Atmospheric Chemistry And Risk Assessments Of Hazardous Air Pollutants

As discussed in Chapter 2.G, risk assessment and risk management are two separate elements of developing cost-effective control strategies for hazardous air pollutants, HAPs. The risk assessment portion ideally provides a complete understanding of the chemical and physical processes that apply to the HAP once it is emitted into the atmosphere. Thus, as we shall see in the examples that follow, some HAPs react in the atmosphere to form less toxic substances (sometimes referred to as "environmental deactivation"), whereas others form more toxic compounds ("environmental activation"). Understanding the atmospheric reactions of HAPs is clearly essential to developing appropriate health risk assessments upon which cost-effective control strategies can be based in the risk management phase.

In addition to the role of atmospheric reactions in the fate of airborne HAPs, atmospheric chemistry also plays a role in the formation of some of them, most notably formaldehyde and acetaldehyde. Thus, the potential formation of such compounds from the oxidation of precursors in the atmosphere must also be taken into account in their risk assessments.

In the United States, the Clean Air Act Amendments of f990 defined a list of 189 compounds or mixtures of compounds as "hazardous air pollutants," shown in Table 16.15 (Kao, 1994; Kelly et al., 1994). These include a range of chemicals, such as hydrocarbons, halogenated, oxygenated, and nitrogen- and sulfur-containing organics, pesticides, and inorganics. Although most are individual compounds, some such as "polycyclic organic matter" represent complex mixtures (see Chapter 10).

A detailed review of the sources, atmospheric chemistry, and fates of HAPs is beyond the scope of this book, instead, we concentrate here on several organic HAPs to illustrate the role of atmospheric chemistry in risk assessments. For other species, reviews and summaries are available in the literature (e.g., see California Air Resources Board (1997) for summaries of 243 species and Lin and Pehkonen (1999) for a review of the atmospheric chemistry of mercury).

The California Air Resources Board has prepared risk assessments for a number of toxic airborne compounds and mixtures, designated as "toxic air contaminants," TACs (Table 16.15). For example, risk assessments for individual compounds such as benzene, benzo[«]pyrene (see Chapter 10), formaldehyde, and vinyl chloride have been carried out, in addition to complex mixtures such as diesel exhaust (California Air Resources Board, 1997a) and environmental tobacco smoke (California Environmental Protection Agency, 1997). These risk assessment documents form the basis for controls imposed as part of the risk management process (e.g., see Seiber, 1996).

Kelly et al. (1994) and Kao (1994) have reviewed what is known about ambient concentrations of the HAPs and their reactions in air. Not surprisingly, given the number of compounds involved, the concentrations and atmospheric reactions and fates of many of these species are not known, precluding the development of accurate risk assessments. Also complicating such risk assessments are the wide variety of sources and their temporal and spatial variations, especially in urban areas (e.g., see Spicer et al., 1996; and Mukund et al., 1996). Some of the atmospheric fates of selected VOCs are treated in a review by Pitts (1993), in a series of papers by Grosjean (Grosjean, 1990a-c; f991a-d) and the f997 California Air Resources Board report. We examine here selected HAPs and related compounds,

TABLE 16.15 Compounds or Mixtures Designated as Hazardous Air Pollutants (HAPs) in the United States" and Toxic Air Contaminants (TACs)'' in the State of California'' '

Saturated hydrocarbons Hexane


Unsaturated hydrocarbons *1,3-Butadiene

Saturated halogenated hydrocarbons Bromoform *Carbon tetrachloride 'Chloroform l,2-Dibromo-3-chloropropane Ethyl chloride (chloroethane) 'Ethylene dibromide (1,2-dibromoethane) 'Ethylene dichloride (1,2-dichloroethane) Ethylidene dichloride (1,1-dichloroethane) Hexachloroethane Methyl bromide (bromomethane) Methyl chloride (chloromethane)

Aromatic hydrocarbons 'Benzene Biphenyl Catechol

Coke oven emissions




Polycyclic organic matter



Xylenes (isomers and mixture) o-Xylene m -Xylene p-Xylene

Halogenated aromatic hydrocarbons Benzotrichloride Benzyl chloride Chlorobenzene 1,4-Dichlorobenzene

I. Aliphatic and cyclic hydrocarbons

II. Aromatic compounds

Methylchloroform (1,1,1-trichloroethane) Methyl iodide (iodomethane) 'Methylene chloride (dichloromethane) Propylene dichloride (1,2-dichloropropane) 1,1,2,2-Tetrachloroethane 1,1,2-Trichloroethane

