Tom Hardie Memorial Award

No. 40

Cleaning the Air We Breathe: Science and Policy Recommendations to Reduce Concentrations of Secondary Organic Aerosols  

May 2012 Lauren Goldstein-Kral ’12

THE TOM HARDIE ’78 MEMORIAL AWARD

Each year the Center for Environmental Studies awards the Tom Hardie ‘78 Memorial Award to the student whose work best demonstrates excellence in environmental studies. The award consists of a photograph of Tom Hardie and the publication of his/her work as part of a monograph series which is made available at Harper House or on-line at ces.williams.edu under “publications”.

This award was created in remembrance of Thomas G. Hardie III ’78 (1956-1975).

Tom Hardie was a country boy who knew his way around a city--especially if there was an art museum nearby. But nature was his first love. When a student at Gilman School in Baltimore, he started the Ecology Recycling Center, as well as Operation GreenGrass, bringing inner city students out to the country. As a Williams freshman he expected to major in Pre-Med, planning to be a doctor, but the heart of his year was the Center for Environmental Studies. It was here he felt most at home. Tom probably thought he was giving, but the Center gave him the most meaningful, happiest part of his one year at Williams. In return for this gift to their son, his family has established the Thomas Hardie Memorial Award for others, like Tom, who want to help their world without being asked.

At the end of this publication is a list of all the past Tom Hardie ’78 Memorial Awards. Free copies of each are available upon request at the Center for Environmental Studies. Some are available online at ces.williams.edu.

 

Cleaning the Air We Breathe: Science and Policy Recommendations to Reduce Concentrations of Secondary Organic Aerosols  

Lauren Goldstein-Kral

Capstone Paper for ENVI 402 Professor Kohler

 

Introduction  

Small atmospheric particles (smaller than one thirtieth the diameter of a piece of human

hair1) are among the most dangerous air pollutants for human health. These particles and the gas

in which the particles are suspended are referred to as aerosols. Heart and lung diseases resulting

from aerosol inhalation are responsible for the deaths of approximately 50,000 people in the

United States annually.2

Aerosols can be divided into two categories, primary aerosols and secondary aerosols.

Primary aerosols are suspensions of small particles in gas that are emitted directly into the

atmosphere. Major sources of primary aerosols are sea salt released from breaking waves, dust

from natural and agricultural sources, particles from the burning of biomass (from both natural

and anthropogenic sources), and particles from volcanic eruptions. Secondary aerosols are

suspensions of particles in gas that are formed by chemical reactions that take place in the

atmosphere. Some types of molecules that are involved in the formation of secondary aerosols

are sulfates, nitrates, nitrites, alkanes, alkenes, and aromatic molecules. Major sources of these

pollutants are car and factory emissions especially from the use of petroleum. Carbon-based

aerosols that are formed in the atmosphere are a subset of secondary aerosols called secondary

organic aerosols (SOAs).

Recent scientific breakthroughs have brought SOAs to the forefront of atmospheric

environmental chemistry. Nawrot et al (2007) showed that there is a greater association between

mortality rates and particulate pollution in the summer when SOA concentrations are high than

                                                                                                               1 Barringer, Felicity. "Scientists Find New Dangers in Tiny but Pervasive Particles in Air Pollution." New York Times. 18 Feb. 2012. Web. 24 Feb. 2012. <http://www.nytimes.com/2012/02/19/science/earth/scientists-find-new-dangers-in-tiny-but-pervasive-particles-in-air-pollution.html>. 2 Ibid.

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in the winter when SOA concentrations are low.3 These findings were highly significant (p <

0.001) and suggest that SOAs have greater health effects than most other forms of particulate

pollution.4 The dangers of SOAs were confirmed by an experiment that exposed a culture of

lung cells to SOAs and measured the production of interlukin-8, an indicator of biological

stress.5 Interlukin-8 expression increased in cells exposed to SOAs showing that SOAs have

health effects at a cellular level.6 Because this is still a relatively new area of research,

experiments are currently in progress that are attempting to explain the mechanism by which

SOAs affect human cells.

There have also been breakthroughs in our understanding of atmospheric concentrations

of SOAs. Volkamer et al. (2006) showed that SOA concentrations in the environment are

greater than the concentrations predicted by models7, and Perraud et al. (2011) discovered that

current models made incorrect assumptions about the rate of SOA formation from nitrates.8

When the models are corrected for this mistake, predicted values become closer to the measured

values. In light of these recent scientific breakthroughs, the EPA decided in February 2012 to

                                                                                                               3 Nawrot, T. S., R. Torfs, F. Fierens, S. de Henauw, P. H. Hoet, G. van Kersschaever, G. de Backer, and B. Nemery. “Stronger associations between daily mortality and fine particulate air pollution in summer than in winter: evidence from a heavily polluted region in western Europe.” J. Epidemiology Community Health, 61.2 (2007): 146–149. Web. 26 Feb. 2012. <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2465652/>. 4 Ibid. 5 Jang, M., A. J. Ghio, and G. Cao. "Exposure of BEAS-2B Cells to Secondary Organic Aerosol Coated on Magnetic Nanoparticles." Chemical Research in Toxicology, 19.8 (2006): 1044-050. Web. 5 Mar. 2012. <http://pubs.acs.org/doi/full/10.1021/tx0503597>. 6 Ibid. 7 Volkamer, R., J. Jimenez, F. Martini, K. Dzepina, Q. Zhang, D Salcedo, L. Molina, D. Worsnop, and M. Molina. "Secondary Organic Aerosol Formation from Anthropogenic Air Pollution: Rapid and Higher than Expected." Geophysical Research Letters, 33 (2006): 1-4. Web. 26 Feb. 2012. <128.138.136.5/jimenez/Papers/2006GL026899_published.pdf>. 8 Perraud, V., E.A. Bruns, M.J. Ezell, S.N. Johnson, Y. Yu, L. Alexander, A. Zelenyuk, D. Imre, W.L. Chang, D. Dabdub, J.F. Pankow, and B.J. Finlayson-Pitts. "Nonequilibrium Atmospheric Secondary Organic Aerosol Formation and Growth." PNAS, 109.8 (2011): 2836-841. Web. 26 Feb. 2012. <www.pnas.org/content/109/8/2836.abstract>.

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reevaluate its models and policies related to SOAs.9 This paper will provide a background of the

science behind SOA formation, examine the accuracy of the current models, and discuss the

current policies that affect SOA concentrations. Based on this information, this paper provides

scientific recommendations and examines both a voluntary and a mandatory policy approach.

This paper presents the final recommendation of immediately implementing a voluntary policy

approach with a focus on reducing SOA concentrations while scientists work to improve models

that can eventually be used to guide a mandatory policy.

Background

What are secondary organic aerosols (SOAs) and how do they form?

Secondary organic aerosols (SOAs) are suspensions of carbon-containing particles in air

that are formed by chemical reactions in the atmosphere. Because SOAs are formed in the

atmosphere and not emitted directly, it is difficult to regulate their concentrations. Furthermore,

it is difficult and expensive to measure the concentrations of SOAs, so SOA concentrations in

most areas are predicted by models instead of being measured directly. These models use

emission rates for SOA precursors to predict atmospheric SOA concentrations. Currently, there

are 58 different SOA precursors that are used in models, but many more SOA precursors remain

unknown.10 Most precursors can undergo more than one set of chemical reactions in order to

form more than one type of SOA molecule.11 Thus, the actual number of different SOA

                                                                                                               9 Barringer, F. "Scientists Find New Dangers in Tiny but Pervasive Particles in Air Pollution." New York Times. 18 Feb. 2012. Web. 24 Feb. 2012. <http://www.nytimes.com/2012/02/19/science/earth/scientists-find-new-dangers-in-tiny-but-pervasive-particles-in-air-pollution.html>. 10 Robinson, A. L., N. M. Donahue, M. K. Shrivastava, E. A. Weitkamp, A. M. Sage, A. P. Grieshop, T. E. Lane, J. R. Pierce, and S. N. Pandis. "Rethinking Organic Aerosols: Semivolatile Emissions and Photochemical Aging." Science 315.5816 (2007): 1259-262. Web. 20 Apr. 2012. <http://www.sciencemag.org/content/315/5816/1259.full#F2>. 11 Ibid.  

