of the PM2.5 (ref 28). 

The NARSTO Fine Particle Assessment indicates that sulfates form preferentially over nitrates 

and that particle nitrate formation is affected by a number of factors, including the availability of 

sulfates, NOx, ammonia, nitric acid, OH groups, and, in general, VOCs (ref 29). The report also 

notes that implementing decreasing ammonia emissions where sulfate concentrations are high 

can reduce PM2.5 mass concentrations, but may also increase particle and precipitation acidity 

(more sulfuric acid and ammonium sulfate). As noted above, this acidification of particles may 

result in an increase in the formation of secondary organic compounds. Moreover, the 

relationship between ammonia and sulfate-nitrate equilibrium may also impact SOA formation,

although this link is not well understood. Recent studies of ammonia sources and possible 
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emission reduction measures indicate that ammonia controls are a maturing science, but that 

ongoing research will greatly improve our understanding of such control measures.

The same can be said of our understanding of the role of ammonia in aerosol formation. Based 

on the above information and further insights gained from the NARSTO Fine Particle 

Assessment, it is apparent that the formation of sulfate, nitrate & SOA compounds is a complex, 

nonlinear process. The control techniques for ammonia and the analytical tools to quantify the 

impact of reducing ammonia emissions on atmospheric aerosol formation are both evolving 

sciences. Also, there are indications that there may be considerable ambiguity concerning the 

results of reducing ammonia emissions and in some cases, there may be undesired consequences 

of ammonia reductions. Therefore, based on our current understanding the role of ammonia in 

these complex precursor interactions and emission reduction processes, it seems prudent to 

continue research on ammonia control technologies and the ammonia - sulfate - nitrate - SOA 

equilibrium before one undertakes broad national programs to reduce ammonia emissions. 

REFERENCES

References: Sections I and II

Appel, K. W., Pouliot, G. A., Simon, H., Sarwar, G., Pye, H. O. T., Napelenok, S. L., Akhtar, F., 

and Roselle, S. J. (2013), Evaluation of dust and trace metal estimates from the Community 

Multiscale Air Quality (CMAQ) model version 5.0, Geosci. Model Dev. Discuss., vol 6, pp 

1859-1899.

S.H. Chu, J.W. Paisie and B.W.L. Jang (2004), PM data analysis – a comparison of two urban 

areas: Fresno and Atlanta, Atmospheric Environment, pp 3155-3164.

J.L. Hand, B.A. Schichtel, M. Pitchford, W.C. Malm and N.H. Frank (2012), Seasonal 

Composition of Remote and Urban Fine Particulate Matter in the United States, Journal of 

Geophysical Research, vol 117, doi:10.1029/2011JD017122.

N. Frank (2006), Retained Nitrate, Hydrated Sulfates, and Carbonaceous Mass in Federal 

Reference Method Fine Particulate Matter for Six Eastern U.S. Cities, Journal of the Air and 

Waste Management Association, vol 56, pp 500-511.

W.C. Malm, B.A. Schichtel and M.L. Pitchford (2011), Uncertainties in PM2.5 Gravimetric and 

Speciation Measurements and What We Can Learn from Them, Journal of the Air and Waste 

Management Assciation, vol 61, pp 1131-1149.

A. Reff, S.I. Eberly and P.V. Bhave, Receptor Modeling of Ambient Particulate Matter Data 

Using Positive Matrix Factorization: Review of Existing Methods, Journal of the Air and Waste 

Management Association, vol 57, pp 146-154.

H. Simon, P.V. Bhave, J.L. Swall, N.H. Frank and W.C. Malm (2011), Determining the spatial 

and seasonal variability in OM/OC ratios across the US using multiple regression. Atmospheric 

Chemistry and Physics, vol 11, pp 2933-2949.
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Sorooshian, A., Shingler, T., Harpold, A., Feagles, C. W., Meixner, T., and Brooks, P. D. (2013), 

Aerosol and precipitation chemistry in the southwestern United States: spatiotemporal trends 

and interrelationships, Atmos. Chem. Phys., vol 13, pp 7361-7379.

B.J. Turpin and J.J. Huntzicker (1995), Identification of Secondary Organic Aerosol Episodes 

and Quantitation of Primary and Secondary Organic Aerosol Concentrations During SCAQS, 

Atmospheric Environment, Vol 29, pp 3527-3544.

B. J. Turpin and H.J. Lim (2001), Species Contributions to PM2.5 Mass Concentrations: 

Revisiting Common Assumptions for Estimating Organic MassAerosol Science and Technology, 

vol 35, pp 602-610.

S. Yu, R. L. Dennis, P.V. Bhave, and B.K. Eder (2004), Primary and secondary organic aerosols 

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carbon (EC), and air quality modeled primary OC/EC ratios, Atmospheric Environment, vol 38, 

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References: Section III

1. S. N. Behera and M. Sharma, (2010) Reconstructing Primary and Secondary Components 

of PM2.5 Composition for an Urban Atmosphere. Aerosol Science and Technology 44:11, pages 

983-992

2. EPA’s Report to Congress on Black Carbon, EPA-450/S-12-001, March 2012. Available 

at: http://epa.gov/blackcarbon/

3. U.S. EPA. Integrated Science Assessment for Particulate Matter (Final Report). U.S. 

Environmental Protection Agency, Washington, DC, EPA/600/R-08/139F, 2009. 

http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=216546)

4. North American Research Strategy for Tropospheric Ozone and Particulate Matter 

(NARSTO) (2004) Particulate Matter Assessment for Policy Makers: A NARSTO Assessment. 

P. McMurry, M. Shepherd, and J. Vickery, eds. Cambridge University Press, Cambridge, 

England. ISBN 0 52 184287 5. For more information, see http://www.cgenv.com/NARSTO. 

See also supporting technical information for the Clear Skies Act, 

http://www.epa.gov/clearskies/, and for the Clean Air Interstate Rule, 

http://www.epa.gov/cleanairinterstaterule. 

5. Rao, V.; Tooly, L.; and Drukenbrod, 2013. 2008 National Emissions Inventory: Review 

Analysis and Highlights. US Environmental Protection Agency, Office of Air Quality Planning 

and Standards, Air Quality Assessment Division, EPA-454/R-005, May 2013. 

http://www.epa.gov/ttn/chief/net/2008inventory.html

6. Sievering, H., Boatman, J., Gorman, E., Kim, Y., Anderson, L., Ennis, G., Luria, M., 

Pandis, S.N., 1992. Removal of sulfur from the marine boundary layer by ozone oxidation in seasalt. Nature 360, 571-573.

7. McHenry, J.N., Dennis, R.L., 1994. The relative importance of oxidation pathways and 

clouds to atmospheric ambient sulfate production as predicted by the Regional Acid Deposition 
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Model. Journal of Applied Meteorology 33, 890-905. Also: Langner, J., Rodhe, H., 1991. A 

global three dimensional model for the tropospheric sulfur cycle. Journal of Atmospheric 

Chemistry 13, 225-263.

8. “Theoretical and Modeling Studies of the Atmospheric Chemistry of Sox and HOx 

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