Unsaturated halogenated hydrocarbons Allyl chloride Chloroprene 1,3-Dichloropropene Hexachlorobutadiene Hexachlorocyclopentadiene 'Tetrachloroethylene (perchloroethylene) 'Trichloroethylene Vinyl bromide 'Vinyl chloride

Vinylidene chloride (1,1-dichloroethylene)

Hexachlorobenzene Polychlorinated biphenyls (aroclors)



Phenolic compounds

Cresols/cresylic acid (isomers and mixtures)

o-Cresol m-Cresol p-Cresol

Pentachlorophenol Phenol




Bis(2-ethylhexyl) phthalate (DEHP) Dibutyl phthalate Dimethyl phthalate Phthalic anhydride

Acetamide Acetonitrile





Aniline o-Anisidine






3.3-Dimethoxybenzidine (Dimethylamino)azobenzene 3,3'-Dimethylbenzidine Dimethylcarbamoyl chloride Dimethylformamide

1.1-Dimethylhydrazine 4,6-Dinitro-o-cresol and salts

2.4-Dinitrophenol 2,4-Dinitrotoluene

1.2-Diphenylhydrazine Ethyl carbamate (urethane) Ethylene imine (aziridine)

III. Nitrogenated organic compounds


Hexamethylene 1,6-diisocyanate



Methyl hydrazine

Methyl isocyanate


Methylene(diphenyl diisocyanate) (MDI)









Pentachloronitrobenzene (quintobenzene)


1,2-Propylene imine (2-methylaziridine)


2,4-Toluene diamine 2,4-Toluene diisocyanate o-Toluidine Triethylamine

TABLE 16.15 (continued)

Alcohols Methanol

Aldehydes *Acetaldehyde '"Formaldehyde Propionaldehyde a, [¡-Unsaturated carbonyls Acrolein

Carboxylic acids Acrylic acid Chloroacetic acid


Ethyl acrylate Methyl methacrylate Vinyl acetate


Bis(chloromethyl) ether Chloromethyl methyl ether Dibenzofurans

Dichloroethyl ether (bis(2-chloroethyl) ether) 1,4-Dioxane (1,4-diethylene oxide)

IV. Oxygenated organic compounds

Glycol ethers

Methyl fert-butyl ether (MTBE)


Methyl ethyl ketone (2-butanone) Methyl isobutyl ketone (hexone)


Epichlorohydrin (l-chloro-2,3-epoxypropane) 1,2-Epoxybutane ^Ethylene oxide Propylene oxide Styrene oxide

Other carbonyls and oxygenates Acetophenone Caprolactam 2-Chloroacetophenone Ethylene glycol Hydroquinone Isophorone Maleic anhydride Phosgene

1,3-Propane sultone



V. Pesticides and herbicides

Captan Heptachlor

Carbaryl Lindane (all isomers)

Chloramben Methoxychlor

Chlordane Parathion''

Chlorobenzilate Propoxur (baygon)

2,4-Dichlorophenoxyacetic acid Toxaphene (chlorinated camphene)

Dichlorodiphenyldichloroethylene (DDE) Trifluralin Dichlorvos

VI. Inorganic compounds

Antimony compounds

* Arsenic compounds (including arsine)

* Asbestos

Beryllium compounds *Cadmium compounds Calcium cyanamide Carbon disulfide Carbonyl sulfide Chlorine

*Chromium compounds Cobalt compounds Cyanide compounds

Hydrochloric acid Hydrofluoric acid *Lead compounds Manganese compounds Mercury compounds Mineral fibers (fine) *Nickel compounds Phosphine Phosphorus

Radionuclides (including radon) Selenium compounds Titanium tetrachloride

VII. Sulfates

Diethyl sulfate

Dimethyl sulfate

6 Asterisks indicate HAPs for which the State of California has prepared detailed risk assessments and identified them as Toxic Air Contaminants (TACs).

' For excellent summaries of general exposure and health effects data for 243 substances, see the California Air Resources Board report (1997b). Each summary describes the physical properties, sources, and concentrations both outdoors and indoors, atmospheric persistence, health effects, and other risk assessment information.

'' For a risk assessment, see "Evaluation of Ethyl Parathion as a Toxic Air Contaminant," California Department of Food and Agriculture (1988).

and the role of atmospheric chemistry in their risk assessments.

