Organic Acids

While the focus in terms of acid deposition has been on sulfuric and nitric acids, it has been increasingly recognized that organic acids can also contribute significantly to the acidity of both the gas and aqueous phases in both urban and remote regions. A review of carboxylic acids in the atmosphere is given by Chebbi and Carlier (1996).

The major organic acids found in the gas phase are formic acid (HCOOH) and acetic acid (CH3COOH), with smaller contributions from larger aliphatic acids and multifunctional acids such as pyruvic acid (CH3COCOOH) and glyoxalic acid (COOHCHO) (e.g., see Kawamura et al., 1985; and Khwaja, 1995). Formic and acetic acids have sufficiently high vapor pressures that they are found almost totally in the gas phase rather than in particles (e.g., see Kawamura et al., 1985; Talbot et al., 1988; Grosjean, 1989; and Khwaja, 1995). Concentrations of HCOOH in urban areas are typically a few ppb (e.g., see Dawson et al., 1980; and summaries by Lawrence and Koutrakis, 1994; and Khwaja, 1995) although levels up to 45 ppb have been observed (Lawrence and Koutrakis, 1994). CH3COOH is also present in urban areas in the low-ppb range, with reported concentrations as high as 15 ppb in the Los Angeles area (Grosjean, 1989).

In semirural and rural areas, the concentrations of HCOOH and CH3COOH are somewhat smaller but still tend to be in the range around a ppb (e.g., San-hueza et al., 1996; Kumar et al., 1996; Granby et al., f997a, 1997b). For example, at a rural site in Virginia, concentrations of formic and acetic acids are typically ~f and 0.5 ppb, respectively, although peak formic acid concentrations as high as fO ppb have been measured (Talbot et al., 1995; Keene et al., 1995). in the northern Congo, ground-level concentrations of ~0.5 ppb each have been measured for these two acids, but the levels were as high as ~3-4 ppb in the boundary layer above the surface (Helas et al., 1992).

In marine remote regions, concentrations of formic and acetic acids are typically about 0.1-0.3 ppb, although much higher levels have been observed in stable layers of air at higher altitudes above the ocean (e.g., Chapman et al., 1995).

Formic and acetic acids may constitute a large fraction of the gas-phase acidity. For example, Grosjean (1990) measured the concentrations of these two organic acids as well as the inorganic acids HN03 and HC1 in southern California during a smog episode in

FIGURE 8.22 Calculated contributions to S(IV) oxidation in a cloud of the iron-catalyzed oxidation by 02/Fe(III), by H202 and OH in solution, and by OH in the gas phase, OH(g), expressed in terms of rate of production of column S(IV) (adapted from Jacob et al, 1989).

Month

FIGURE 8.23 Average concentrations of gas-phase acids at eight sites in southern California in 1986 (adapted from Nolte et al., 1997).

Month

FIGURE 8.23 Average concentrations of gas-phase acids at eight sites in southern California in 1986 (adapted from Nolte et al., 1997).

August 1986. The two organic acids taken together represented 44-93% of the total gas-phase acids.

Similarly, Fig. 8.23 shows the monthly average concentrations of HCOOH, CH.COOH, HC1, and HNO, measured at eight sites in southern California in 1986 (Nolte et al., 1997). Formic and acetic acids are seen to be the major acids, consistently exceeding HN03.

There are a number of potential sources, both primary and secondary, of formic and acetic acids in urban areas (e.g., see review by Chebbi and Carlier, 1996). Clearly the oxidation of organics can lead to the formation of acids. For example, as discussed in Chapter 6.E.2, the reaction of 03 with alkenes generates a Criegee intermediate that can react with water vapor to generate a carboxylic acid. Aqueous-phase oxidations, e.g., of HCHO to HCOOH, followed by evaporation from the condensed phase may also potentially contribute. Direct emissions (i.e., primary sources) include automobile exhaust, biomass combustion, stationary source emissions, for example, from vinegar manufacturing and cooking of food, and possibly natural emissions from vegetation (e.g., see Kawamura et al., 1985; Talbot et al., 1988, 1995; Dawson and Farmer, 1988; and Keene and Galloway, 1988).

The relative contributions of these sources depends, not surprisingly, on the particular location. For example, Granby et al. (1997a, 1997b) found that the concentrations of HCOOH and CH3COOH were similar in central Copenhagen and at a semirural site 30 km away, where the NO concentrations were an order of magnitude smaller. In addition, they correlated with the photochemical species such as 03, suggesting that the acids were formed by chemical reactions (e.g., of 03 with alkenes) during long-range transport. In southern California, these two acids were correlated with elemental carbon, used as an indicator of primary emissions, at upwind site close to the coast but not at downwind sites, suggesting that local primary emissions dominated upwind (Nolte et al., 1997). Conversely, the correlation between formic and acetic acids with both 03 and particle sulfate increased downwind, suggesting a significant contribution to their formation from chemical reactions both in the gas phase and in solution (where most of the sulfate is formed). Similarly, in Venezuela, atmospheric oxidation of organics appears to be the major source of these acids, perhaps with some contribution from emissions from soils during the dry season (Sanhueza et al., 1996).

Grosjean (1989, 1992) estimated that about 40% of the HCOOH and -75% of the CH3COOH measured at Claremont, just east of Los Angeles, are due to direct emissions. This is qualitatively consistent with carbon isotope measurements of l3C/l2C of formic and acetic acids in rainwater collected in this area, which show HCOOH is from a combination of direct emissions and secondary oxidation chemistry and CH3COOH is primarily from direct emissions (Sakugawa and Kaplan, 1995).

A variety of dicarboxylic acids have been measured in air, including, for example, oxalic acid [(COOH)2], succinic acid [HOOCCH2CH2COOH], and malonic acid [HOOCCH2COOH], as well as larger straight-and branched-chain carboxylic acids; unsaturated and aromatic acids such as phthalic acid are also observed in smaller concentrations (e.g., see Kawamura et al., 1996a, 1996b). Because of their lower vapor pressures, they are found predominantly in particles (see Chapter 9).

As discussed in Chapters 6 and 9, these dicarboxylic acids are believed to result in part from the oxidation of organics in air. However, Kawamura and Kaplan (1987) have also shown that automobile exhaust can be a significant source as well.

Organic acids can be removed by reaction with OH (see Chapter 6.H) as well as by wet or dry deposition. As a result, these acids are a common component of rain, clouds, fogs, and dews as would be expected from their large Henry's law constants, ~103-f04 mol L_l atm~' (see Keene et al., 1995), and are found in the condensed phase from remote to highly polluted urban areas (e.g., see Norton, 1985; Keene and Galloway, 1986, 1988; Likens et al., 1987; Weathers et al., 1988; Muir, 1991; Sakugawa et al., 1993; Keene et al., 1995; and Khwaja et al., 1995).

In short, while the focus has been primarily on sulfuric and nitric acids as a source of acid deposition, it is clear that organic acids can also contribute significantly. The gas-phase concentrations of the simplest carboxylic acids, formic acid and acetic acid, are relatively high even in remote regions, of the order of a ppb. Both natural and anthropogenic sources have been proposed, but the nature of the individual sources and their relative contributions are not well established.

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