1

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 PM 2.5 SIP Requirements Rule
EPA

- HQ

- OAR

- 2013

- 0691

Subject: Background on atmospheric fine particulate matter composition and formation chemistry

I. Overview of PM and PM 2.5

.

Particulate matter (PM) refers to airborne particles of solid and liquid material in the ambient atmosphere. A major distinguish ing 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 typic ally ranges in aerodynamic
diame t ers 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, m eaning 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 de tritus; a major formation mechanism of fine PM is oxidation of SO2 and NH3 gases to form particulate ammonium sulfate.

PM 2.5 technically refers to PM with an aerodynamic diameter under 2.5  m , but filter

- based sampling devices, including the Federal Ref erence Method (FRM), are not able to create a perfect cutoff between PM 2.5 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 d uring sampling on warm days and changes in particle

- bound water during gravimetric measurement. For regulatory purposes, PM 2.5 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. PM 2.5 can sometimes contain significant amounts of primary PM, although it is often secondary in origin. PM 2.5 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 PM 2.5 concentrations as measured by the FRM

.

II. Composi tion, emissions, and spatial distribution of PM 2.5 components.

PM 2.5 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

compou nd 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,2

which depend on the mixture of hygroscopic compounds. T race elements are a lso 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) PM 2.5 . Some discussion of each of the four major components is given below.

Ions

The i onic fraction of PM is mostly composed of sulfate (SO4), nitrate (NO3), and ammonium (NH4). Example compo unds include ammonium sulfate ( (NH4)2SO4) an d ammonium nitrate (NH4NO3). Sodium chloride (NaCl) is seen in marine and coastal environments. Less

com monly measured compounds include ammonium bisulfate (NH4HSO4), sodium sulfate (Na2SO4), sulfuric acid (H2SO4), and other ions such as chlor ide (Cl), potassium (K) and carbonate (CO3). SO4, NO3, and NH4 originate through both primary and secondary
mechanis ms. Atmospheric chemistry convert s 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 th e 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, oxyg en, 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 substantia lly 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 c ompounds, 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 PM 2.5 mass to its constituents have also demons trated 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 hydr ogen that occur in OM

. However, in practice, it typically refers to the portion of measured total carbon that is not classified as organic. This fr action 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 fo ssil fuels

. The latter includes anthropogenic sources such as boilers and waste3

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,

esp ecially of coal. Most of the mass of crustal PM resides in size ranges larger than that of
PM 2.5 , 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 PM 2.5 samples. Crustal elements and their oxides can also be emitted from various industrial sources and will be included in the
estimate of PM 2.5 crustal material, although receptor modeling techniques can attempt to resolve

separ ate 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
PM 2.5 concentrations, which is due to: 1) loss of water and NO3 due to volatilization in the FRM analy sis, 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 PM 2.5 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 ammoniu m 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 t he remainder of the PM 2.5 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 PM 2.5 mass. It can be reported as part of sulfate and nitrate mass which also in clude 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 PM 2.5 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 spatia l patterns in PM 2.5 and its bulk composition. For example, PM 2.5 concentrations are highest in the West and South East regions. Sulfate mass (the sum of SO4, NH4 and water) comprises a larger fraction of
PM 2.5 than nitrate in the North Eastern U.S., whereas nitrate has an almost equal contribution to4

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 co ncentrations, 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 surro unding rural values . UI values for PM 2.5 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 PM 2.5 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 PM 2.5 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 PM 2.5 concentrations in most regions , which suggests that transport of PM 2.5 from rural areas is the domin ant source of
ambient PM 2.5 . A substantial fraction of this rural PM 2.5 is estimated to be secondary in origin,
as discussed in the next section.