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molecules far exceeds 58, which further complicates the issue of accounting for and regulating

SOAs.12

All precursors for SOA formation are volatile or semi-volatile organic molecules.

Volatility refers to the ability of a molecule to enter into the gas phase. Thus, the precursors for

SOA formation are molecules that can enter into the gas phase. The word “organic” in

secondary organic aerosols means that the molecule contains carbon atoms. Molecules that form

the basic structures of living beings contain carbon. Because of their similarity to molecules

within living beings, pollutants that contain carbon frequently are associated with negative health

consequences. In the case of SOA precursors, molecules generally contain a carbon ring or a

carbon chain. Three well-characterized SOA precursors are toluene (Fig. 1a), α-pinene (Fig. 1b),

and isoprene (Fig. 1c).

Figure 1. Well-characterized precursors to SOAs: Toluene (a), α-pinene (b), Isoprene (c).

The exact mechanism of SOA formation depends on the specific precursor, but there are

general characteristics that occur in almost all pathways (shown in Figure 2). The first step in

SOA formation is light-dependent oxidation of the SOA precursors. Oxidation is a loss of

electrons from the core of a molecule generally due to loss of hydrogen atoms or addition of an

oxygen atom or hydroxyl (OH) group. In the case of SOA formation, oxidation generally is

facilitated by OH, NO3, NO2, or O3. It is light-dependent because the reaction requires energy

                                                                                                               12 Ibid.

a)   b)   c)  

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from the sun in order to proceed. After the oxidation step, the “first generation products” are

created. These products depend on the specific precursor and can either condense to become part

of an SOA or they can undergo additional chemical reactions. If the products undergo additional

chemical reactions then they are called “second generation products.” Those second generation

products then condense to become part of an SOA. A product can condense on a floating

particle in the air in order to start the formation of an SOA or a product can condense on an

already formed SOA and make it larger. (Multiple first generation and/or second generation

products can also condense together to form an SOA molecule, but this mechanism is not well

understood.) The diameter of the SOA depends partly on the specific SOA present and partly on

the amount of time since the particle has been formed.13 When SOAs are formed from α-pinene

precursors, the diameters of the particles generally range from 120 nm to 200 nm.14

Figure 2. Overview of pathways common in SOA formation.

Floating particles that can serve as a condensation site for SOAs are called seed aerosols.

There are many different types of seed aerosols, which complicates the issue of accounting for

                                                                                                               13  Vaden, T.D., D. Imre, J. Beranek, M. Shrivastava, A. Zelenyuk. “Evaporation kinetics and phase of laboratory and ambient secondary organic aerosol.” Proc Natl Acad Sci, 108 (2011): 2190–2195. Web. 20 Apr. 2012. < http://www.pnas.org/content/109/8/2836.full.pdf+html>.  14 Ibid.

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and regulating SOAs. Research regarding seed aerosols has been limited, but it is assumed that

most primary organic aerosols can serve as seed aerosols. Four seed aerosols that have been

studied in laboratory settings are ammonium sulfate, ammonium nitrate, sodium silicate, and

calcium chloride.15 The exact effect of seed aerosols on SOA formation depends on a variety of

factors including concentration of the seed aerosol, concentration of molecules used for SOA

formation, time since these molecules were released into the environment (or into the smog

chamber for controlled scientific research)16, and relative humidity.17 In spite of these different

factors that affect SOA formation, one trend that remains constant is that seed particles cause

SOAs to form much more quickly than they would in the absence of seed particles. (It is unclear

what causes SOAs to condense in the absence of seed particles, but in controlled smog chambers,

SOAs can be formed without seed particles.18) When components necessary for SOA formation

are put in a smog chamber with ammonium sulfate, ammonium nitrate, sodium silicate, or

calcium chloride at a concentration of 9000 pt/cm3, SOAs can be detected immediately. The

readings for concentrations of SOAs immediately after the components are added to the smog

chamber are 1000 particles/cm3 if calcium chloride is used as the seed aerosol or 500

particles/cm3 if one of the other three seed aerosols is used. In the absence of seed aerosols,

SOAs do not reach a level of 500 particles/cm3 until approximately 75 minutes after the

components are added to the smog chamber.19 However, by 125 minutes after the components

have been added to the smog chamber the smog chamber with no seed aerosols shows a greater

                                                                                                               15 Li-qing, H., W. Zhen-ya, H. Ming-qiang, F. Li, Z. Wei-jun. “Effects of seed aerosols on the growth of secondary organic aerosols from the photooxidation of toluene.” Journal of Environmental Science, 19 (2007): 704-708. Web. 12 May 2012. <http://ac.els-cdn.com/S100107420760117X/1-s2.0-S100107420760117X-main.pdf?_tid= 056a086512f32041c887d76705b06123&acdnat=1336856570_23be83a1917c215dae6642837106cabd>. 16 Ibid. 17 Cocker, D. R., S. L. Clegg, R. C. Flagan, J. H. Seinfeld. “The effect of water on gas-particle partitioning of secondary organic aerosol. Part I: α-pinene/ozone system.” Atmospheric Environment, 35 (2001): 6049-6072. Web. 12 May 2012. <www.engr.ucr.edu/~dcocker/JA9.pdf>. 18 Ling-qing, 2007. 19 Ibid.

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concentration of SOAs than all other smog chambers except the smog chamber with calcium

chloride. A possible explanation for this finding is that it takes longer for SOAs to condense

without seed particles, but SOAs that form without seed particles form closer interactions with

each other and are less likely to evaporate back into the gas phase than SOAs that form on seed

particles. The exception is calcium chloride where the presence of calcium chloride ultimately

results in more SOA formation than if seed particles are not present. An explanation for this

finding is that calcium chloride interacts with SOAs more strongly than SOA molecules interact

with each other. Thus, when calcium chloride is used as a seed aerosol, particles are less likely

to evaporate than when there is no seed aerosol. Because ammonium sulfate, ammonium nitrate,

sodium silicate, and calcium chloride are only a small fraction of the actual seed aerosols present

and because seed aerosols are impacted by many factors (mentioned earlier), there currently is

not enough information to incorporate seed aerosols into models for SOAs.

Another complexity involved in modeling SOA concentrations is that 58 precursors to

SOA are currently used in models, but there are many more that are unknown and not included in

models.20 In addition, for the precursors that are known, the process of SOA formation is

complex and many of the first generation and second generation products remain unknown.

Toluene is one of the most well studied SOA precursors, and even for toluene, many of its

second generation products remain unknown. Figure 3 shows known first generation products

and helps demonstrate the complexity of these reactions. Each first generation product can

undergo numerous additional reactions, so deducing all of the second generation products would

be incredibly complex.

                                                                                                               20 Robinson, 2007.

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Figure 3. First generation products from toluene in the formation of SOAs.21

It is not feasible to examine and regulate emissions of all 58 SOA precursors, so this

paper will focus on three precursors: toluene, α-pinene, and isoprene. These three precursors are

well studied and are considered to be major precursors contributing to SOA formation.22 In field

studies in Maryland and North Carolina conducted by the EPA, SOAs from toluene, α-pinene,

and isoprene were identified in significant concentrations in the ambient air.23

                                                                                                               21 Kamens, R. M. "Final Report: Secondary Aerosol Formation from Gas and Particle Phase Reactions of Aromatic Hydrocarbons." EPA, 31 Dec. 2007. Web. 22 Apr. 2012. <http://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.abstractDetail/abstract/6612/report/F>. 22 Edney, E.O. "What Are the Precursors and Formation Processes for Secondary Organic Aerosol?" EPA, 2011. Web. 3 Mar. 2012. <http://www.epa.gov/nheerl/ download_files/posters/S2_011_Edney_PM_BOSC05.pdf>. 23 Ibid.  