Formaldehyde is a designated HAP/TAC. As we have seen on many occasions throughout this book, it not only is directly emitted by a number of sources, including by motor vehicles outdoors and by building materials indoors (see Chapter 15.D), but is also formed in air from the oxidation of both anthropogenic and biogenic organics.

These secondary sources of HCHO vary geographically, seasonally, and diurnally and include the oxidation of anthropogenically emitted VOC as well as biogenic organics such as isoprene. In California, for example, oxidation of VOC is estimated to generate most of the HCHO (about 150,000 tons per year, with a large uncertainty of 50%), whereas direct emissions account for only about 10% of the total, i.e., for ~ 18,000 tons per year (Cal EPA, 1992b; Harley and Cass, 1994). The importance of secondary formation of HCHO (and CH3CHO) in the atmosphere is not unique to California (Kao, 1994). However, the relative amounts of direct emissions are undoubtedly much larger in urban areas where vehicles are responsible for ~80% of the direct emissions.

Formaldehyde reacts rapidly in air through photolysis (see Chapter 4.M) and through attack by OH, N03, and, in coastal areas, likely chlorine atoms as well:

Both H and HCO then react with 02 to generate H02.

Formaldehyde is therefore an example of a HAP that has both primary and secondary sources and that is relatively rapidly "deactivated" in the atmosphere through photolysis and atmospheric reactions (see Problem 3).

A number of pesticides are listed as HAPs (see Table 16.16). These can be transported significant distances from their point of application, and during that time they undergo chemical transformations as well as deposition (Kurtz, 1990). Table 16.17 shows some pesticides and their transformation products in air (Seiber and Woodrow, 1995).

One example of a reactive HAP that is a pesticide is 1,3-dichloropropene, used as a soil fumigant for nematodes. Both the cis and trans forms react with OH and 03 (Tuazon et al., 1984), although the OH reaction is sufficiently fast that this reaction is expected to be the major atmospheric fate (see Problem 5). Thus, the rate constants for OH reaction with the cis and trans forms are 0.774 X 10"" and 1.31 X 10"11 cm3 molecule"1 s ~1, respectively, giving lifetimes with respect to OH at

2 X 10h radicals cm-3 of f8 and ff h. The products of the OH reactions are formyl chloride (HC(O)Cl) and chloroacetaldehyde (ClCH2CHO), with one molecule of each formed for each molecule of 1,3-dichloropro-pene reacted (Tuazon et al., 1984) (see Problem 6).

A number of the pesticide HAPs are organophosphorous compounds. Seiber and co-workers have measured some organophosphorus pesticides, including di-azinon, parathion, chlorpyrifos, and methidathion, in air and fogs and wet deposition in a variety of locations in California (e.g., Glotfelty et al., 1990; Schomburg et al., 1991; Zabik and Seiber, 1993; Seiber et al., 1993; Baker et al., 1996). Both the parent pesticides, all containing reactive P = S groups, and their oxidation products, the corresponding oxon (P = 0), have been identified and quantified. The ratio of the oxons to the corresponding parent compounds increased with distance from the site of release and also increased after dawn, suggesting photochemical reactions were involved in the conversion.

These photochemical reactions likely involve OH chemistry. For example, Atkinson et al. (1989) reported that OH reacted with (CH30)3P = S to form (CH30)3P=0 and, similarly, that (CH30)2P(S)CH3 formed (CH30)2P(0)CH3, in yields of 0.28 and 0.13, respectively. The reaction is believed to involve addition to the P=S bond, followed by secondary chemistry to generate the oxon (Goodman et al., f988).

Another example of this conversion of P = S found in pesticides to P=0 is the oxidation of malathion in the atmosphere. Malathion itself is not a HAP and has relatively low acute mammalian toxicity because it is degraded by mammalian carboxylesterases. It is effective as a pesticide because in insects, it is activated to malaoxon, an acetylcholinesterase inhibitor. However, malathion itself typically contains impurities such as isomalathion whose mammalian toxicities are greater

TABLE 16.16 Some Pesticides and Pesticide Transformation Products Listed as Hazardous Air Pollutants"

1.3-Dichloropropene 2,3,7,7-Tetrachlorodibenzodioxin 2,4,5-Trichlorophenol

2.4-D salts and esters 4,6-Dinitro-o-cresol and salts Acrolein





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