III

. Secondary formation of PM 2.5 from Gaseous Precursors

Overview of Atmospheric Chemistry

As noted above, PM 2.5 composition is complex and highly variable. This is due in part to the strong contribution of secondary PM 2.5 to the total atmospher ic burden, and the complexity of the secondary formation processes. A very large number of possible chemical reactions, often non

linear in structure, can convert SO2, NOx, NH3, and VOC precursors to PM 2.5 . 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 par ameters 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 im portant 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 o nly in the daytime. The hydroxl radical is an important5

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
sul fate 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 maj or 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 ther e 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) a nd 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 contr ibute 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 ab out 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 ammon ia, sea salt, or dust) to form a neutralized salt, it will volatize in the atmosphere and not be measured using standard PM 2.5 chemical speciation measurement6

techniques (ref 13). As mentioned earlier, further volatilization of ammonium nitrate can occur
d uring the sampling for PM 2.5 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 typica lly 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 comp lex 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. sec ondary 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 v apors 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 forma tion 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 whic h 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 re ferred 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 o f 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) sho ws 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).7

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 suc h 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 atm ospheric 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 sesqui terpenes 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 res earch 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 PM 2.5 reduction purposes?” The traditional notion that biogenic SOA cannot be
controlled for PM 2.5 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

- Fo rming Organic Gases Anthropogenic

- Aromati cs (esp. toluene,

xylenes, trimethyl

- benzenes, benzene)

- Higher alkanes (>6 C atoms) - Lower alkanes <6 C atoms (ethane to hexane isomers)

- Lower MW car bonyls, chlorinated compounds and oxygenated solvents Biogenic

- Terpenes (esp. α

- and

β

- pinene, limonene, carene)

- Sesquiterpenes

- Isoprene8

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 8 0% of the organic aerosol in Southern California on a mo nthly 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 PM 2.5 . The reaction s 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 co mpounds 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 (7 3 % ) 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 t hat 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 fro m 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 c ontributors 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 whi ch 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 PM 2.5 mass. The ammonium itself roughly accounts for between 5% and 20 % of the PM 2.5 (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 grou ps, and, in general, VOCs (ref 29). The report also notes that implementing decreasing ammonia emissions where sulfate concentrations are high
can reduce PM 2.5 mass concentrations, but may also increase particle and precipitation acidity (more sulfuric ac id 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 possible9

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 & S OA 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 t here 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 precur sor 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 (C MAQ) model version 5.0, Geosci. Model Dev. Discuss., vol 6, pp
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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

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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 Sulfate s, 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 PM 2.5 G ravimetric and Speciation Measurements and What We Can Learn from Them, Journal of the Air and Waste Management Assciation, vol 61, pp 1131

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A. Reff, S.I. Eberly and P.V. Bhave, Receptor Modeling of Ambient Particulate Matter Data Using Positive Matri x Factorization: Review of Existing Methods, Journal of the Air and Waste Management Association, vol 57, pp 146

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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

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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 St ates: spatiotemporal trends and interrelationships, Atmos. Chem. Phys., vol 13, pp 7361

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B.J. Turpin and J.J. Huntzicker (1995), Identification of Secondary Organic Aerosol Episodes and Quantitation of Primar y and Secondary Organic Aerosol Concentrat ions During SCAQS, Atmospheric Environment, Vol 29, pp 3527

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B. J. Turpin and H.J. Lim (2001), Species Contributions to PM 2.5 Mass Concentrations: Revisiting Common Assumptions for Estimating Organic MassAerosol Science and Technology, vol 35, pp 602

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S. Yu, R. L. Dennis, P.V. Bhave, and B.K. Eder (2004), Primary and secondary organic aerosols over the United States: estimates on the basis of observed organic carbon (OC) and elemental carbon (EC), and air quality modeled primary OC/EC ratios, Atmos pheric Environment, vol 38, pp 5257

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References: Section III

1. S. N. Behera and M. Sharma, (2010) Reconstructing Primary and Secondary Components
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2. EPA ’s Report to Congress on Black Carbon, EPA

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3. U.S. EPA. Integrated Science Assessment for Particulate Matter (Final Report). U.S. Environmental Protection Agency, Washington, DC, EPA/600/ R

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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.

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11. Ying

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