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What are major sources of SOA precursors and how are they regulated?

Toluene:

Petroleum fractions are the source of 99.5% of the toluene released in the United States,

and 87% of this toluene comes from the process of catalytic reforming.24 Approximately 70% of

petroleum is ultimately used for transportation.25 Petroleum is also used in heating and in the

creation of some types of plastic, rubber, or other materials such as surgical equipment,

pesticides, and toothpaste.26 Catalytic reforming, the process most responsible for toluene

formation, occurs in the petroleum refining process after the petroleum has been distilled and

desulfurized. In the process of catalytic reforming, the carbon-based chains of petroleum

molecules are restructured to create more complex shapes that are better suited for fuel. This

process increases the octane rating, but is also responsible for the production of byproducts such

as toluene, methane, ethane, propane, and butane.

Toluene is currently regulated by the EPA under the Toxic Substances Control Act, Clean

Air Act, Comprehensive Environmental Response, Compensation, and Liability Act

(Superfund), Clean Water Act, and Safe Drinking Water Act.27 The act that is most relevant to

SOA formation is the Clean Air Act. The rationale behind toluene regulation by the EPA is that

toluene (independent of SOA formation) is associated with negative health consequences.

Breathing large amounts of toluene has been shown to affect the nervous system, kidneys, liver,

and heart.28 Furthermore, the continued inhalation of toluene can result in brain damage.29 In

                                                                                                               24 TRC Environmental Corporation. "Locating and Estimating Air Emissions from Sources of Toluene." Environmental Protection Agency, Sept. 1993. Web. 5 Apr. 2012. <http://www.epa.gov/ttnchie1/le/toluene.pdf>. 25 "How the US Uses Oil." Pro Con, 3 Aug. 2009. Web. 22 Apr. 2012. <http://alternativeenergy.procon.org/view.resource.php?resourceID=001797>. 26  Ibid.  27 Office of Pollution Prevention and Toxics. "Chemicals in the Environment: Toluene." Environmental Protection Agency, Aug. 1994. Web. 05 Mar. 2012. <http://www.epa.gov/chemfact/f_toluen.txt>. 28 Ibid. 29 Ibid.  

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the case of a pregnancy, toluene can harm the fetus.30 Because of the shortage of data available,

the negative health effects from SOAs derived from toluene remain unknown, and the role of

toluene in SOA formation is not considered in any EPA regulations.

α-pinene:

The major anthropogenic source of α-pinene is turpentine. Turpentine is an oil that is

separated from the wood chips of pine trees. It is used in the pharmaceutical industry, in the

production of oils such as perfumes, and as a disinfectant.31 Because α-pinene is volatile,

significant amounts of α-pinene are released into the atmosphere during processing of turpentine.

α-pinene is also released naturally from leaves, but there has been little research about the

types of plants and trees that release α-pinene and the total amount of α-pinene released from

natural biological emissions is unknown. One known natural source of α-pinene is the

Mediterranean holm oak, Quercus ilex L.32 The Mediterranean holm oak is found mainly in the

Mediterranean basin of southern Europe, western Asia, and northern Africa, so it is not a major

source of α-pinene in the United States.

α-pinene is not currently regulated in the United States. Even though α-pinene is not

regulated in the United States, it is one of the most common precursors used in experiments

involving SOAs that are conducted or funded by the EPA.33 Thus, the EPA is aware of α-pinene

and its role in SOA formation even though the EPA is not involved in its regulation.

                                                                                                               30 Ibid. 31 "Turpentine Production and Processing." New Zealand Institute of Chemistry. Web. 22 Apr. 2012. <http://nzic.org.nz/ChemProcesses/forestry/4F.pdf>.  32 Loreto, F., P. Ciccioli, E. Brancaleoni, M. Frattoni, S. Defline. “Incomplete 13C labeling of α-pinene content of Quercus ilex leaves and appearance of unlabeled C in α-pinene emission in the dark.” Plant, Cell and Environment, 23 (2000): 229-234. Web. 12 May 2012. <http://onlinelibrary.wiley.com/store/10.1046/j.1365-3040.2000.00536.x /asset/j.1365-3040.2000.00536.x.pdf;jsessionid=B866C466FFBBF4860B71C4063EE98332.d04t01?v=1&t=h2599 xra&s=3e55f7a4d3511f95f828a00eeaf539b00fbdb9d5>. 33 Edney, E. O. "What Are the Precursors and Formation Processes for Secondary Organic Aerosol?" EPA, 2011. Web. 3 Mar. 2012. <http://www.epa.gov/nheerl/download_files/posters/S2_011_Edney_PM_BOSC05.pdf>.

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Isoprene

In the United States, the major source of isoprene is believed to be natural processes in

forests. Isoprene is formed in the chloroplasts of leaves (especially willows) and is also formed

to a lesser extent by bacteria.34 Terrestrial sources account for the release of 500 million tons of

isoprene into the environment each year, which is similar in quantity to methane emissions.35

The specific reason for isoprene production and emission from vegetation is currently under

debate, but a leading theory is that isoprene protects the leaves of the plant from thermal stress.36

Isoprene has recently been the subject of much research funded by the EPA due to its role in

ground level ozone production. Isoprene has also been studied to a lesser extent for its role in

SOA formation.37

In the United States, there are currently no major recognized anthropogenic sources of

isoprene. However, a study conducted by the United Kingdom National Centre for Atmospheric

Science showed that in equatorial regions, oil palm plantations release more isoprene than

rainforests.38 Palm oil is from the fruit of the oil palm tree, and it is used in cooking. Because

oil palm trees grow in equatorial climates and are not present in the United States, the EPA does

not regulate anthropogenic isoprene. However, there are US firms that build oil palm plantations

in other countries. In 2011, Herakles Farms, an agricultural company from New York,

                                                                                                               34 Fall, R., and S. D. Copley. "Bacterial Sources and Sinks of Isoprene, a Reactive Atmospheric Hydrocarbon." Environmental Microbiology, 2.2 (2000): 123-30. Web. 22 Apr. 2012. <http://onlinelibrary.wiley.com/store/10.1046/j.1462-2920.2000.00095.x/asset/j.1462-2920.2000.00095.x.pdf?v=1&t=h1dhxgsu&s=69513253369b2bce5195eb56ce99840d5dcab215>. 35 Ibid.  36 "The Role Of Isoprenes In Protecting Leaves From High Ambient Temperature." ScienceDaily, 26 July 2007. Web. 22 Apr. 2012. <http://www.sciencedaily.com/releases/2007/07/070726104845.htm>. 37  Edney, 2012.  38 Warkwick, N., R. Pike, G. Carver, J. Pyle, and A. Archibald. "Land Use Change and Isoprene Chemistry." National Centre for Atmospheric Science, 2010. Web. 22 Apr. 2012. <http://climate.ncas.ac.uk/researchhighlights2010/217-land-use-change-and-isoprene-chemistry>.

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announced plans to build a palm oil plantation in Cameroon.39 This project has been met with

considerable criticism from environmentalists because palm oil trees store less carbon than the

rainforest that they will replace.40 In addition, this palm oil plantation will result in isoprene

emissions that will increase ground level ozone and SOA concentrations in the region.

What factors increase the rate of SOA formation?

NOx and SOx are not officially studied as precursors to SOAs because they are small

molecules and are not the major source of the atoms ultimately found in SOAs. Nevertheless,

NOx and SOx are involved in the SOA formation process. Edney (2011) showed that in a smog

chamber (which was made to simulate environmental conditions) a mixture of isoprene, NOx,

and light resulted in little SOA formation, but when SO2 was added to the smog chamber, large

amounts of SOAs were formed.41 This finding shows that SO2 has a major role in increasing the

rate of SOA formation. However, the specific mechanism by which SO2 is involved in SOA

formation is currently unknown.

The relationship between NOx and SOA formation is slightly more complicated than the

relationship between SO2 and SOA formation. The effect of NOx on SOA formation depends on

the form that NOx takes in the atmosphere. If NOx is in the form of NO (as it is in molecules like

HONO and CH3ONO) then the SOA precursors generally undergo a chemical reaction that

results in highly volatile products.42 These highly volatile products do not readily condense on

                                                                                                               39 Butler, R., and J. Hance. "A Huge Oil Palm Plantation Puts African Rainforest at Risk." Yale University, 12 Sept. 2011. Web. 22 Apr. 2012. <http://e360.yale.edu/feature/huge_oil_palm_plantation_puts_africa_rainforest_at_risk/2441/>. 40 Ibid. 41 Edney, 2011. 42 Chan, A., M. Chan, J. Surratt, P. Chhabra, C. Loza, J. Crounse, L. Yee, R. Flagan, P. Wennberg, J. Seinfeld. "Role of Aldehyde Chemistry and NOx Concentrations in Secondary Organic Aerosol Formation." Atmospheric Chemistry and Physics, 10 (2010): 7169-188. Web. 22 Apr. 2012. <http://www.atmos-chem-phys.net/10/7169/2010/acp-10-7169-2010.pdf>.

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particles to form SOAs.43 An example of this mechanism for an aldehyde is shown in Figure 4.

However, if NOx is in the form of NO2 then the SOA precursors undergo a different set of

chemical reactions that produce less volatile products that do condense to form SOAs.44 Thus,

the ratio of NO2/NO can be used to analyze the effect of NOx on SOA formation. The reason for

these different mechanisms is that after the oxidation step in SOA formation, the resulting

compound frequently contains an additional oxygen. NO is less stable than NO2. Thus, if there

is a high concentration of NO in the atmosphere then it will extract an oxygen from the

secondary organic aerosol precursor in order to form NO2. When the SOA precursor loses the

oxygen its volatility generally increases. It then undergoes a different set of reactions than the

set of reactions that would lead to SOA formation.

When there is a high ratio of NO2 to NO, then NO2 instead of NO is more frequently

involved in the step after oxidation of an SOA precursor. NO2 is a polar molecule where

nitrogen is slightly positively charged and each of the oxygens is slightly negatively charged.

After the SOA precursor is oxidized, the positively charged nitrogen on NO2 is attracted to the

negatively charged oxygen on the SOA precursor, and the NO2 adds to the molecule. Through a

series of subsequent reactions involving other atmospheric gases such as O2, H2O, HNO3, or NO

this product is converted to an SOA (details shown in Fig. 4).

                                                                                                               43 Ibid. 44 Ibid.  

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Figure 4. The effect of the NO2/NO ratio on SOA formation.45

The most significant implication of this research is that an increase in NO2 emissions

results in an increase in the NO2/NO ratio, which results in an increase in SOA formation. Thus,

in order to limit SOA formation, one possible approach is to reduce NO2 emissions. This would

result in a reduction in the NO2/NO ratio, and would increase the likelihood that oxidized SOA

precursors undergo the set of reactions facilitated by NO that result in volatile compounds than

the set of reactions facilitated by NO2 that result in SOAs.

What are sources of NO2 and SO2?

NO2 has both natural and anthropogenic sources. Natural sources include volcanic

activity, forest fires, and lightning.46 Anthropogenic sources are generally from fossil fuel

                                                                                                               45 Ibid. 46 Hesterberg, T. W., W. B. Bunn, R. O. McClellan, A. K. Hamade, C. M. Long, and P. A. Valberg. "Critical Review of the Human Data on Short-term Nitrogen Dioxide (NO2) Exposures: Evidence for NO2 No-effect

14

 

combustion largely for heating and transportation uses. On a smaller scale, a source of NO2 is

gas used for cooking.47

Almost all sources SO2 are anthropogenic. The largest source of SO2, making up 73% of

all SO2 emissions, is the combustion of fossil fuels at power plants.48 The second largest source

is industrial fossil fuel combustion comprising 20% of the total SO2 emissions.49 Minor sources

include extracting metal from ore and combustion of fuels used for locomotives and ships.50

How are NO2 and SO2 regulated?

NO2 and SO2 are both regulated by the Clean Air Act. The Clean Air Act was first

passed in 1963 and was strengthened in 1970 in conjunction with the creation of the EPA.51

Since 1970, the EPA has been involved in the enforcement and analysis of the Clean Air Act.

The Clean Air Act was created in response to health concerns.52 Two major events that made

these health concerns apparent were a fog in Donora, Pennsylvania that resulted in the death of

20 people and sickness of 6,000 people and the “Killer Fog” in London that resulted in the death

of 3,000 people.53 The Clean Air Act is regularly re-evaluated by the EPA to account for

scientific findings that link air pollutants to human health. Because recent research has shown

negative human health consequences resulting from exposure to SOAs, the Clean Air Act is a

relevant piece of legislation to re-evaluate. Originally, NO2 and SO2 were regulated by the Clean

                                                                                                                                                                                                                                                                                                                                                                     Levels." Crit Rev Toxicol., 39.9 (2009): 743-81. Web. 23 Apr. 2012. <http://www.ncbi.nlm.nih.gov/pubmed/19852560>. 47 Ibid. 48 "Sulfur Dioxide." Environmental Protection Agency. Web. 23 Apr. 2012. <http://www.epa.gov/air/sulfurdioxide/>. 49 Ibid. 50 Ibid.  51 "Plain English Guide to The Clean Air Act." EPA. Environmental Protection Agency. Web. 22 Apr. 2012. <http://www.epa.gov/air/peg/index.html>. 52 Ibid. 53 Ibid.  

15

 

Air Act because they are associated with negative cardiovascular and respiratory affects.54 The

Clean Air Act does not currently consider adverse health effects from SOAs when creating their

regulation standards.

The current regulation of NO2 under the Clean Air Act is that NO2 cannot be present at

an average concentration that exceeds 100 ppb for more than an hour and the average yearly NO2

concentration cannot exceed 0.053 ppm. The standard for SO2 emissions under the Clean Air

Act is that SO2 concentrations cannot exceed an average of 75 ppb during an hour period.55

Title IV of the 1990 Amendments to the Clean Air Act requires that the EPA create

regulations to limit acid rain deposition.56 In response to this title, the EPA created the Acid

Rain Program. Acid rain causes acidification of bodies of water resulting in damage to aquatic

ecosystems. Acid rain also damages sensitive forests, soils, building materials, paints, and

sculptures. Most significantly, SO2 and NOx gases, the reason for acid rain, are associated with

fine particulate matter and ground level ozone, which are public health threats.57 The Acid Rain

Program sets allowances for SO2 emissions and allowances can be bought, sold, or banked for

use in future years. The overall cap is currently at 8.95 million tons of SO2 per year.58 The

regulation of NOx under the Acid Rain Program is less strict than the regulation of SO2. NOx is

regulated through emission limits for individual boilers, but there is no overall cap for NOx

emissions.59 Overall, EPA programs aimed at reducing SO2 and NOx emissions have been

successful with decreases in ambient SO2 concentrations of approximately 60% from 1989 to

                                                                                                               54 "Regulatory Actions." Environmental Protection Agency, 22 Mar. 2012. Web. 06 Apr. 2012. <http://www.epa.gov/oaqps001/nitrogenoxides/actions.html>. 55 Ibid. 56 "Acid Rain Program." Environmental Protection Agency, 2 Mar. 2011. Web. 13 May 2012. <http://www.epa.gov/airmarkets/progsregs/arp/basic.html>. 57 Ibid. 58 Ibid. 59 Ibid.

16

 

2007 and decreases in ambient nitrate concentrations of approximately 35% during the same

time.60

Managing for SOA Concentrations: SOA Models

Measuring SOA concentrations in the atmosphere is a time-consuming process and is not

feasible on a large scale. SOA concentrations also cannot be explained based on emissions data

because they are formed in the atmosphere and not emitted directly. Thus, the most widely used

method to explain SOA concentrations in the atmosphere is modeling. The model used by the

EPA is the CMAQ Aerosol Module. This module is constantly being re-evaluated in light of

experimental findings and currently exists as version 4.7.61 This model accounts for SOAs

formed from seven major classes of molecules: long alkanes, high-yield aromatics, low-yield

aromatics, benzene, monoterpene, sesquiterpenes, and isoprene.62 However, one major weakness

of the model is that it relies on broad generalizations about the behavior of molecules in those

classes. Another major weakness is that it assumes that the molecules exhibit unhindered

equilibrium behavior. The implications of this assumption will be explained in more detail in the

next section.

Based on experimental evidence from field studies, it is clear that current models do not

accurately predict atmospheric SOA concentrations. Volkamer et al. (2006) used models to

predict SOA concentrations and then used field measurements to compare the predicted SOA

concentrations to the actual SOA concentrations.63 They calculated the ratio of measured SOA

                                                                                                               60 "2009 Environmental Results." Environmental Protection Agency, 10 Feb. 2011. Web. 13 May 2012. <http://www.epa.gov/airmarkets/progress/ARP09_3.html>.  61 "CMAQ Aerosol Module." Environmental Protection Agency. Web. 23 Apr. 2012. <http://www.epa.gov/AMD/ModelDevelopment/aerosolModule.html>. 62 Ibid. 63 Volkamer, Rainer, Jose Jimenez, Federico Martini, Katja Dzepina, Qi Zhang, Dara Salcedo, Luisa Molina, Douglas Worsnop, and Mario Molina. "Secondary Organic Aerosol Formation from Anthropogenic Air Pollution:

17

 

concentration to modeled SOA concentration for sampling sites in Mexico, the United States, the

United Kingdom, and three locations in Asia.64 These ratios ranged from 4 to 100, which shows

that at best, the model predicted concentrations that were four times lower than measured and at

worst, the model predicted concentrations that were one hundred times lower than measured.65

This result and the results of similar experiments led researchers to analyze SOA models more

closely in order to identify the major sources of error.

Sources of Uncertainty in SOA Models

Currently, SOA models focus on precursors that are highly volatile. Volatility is a

measure of how likely a chemical is to enter the gas phase, and volatility is typically measured in

µg m-3 (a mass-per-volume measurement).66 If a compound has a volatility of 100,000 µg m-3 or

greater than it is considered highly volatile. In reality, there are many organic compounds that

are found in the atmosphere at significant levels but have volatilities less than 100,000 µg m-3.67

These compounds are called intermediate-volatility organic compounds (IVOCs) or semivolatile

organic compounds (SVOCs) depending on their volatility. IVOCs have a volatility of between

1,000 µg m-3 and 100,000 µg m-3 while SVOCs have a volatility between 0.1 µg m-3 and 1,000

µg m-3.68 A major source of IVOCs and SVOCs is diesel fuel. To experimentally analyze the

effect of IVOCs and SVOCs on SOA formation, Robinson et al. (2007) burned diesel fuel in a

chamber with UV light. The UV light was used to simulate environmental conditions and was

                                                                                                                                                                                                                                                                                                                                                                     Rapid and Higher than Expected." Geophysical Research Letters, 15 May 2006. Web. 26 Feb. 2012. <128.138.136.5/jimenez/Papers/2006GL026899_published.pdf>. 64 Ibid. 65 Ibid.  66 Robinson, A. L., N. M. Donahue, M. K. Shrivastava, E. A. Weitkamp, A. M. Sage, A. P. Grieshop, T. E. Lane, J. R. Pierce, and S. N. Pandis. "Rethinking Organic Aerosols: Semivolatile Emissions and Photochemical Aging." Science, 315.5816 (2007): 1259-262. Web. 20 Apr. 2012. <http://www.sciencemag.org/content/315/5816/1259.full#F2>. 67 Ibid. 68 Ibid.  

18

 

also critical for this experiment because the formation of SOAs is mainly through photochemical

oxidation, which requires UV light. Robinson et al. calculated the amount SOAs that would be

expected based on current models and measured the amount of SOA formation in their chamber

using an aerosol mass spectrometer. Their results show that the amount of SOAs predicted by

the model is less than ¼ of the actual mass of SOAs formed in the chamber (Fig. 5). Because

diesel is high in IVOCs and SVOCs, Robinson et al. (2007) believe that the difference between

the predicted amount and observed amount of SOAs is largely because IVOCs and SVOCs are

underrepresented in SOA models.

Figure 5. Contributions of primary aerosols (grey), SOAs predicted by models (red), and SOAs that are unexplained by models (blue) to total aerosol mass in a chamber with UV light and compounds from combusted diesel.69 Another major error that has recently been discovered in the current models is that the

formation of SOAs from nitrates is a relatively irreversible process and is not consistent with an

equilibrium between gas and liquid nitrates as was previously assumed. In general, if particles of

a volatile or semi-volatile compound are added to a chamber, some of the particles will enter into

                                                                                                               69 Ibid.

19

 

gas form. Eventually the system will reach an equilibrium, which means that the ratio between

the amount of compound in the gas and particle phases is a constant. This constant (Kp,i) is

expressed by Raoult’s Law (equation 1) where Kp,i is the equilibrium coefficient, Fi is the

concentration of the specific compound in the particle phase, Ai is the concentration of the

specific compound in the gas phase, and M is the concentration of particulate material.70 All of

these concentration are in the units of µg m-3, so Kp,I is in the units of m3 µg-1.71

Thus, Kp,i gives

the amount of air occupied by 1 µg of the compound.

Kp,i= (Fi/M) [1] Ai

However, in order for this equilibrium equation to be true, the compound must be able to

freely change phases. The equilibrium between a particle and a gas is called a dynamic

equilibrium because particles instantaneously change phase largely based on small fluctuations

in their immediate environment. Nevertheless, the system remains in equilibrium because each

phase change in one direction is compensated by a phase change in the opposite direction. For

example, if one molecule were to change from the particle to gas phase, then another molecule

would change from the gas to the particle phase. This fundamental assumption that the

molecules can freely change phases is violated in the case of SOAs. Perraud et al. (2011)

conducted experiments with α-pinene (an SOA precursor) and six different concentrations of

NO2 and measured the amounts of intermediate and final products in SOA formation in order to

show that the equilibrium suggested by Raoult’s Law does not apply. These experiments were

conducted under normal atmospheric conditions and using liquid polyethylene glycol (PEG).

The samples in PEG were a control because the intermediate and final SOA products are known

to dissolve in PEG in the equilibrium described by Raoult’s Law. The results of these                                                                                                                70 Ibid. 71 Ibid.  

20

 

experiments showed that the relationship between Ai and Fi/M was linear in PEG as predicted by

equation 1 (Fig. 6). However, the relationship between Ai and Fi/M for SOAs was closer to

exponential than linear (Fig. 6). These findings show that the predicted equilibrium between the

particle and gas phase for nitrates in SOAs is incorrect.

Figure 6. The relationship between Fi/M and Ai is linear for SOA precursors in PEG as expected by Raoult’s Law, but the relationship between Fi/M and Ai for SOAs is not linear indicating that the predicted equilibrium between the particle and gas phases of nitrates in SOAs is incorrect.72 A reason that nitrates in SOAs do not follow the expected equilibrium is because of the

structure of an SOA molecule. Vaden et al. (2010) examined the role that primary organic

aerosols (POA) have in the structure formation of SOAs.73 SOAs are hydrophilic which means

they dissolve readily in water while POAs are hydrophobic which means that they clump in

water and do not dissolve. Based on solubility principles, substances with similar physical

properties dissolve while substances with different physical properties (such as a hydrophobic

and a hydrophilic substance) remain distinct. Vaden et al. (2010) mixed SOAs with hydrophobic

                                                                                                               72 Perraud et al., 2011. 73 Vaden, T. D., C. Song, R. A. Zaveri, D. Imre, and A. Zelenyuk. "Atmospheric Chemistry Special Feature: Morphology of Mixed Primary and Secondary Organic Particles and the Adsorption of Spectator Organic Gases during Aerosol Formation." Proceedings of the National Academy of Sciences, 107.15 (2010): 6658-663. Web. 22 Apr. 2012. <http://www.pnas.org/content/early/2010/02/22/0911206107.full.pdf+html>.

21

 

dioctyl phthalate (DOP) (a molecule with similar properties to POAs) and analyzed the resulting

mixture using a single-particle mass spectrometer. A single-particle mass spectrometer uses a

laser to ionize and split a molecule. Based on the fragmentation pattern and masses of the

fragments, the identity of the molecule can be determined. Vaden et al. (2010) used a strong

laser to break the entire molecule into fragments to show that one molecule contains both POAs

and SOAs. Vaden et al. (2010) then used a weak laser so that only the surface components of

the molecule would be analyzed. The results of these experiments showed that the surface of a

given molecule would have either DOP or SOAs but would not have a mixture of the two. These

results suggest that SOAs are present in a three dimensional structure with other SOAs and with

hydrophic atmospheric molecules. This structure contains layers of SOAs alternating with layers

of hydrophobic molecules. Once this three dimensional structure forms, it is relatively stable.

This stability is believed to be the reason why NO2 did not exhibit the expected equilibrium

behavior in the experiments by Perraud et al. (2011). Once NO2 was incorporated into a three-

dimensional structure containing SOAs it was attracted to hydrophilic SOA layer and repelled

from the hydrophobic layers. The forces of attraction within the SOA layer and the shielding

from the atmosphere provided by outer layers in the three-dimensional structure likely prevented

NO2 from freely returning to gas phase.

Because Raoult’s Law has been used in SOA models but does not accurately describe the

formation of SOAs in the environment, those models grossly underestimate actual SOA

concentrations. Current SOA models assume that SOA molecules form and dissociate in an

unhindered dynamic equilibrium. In reality, once SOAs form, they are generally incorporated

into larger three-dimensional structures and are subjected to hydrophilic attractions and

hydrophobic shielding. These forces strengthen the three-dimensional structure and make

22

 

evaporation back into the gas phase less favorable. The nonequilibrium behavior of the

formation of SOAs from NO2 and α-pinene is likely also present in the formation of SOAs from

other precursors. The finding that the formation of SOAs does not follow traditional equilibrium

behavior was the major reason why the EPA chose to reevaluate their SOA models.74

Possible Policy Approaches

The EPA employs both voluntary and mandatory policy approaches. Voluntary policies

approaches are typically less challenging to create and implement than mandatory policies. Two

major and successful voluntary policies implemented by the EPA were the 33/50 Program and

voluntary Pollution Prevention (P2) Programs. The 33/50 Program was initiated in 1991 and

focused on reducing the release and transfer of 17 priority toxic chemicals by 33% (from 1988

levels) by 1992 and by 50% by 1995.75 This program was highly successful and levels were

reduced by 50% by 1994.76 This project was believed to be successful because the EPA targeted

the 600 companies with the greatest amounts of emissions of the 17 priority toxic chemicals and

focused on forming a positive relationship with these companies.77 The EPA publicly

recognized companies for their participation in the program and created a logo that could be used

by participating companies. This logo made consumers more aware of the participation of

specific companies in the 33/50 Program and thus led to economic benefits for participating

                                                                                                               74 Barringer, F. "Scientists Find New Dangers in Tiny but Pervasive Particles in Air Pollution." New York Times. 18 Feb. 2012. Web. 24 Feb. 2012. <http://www.nytimes.com/2012/02/19/science/earth/scientists-find-new-dangers-in-tiny-but-pervasive-particles-in-air-pollution.html>. 75 "Fact Sheet On EPA'S 33/50 Program." Environmental Protection Agency, Aug. 1999. Web. 14 May 2012. <http://infohouse.p2ric.org/ref/10/09952.htm>. 76 Zatz, M., and S. Harbour. “The United States Environmental Protection Agency's 33/50 Program: the anatomy of a successful voluntary pollution reduction program.” Journal of Cleaner Production, 7.1 (1999): 17-26. Web. 14 May 2012. <http://www.sciencedirect.com/science/article/pii/S0959652698000389>. 77 "33/50 Program: The Final Record." Environmental Protection Agency. Web. 14 May 2012. <http://www.epa.gov/opptintr/3350/33fin04.htm>.

23

 

companies. In addition, participating companies generally made changes that had an initial

economic cost but had long-term economic benefits. For example, Westvaco, a paper-

manufacturing company, installed new cleaning equipment at a cost of 1.7 million dollars in

order to reduce emissions of methyl ethyl ketone (a toxic chemical).78 Despite the high initial

cost, this equipment is ultimately expected to have economic benefits because it results in a

yearly savings of 90 thousand dollars.79 Reviews of the 33/50 Program suggest that the return on

investment (ROI) for participating companies was still negative on average as of 1999, but it is

expected that as time passes the ROI will ultimately become positive.80 No estimates were

provided by the study for the amount of time necessary before the ROI becomes positive.81

The 33/50 Program served as a motivation for many of the voluntary P2 Programs

initiated by the EPA. Currently, the EPA is in the process of implementing its 2010-2014

Pollution Prevention (P2) Program Strategic Plan. This plan is based largely on the use of grants

and technical assistance in order to help companies reduce pollution especially for greenhouse

gases and pollutants that are harmful to human health.82 Because SOAs are a human health

concern, this issue could be addressed by P2 programs. Recently, voluntary P2 programs were

analyzed by the economists Linda Bui and Samuel Kapon.83 These economists concluded that

matching grant programs and technical assistance programs were effective at lowering releases

                                                                                                               78 Zatz and Harbour, 1999. 79 Ibid.  80 Khanna, M., and L. A. Damon. "EPA's Voluntary 33/50 Program: Impact on Toxic Releases and Economic Performance of Firms." Journal of Environmental Economics and Management, 37 (1999): 1-25. Web. 5 Apr. 2012. <http://www.sciencedirect.com/science/article/pii/ S0095069698910579>. 81 Ibid. 82 “U.S. Environmental Protection Agency 2010-2014 Pollution Prevention (P2) Program Strategic Plan.” Environmental Protection Agency, Feb. 2010. Web. 14 May 2012. <http://www.epa.gov/p2/pubs/docs/P2StrategicPlan2010-14.pdf>. 83 Bui, L., and S. Kapon. “The impact of voluntary programs on polluting behavior: Evidence from pollution prevention programs and toxic releases.” Journal of Environmental Economics and Management, Feb. 2012. Web. 14 May 2012. < http://www.sciencedirect.com/science/article/pii/S0095069612000034>.

24

 

of toxic chemicals but educational outreach programs and grants that did not require company

investment were less effective.84

Atmospheric SOA concentrations could be decreased by emphasizing a reduction of NO2

and SO2 emissions in the next P2 Program Strategic Plan. Because of the success of matching

grant and technical assistance programs, these programs should be applied in the future. A

benefit to using a voluntary program to control SOA concentrations is that it could be

implemented relatively quickly (possibly in 2015 if another P2 Program Strategic Plan is

implemented at that time). Another benefit to using a voluntary approach is that a voluntary

approach can be implemented based on the correlation between SOA formation and NO2 and

SO2 emissions, and it does not require concrete quantitative limits for these emissions. Thus,

scientists could continue to improve SOA models in order to deduce appropriate quantitative

limits while a voluntary policy is implemented.

Mandatory regulation of SOAs could be accomplished through revisions or amendments

to the Clean Air Act. The National Ambient Air Quality Standards (NAAQS) were created as

outlined by the Clean Air Act to control pollutants that are harmful to human health and the

environment and currently regulate SO2 and NO2 emissions. One way that mandatory policy

changes could be created is through changes in the limits of SO2 and NO2 emissions in the

NAAQS. These standards can be revised without creating a formal amendment to the Clean Air

Act, and each standard is evaluated at least once every five years.85 NAAQS for NO2 and SO2

were most recently evaluated from January 2010 through March 2012.86 Thus, these emission

                                                                                                               84 Ibid. 85 Scavo, K. “Implementation of National Ambient Air Quality Standards.” EPA State and Local Programs Group. Web. 13 May 2012. <http://epa.gov/apti/video/pdfs/Scavo_Jones.pdf>. 86 “Fact Sheet Final Revisions to the Secondary National Ambient Air Quality Standards for Oxides of Nitrogen and Sulfur.” Environmental Protection Agency. Web. 13 May 2012. <http://www.epa.gov/air/nitrogenoxides/pdfs/20120320factsheet.pdf>.  

25

 

standards should be reviewed again in January 2015. Currently, it is known that increases in

NO2 and SO2 concentrations result in increases in SOA formation, but this effect has not been

quantified. In addition, appropriate limits for NO2 and SO2 concentrations are unknown. It

seems unlikely that models will improve quickly enough to provide these concrete limits by the

time of the 2015 evaluation of the NAAQS for NO2 and SO2, but models may be able to provide

these limits in time for the evaluation in 2020. Thus, a realistic goal for a mandatory policy

approach to limit SOA formation would be to consider SOAs in the 2020 evaluation of the

NAAQS for NO2 and SO2.

Another mandatory policy approach is to create an amendment to the Clean Air Act that

specifically considers SOAs. However, there are many barriers to creating an amendment to the

Clean Air Act. Amendments are created infrequently and the most recent set of amendments

were in 1990.87 Further, amendments must be approved by Congress and are generally brought

to Congress in groups. For example, the Clean Air Act Amendments of 1990 included several

major titles that ranged in scope from urban smog to air toxins to enforcement procedures.88 A

long-term goal for SOA regulation would be to include SOAs in the next set of Clean Air Act

Amendments, but it is unclear when this next set of amendments will be created.

Possible Approaches to Scientific Research

Recent scientific findings have clearly shown that current models of SOA formation are

incorrect and predict SOA concentrations that are 4 to 100 times less than actual SOA

                                                                                                               87 "The Clean Air Act Amendments of 1990." Environmental Protection Agency, 19 Dec. 2008. Web. 13 May 2012. <http://epa.gov/oar/caa/caaa_overview.html>. 88 Ibid.

26

 

concentrations.89 A considerable amount of research about SOA precursors is still needed before

reliable models can be created. This section describes research areas that are essential for the

creation of accurate models and that should be targets for EPA funding.

Robinson et al. (2007) concluded that IVOCs and SVOCs are underrepresented as SOA

precursors through their experiments with the controlled combustion of diesel fuel (refer to

“Sources of Uncertainty in SOA Models”). However, they did not identify specific IVOCs and

SVOCs that are involved in SOA formation. In order to create accurate models of SOAs, it is

important to identify specific IVOCs and SVOCs that are SOA precursors. A good future

experiment would be to release candidate IVOCs and SVOCs one at a time into a chamber that

has similar gas concentrations to the atmosphere (for example, 3.5 µg/m3 of SO290) and that

contains UV light (in order to allow for photochemical oxidation). For each IVOC or SVOC, the

concentrations of SOAs throughout a three hour time period should be measured using an aerosol

mass spectrometer. If the compound is a significant source of SOAs then the aerosol mass

spectrometer should show significant SOA formation during that three hour period.

Furthermore, by releasing a specific amount of a given compound and calculating the aerosol

mass produced, a quantitative relationship can be established between the amount of precursor

and the amount of SOA formed.

In order to improve the accuracy of current models, it will be critical to correctly model

the nonequilibrium behavior between the gas phase and the particle phase for first generation and

second generation products. This relationship will be complicated because it will likely depend

on many factors including the concentration of the precursor, the concentration of molecules that

affect the rate of SOA formation (such as NO2, and SO2), and the amount of sunlight. Even

                                                                                                               89 Volkamer et al., 2006. 90 “2009 Environmental Results." Environmental Protection Agency, 10 Feb. 2011. Web. 13 May 2012. <http://www.epa.gov/airmarkets/progress/ARP09_3.html>.  

27

 

though this relationship is complicated, an understanding of this nonequilibrium behavior is

necessary in order to create accurate models. In order to further examine this nonequilibrium

behavior, graphs of Fi/M vs. Ai should be constructed using major SOA precursors, similar to

that shown in Figure 6. Because it is likely that concentrations of NO2, concentrations of SO2,

and concentrations of individual precursors will all have affects on the relationship between Fi/M

and Ai three graphs should be constructed for each individual precursor. The graphs should

show Fi/M vs. Ai with NO2 concentration, SO2 concentration, or the precursor concentration as

the independent variable. Next, a mathematical equation should be fit to the curve on the graph.

This mathematical equation should be used in models for SOA concentrations instead of the

equilibrium equation proposed by Raoult’s Law.

One limitation that makes it challenging to make informed policy decisions about how to

regulate SOAs is that we do not know the natural relative contribution of each precursor to total

atmospheric SOA concentrations. In order to address this issue, each precursor should be put in

a chamber with light and the required additional molecules in order to form SOAs. Then single

particle mass spectrometry should be performed to create a mass spectrum for the SOAs formed

from that precursor. The mass spectrum of the SOAs formed from each precursor should be

saved, added to current libraries of spectra, and linked to that specific precursor. (A mass

spectrum library is the collection of spectra for known samples. The computer that processes

mass spectrum data can use this library to match an unknown spectrum to a known spectrum to

help identify the unknown.) This library would be a valuable tool for field studies. Once the

library is complete, SOA analysis should be carried out at many locations throughout the globe

and samples should be taken multiple times throughout the year to account for how fluctuations

in the local environment affect the types and distributions of SOAs. Each sample should be

28

 

analyzed using a single particle mass spectrometer and spectra should be matched to library

spectra to deduce the relative contributions of each known precursor to total atmospheric SOA

concentrations.

This experiment would also be a valuable way to ensure that our models account for all

of the major precursors. If a significant amount of mass spectra for SOAs do not match up with

a library spectrum, then it is clear that many of the precursors remain unknown. If a spectrum

seems common and does not match up with a known precursor then analytical methods such as

analysis of the specific mass distributions in the mass spectrum in conjunction with NMR and

FTIR should be used to try to identify the structure of the SOA. From this structure, the

precursor should be determined and verified through addition of the precursor to a smog chamber

with atmospheric gases and light. The SOA products from that chamber should be analyzed to

ensure that they are consistent with the mass spectrum for the unknown. A good goal is to have

a library that accounts for a minimum of 95% of spectra from field samples (based on a 95%

confidence level). The precursors used for all of the library spectra should then be incorporated

into current models, and the relationship between Fi/M and Ai should be calculated for each

precursor as described above instead of relying on Raoult’s Law.

Once this model is complete, there should be extensive field research at both rural and

urban locations around the globe to measure the concentrations of SOAs. The concentrations of

SOAs should also be modeled for these locations. After the research is complete, a graph should

be constructed that has sampling location on the x-axis and the ratio of the measured SOA

concentration to the modeled SOA concentration on the y-axis. If the model is a good

representation of environmental conditions, then the data points should be scattered about the

line y=1. Once scientists are confident in their model then it can be used as the foundation for

29

 

policy decisions. A possible way to establish confidence in a model is to continue working on

the model until the error bars for the ratio of measured SOA/modeled SOA include the value 1

for 95% of samples (based on a 95% confidence level), and the data points for measured

SOA/modeled SOA are randomly scattered about the line y=1.

Final Recommendations

Because there are hundreds of precursors to SOAs, regulating the emissions of precursors

would be an incredibly complicated task. A more feasible approach would be a focus on

decreasing NO2 and SO2 emissions since high concentrations of NO291 and SO2

92 have been

experimentally shown to promote rapid SOA formation. NO2 and SO2 are regulated by the Clean

Air Act, so a good final goal is to modify the Clean Air Act to consider the negative health

effects of SOAs. This can be accomplished by modifying the limits for NO2 and SO2 in the

NAAQS and ultimately through creating an amendment to the Clean Air Act that specifically

considers SOAs. Unfortunately, current models of SOA concentrations do not accurately predict

environmental conditions, and the exact quantitative effect of NO2 and SO2 on the rate of SOA

formation is unknown. Because of these uncertainties, appropriate limits for NO2 and SO2

emissions are currently unclear, and it is unrealistic to modify the Clean Air Act until there are

better models for SOA formation.

An immediate goal should be to increase funding for scientific research that is focused on

improving SOA models. While this research is in progress, another immediate goal should be to

set up a voluntary program that focuses on the reduction of NO2 and SO2 emissions in order to

reduce SOA concentrations. This voluntary program could be created through the EPA P2

                                                                                                               91 Chan et al., 2010. 92 Edney, 2011.  

30

 

Programs. A voluntary program is important because it will result in decreases in SOA

concentrations, which is important for human health, and it will spread awareness about the

dangers of SOAs. Once reliable models are created and policy-makers and the public become

more aware of the negative health affects associated with SOAs, there should be an emphasis on

making changes to the Clean Air Act. Consideration of SOAs by the Clean Air Act would

ensure that this important human health issue will not be overlooked as the types and

distributions of anthropogenic emissions continue to change in the future.

31

TOM HARDIE '78 MEMORIAL AWARDS

#40 May 2012 Cleaning the Air We Breathe: Science and Policy Recommendations to Reduce Concentrations of Secondary Organic Aerosols

Lauren Goldstein-Kral ‘12

#39 May 2011 Keeping It Local: Farm Viability in Berkshire and Bennington Counties

Rooney Charest ‘11

#38 May 2010 An Assessment of Historical PCB Contamination in Arctic Mammals

Benjamin S. Cohen ‘10

#37 May 2009 Exploring the Interrelationships Between Farmers and their Watershed: A Case Study in Human

Ecology from Río La Paz, Costa Rica

Avalon L. Gulley ‘09

#36 May 2008 The Lion and the Lamb: the Struggle Between Conservation and Pastoralism Tanzania

Nora A. Morse ‘08

#35 May 2008 What Drives the Green Car Market? The effects of state tax incentives on hybrid-electric vehicle sales

Whitney A. Leonard ‘08

#34 May 2007 Elites, Regimes, and Growth Machines: The Politics of Parks Development in Chicago and London

Andrew R. Stevenson ‘07

#33 May 2006 Possibilities for Sustainable Tourism in the Indian Himilaya

Katherine M. Majzoub ‘06

#32 May 2006 Community Supported Agriculture: A Model for Combating Distancing

Rachel L. Winch ‘06

#31 May 2005 Lead in the Soils of Pittsfield, Massachusetts: Chemical Analyses and Community Questions

Kathleen A. Carroll ‘05

#30

May 2004

Energy Yield and Visual Impact Studies of the Berlin Wind Project

Samuel M. Arons ‘04

#29

May 2003

Subsistence in Alaska: Balancing Competing Visions of the Land in Fish and Game Management

Judith B. Harvey ‘03

--- May 2002 ------------ None awarded ---------------- ----------------------------

#28 May 2001 Information Flows and the Impact of PCB Contamination on Property Values

Jessica L. Erickson ‘01

#27 May 2000 The Breakdown of Ecosystem Services: Urban Sprawl and Air Pollution in Santiago de Chile

Sarah E. Connolly ‘00

#26 May 1999 Alleys in the American Landscape Ellen Cook '00 32

#25 May 1998 Healing Knowledge and Cultural Practices in a

Modern Tswana Village Catherine Bolten '98

#24 May 1997 A Study of the Past and Present Ecology of the American Chestnut (Castanea dentata [Marsh.] Borkh.) in a Northern Hardwood Forest

Timothy J. Billo '97

#23 May 1997

Is Economic Growth Good for the Environment?

Darby W. Jack '97

#22 May 1997 Alternate Life Amy K. Smith '97

Alternate Life Amy K. Smith '97

#21 May 1996 Farms to Forest: A Naturalist's Guide (published 1995)

Dawn Biehler '97 Daniel I. Bolnick '96

Jonathan C. Cluett '96 Nathaniel G. Gerhart '96 Emilie B. Grossmann '96

J.D. Ho '96 Willard S. Morgan '96

#20 May 1995 A Morphological and Geochemical Comparison of a

Partially Reclaimed Open Mine Pit to a Similar but Undisturbed Site, Cooke City, Montana

Michael Montag '95

#19 May 1994 The International Whaling Commission: An Ineffectual Past, A Controversial Present, and an Uncertain Future

Elizabeth Haven Linen '94

#18 May 1993

Listen to the Mouse's Roar: Disney Architecture and Planning

Damon J. Hemmerdinger '93

#17 May 1992 Parque Y Pueblito: The Combined and Conflicting Interests of The Machalilla National Park and the Village of Casas Viejas

Scott A. Ringgold '92

#16 May 1992 Apartheid's Ecological Legacy and the Political Solutions for South Africa's Environment

Susan C. Donna '92

#15 May 1991 Wilderness Act of 1964 Peter C. Aengst '91

#14 May 1990 Altered Berkshire Landscapes

Tiffany Holmes '90

#13 May 1989 Wetlands Protection Bylaw and Map for Williamstown

James P. Power '90 Mary S. Richardson '91 James A. Simmonds '89

#13 May 1988 A Land-Use Plan for Williamstown, Massachusetts'

Stone Hill

Mary M. Taylor '88

#13 May 1988 Less Mess: Changing Our Ways With Waste Beth A. Stein '88

33

#12 May 1987 Causes of Environmental Degradation in China

Cheryl Lynn Hall '87

#11 May 1986 Williamstown Wetlands

Anne D. Southworth '86

#11 May 1986 Towards a Theory of Ecological Marxism

Nicholas W. Van Aelstyn '86

#10 May 1985 Nature: America's Tragic Heroine

Karla Miller '85

#10 May 1985 To Mine the Sun

L. Hart Hodges III '85

# 9 May 1984 The Shepherd's Well Site: A Site History As It Reflects and Differs From The Land Use History of the 8th Division in the Hopkins Memorial Forest

Julie Woodward '84

# 8 May 1983 Some Conflicting Demands on Energy and Food Production in the Third World

Cecilia Danks '83

# 7 May 1982 Upland Settlement in Williamstown: An Historical and Anthropological Survey

Deborah Gregg '82

# 6 May 1981 Two American Margins and the Path Between

Edward Christian Wolf '81

# 5 May 1980 A Proposal for a National Energy Plan

Students in Chemistry 14

# 4 May 1979 The Toxic Substances Control Act of 1976

Clifford S. Mitchell '79

# 3 May 1978 A Vegetational Survey of Mt. Greylock

A. Christine Reid '78

# 2 Fall 1976 (1977)

Interaction Robin Broad '76

# 1 Spring 1976

Portraits of a Community: Photographs of the Hopper Road

David Chapin Weeks '76

34