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that can form organic films on water do exist in the atmosphere. However, Gill and co-workers (1983) have estimated, based on the limited data available, that only in aerosol particles does sufficient surface-active organic clearly exist to form a film around the particle. They suggest that the presence of surface-active films on cloud droplets, snowflakes, and raindrops is unlikely.

While the focus has been on long-chain surfactantlike organics, Donaldson and Anderson (1999) suggest, based on their measurements of the standard free energy of adsorption of gases onto water, that there may also be significant surface coverages of smaller organics under atmospheric conditions.

There is some field evidence for the existence of organic films on the surfaces of particles. For example, Fig. 9.57 shows the results of electron microscopy of haze aerosol collected in Los Angeles (Husar and Shu, 1975). The droplets are "wrinkled" in appearance, and they suggest this is due to "haze aerosol" droplets being coated with an organic layer that collapsed when the water in the particle evaporated under vacuum. Husar and Shu propose that the wrinkled appearance is due to a nonvolatile layer of organics that shrunk after water evaporated from the particle during analysis;

Hydrophobic group

Water surface Hydrophilic group

FIGURE 9.56 The orientation of surface-active organic molecules at the water surface (adapted from Gill et al., 1983).

Hydrophobic group

Water surface Hydrophilic group

FIGURE 9.56 The orientation of surface-active organic molecules at the water surface (adapted from Gill et al., 1983).

FIGURE 9.57 Electron micrographs of haze aerosol collected in Pasadena in 1973 (A) and of (B) unheated and (C) heated haze particle (adapted from Husar and Shu, 1975).

they described heated particles in Fig. 9.57C as appearing like an "evacuated, thick-walled, rubber ball."

Similarly, Posfai et al. (1998) carried out AFM and TEM studies of aerosol particles collected over the North Atlantic Ocean. The size of the particles measured under the vacuum conditions of the TEM were smaller than those measured by AFM, due to evaporation of water from the particles. However, the TEM images also showed "halos" around the particles at the same diameters as the AFM measurements, suggesting that the particles under ambient conditions had an organic coating. This was supported by energy-dispersive X-ray spectrometry (EDS), which showed that these residues contained S, O, and C (although at least some of the C signal may have been due to the substrate).

Organic films may have some or all of the following effects: (f) reduction of the rate of evaporation of water from the droplets, (2) inhibition of the transport of stable molecules and of highly reactive free radicals such as OH and H02 from the gas phase into the droplet, and (3) reduction of the efficiency with which the particles are scavenged by larger cloud and rain droplets (Gill et al., 1983). Thus, the presence of organic films may increase the lifetime of such particles in the atmosphere compared to those expected if the films were not present (Toossi and Novakov, 1985).

In addition, such films may impede the uptake of species other than water. For example, Jefferson et al.

(1997) showed that a stearic acid coating inhibited the uptake of H2S04 on (NH4)2S04 and NaCl particles; the mass accommodation coefficients for NaCl, for example, decreased from 0.79 to 0,f9-0.3f for different coverages of stearic acid.

There is experimental evidence for a reduced rate of evaporation in the presence of organic films. For example, Shulman et al. (1997) showed that the presence of difunctional oxygenated organics such as oxalic or cis-pinonic acids decreased the rate of evaporation of water from particles. Chang and Hill (1980) exposed water drops of ~f9-/j,m diameter to a stream of air containing the alkene decene (53-220 ppm) and O-, (20 ppm); the rate of water evaporation was reduced by the presence of decene and 03, and a major product of the reaction, nonanal, also reduced the evaporation rate when added separately. Larson and co-workers (Andrews and Larson, 1993; Wagner et al., 1996) reported that coating NaCl with an organic surfactant reduces the uptake of water, although the form of the organic on the particle and how it exerts its effect on water uptake are complex (Hansson et al., 1990; Wagner et al., 1996). Surfactant on carbon black has been reported to increase the hygroscopicity of the particles. However, Hameri and co-workers (1992) reported that a high molecular weight alkane or carboxylic acids added to NaCl particles did not significantly alter the water absorption by the particles.

Uptake of water by aerosol particles is a complex function not only of the presence of surfactants but also of their overall composition, which makes discerning the presence and effects of a surfactant in ambient particles difficult. McMurry and Stolzenburg (f989) give a review of work in this area up to 1989. In their studies in a polluted urban atmosphere, they applied a tandem differential mobility analyzer (TDMA) (see Chapter If.B.2). In essence, this is two differential mobility analyzers (DMA) in which particles of a given size are first selected in one DMA and then exposed to air of a known relative humidity (RH). The change in particle size is measured using the second DMA. The difference in particle volume, obtained from the size change, between a given RH and 0% RH gives the water content. They observed that some particles were nonhygroscopic (i.e., did not absorb water), while others were hygroscopic, and that particles in the 0.4- to 0.5-yu,m range absorbed more water than those in the 0.05- to 0.2-pm range.

Zhang et al. (1993) showed that particles that were more hygroscopic tended to consist largely of sulfates and nitrates, whereas the less hygroscopic species tended to be associated with carbon.

More recently, the uptake of water by tropospheric particles in relatively remote locations near the Grand Canyon and in a polluted urban area near Los Angeles was studied by Saxena et al. (1995) using a TDMA similar to the studies of McMurry and co-workers. Figure 9.58 shows the measured total water content of these particles in Claremont, California, east of Los Angeles, as a function of the water calculated to be associated with inorganics. As already discussed, a vari ety of inorganics such as NaCl, sulfates, and nitrates take up water, and their composition, including the water content, can be calculated from the composition, RH, and temperature. It is seen in Fig. 9.58 that, on average, about 25-35% less water is associated with these particles. Saxena et al. (1995) suggest this may be due to the fact that many of the organics in an urban area such as this are primary in nature, and hence may be hydrophobic. These organics may act as surfactants on the particle surface, inhibiting uptake of water, as suggested by Gill et al. (1983). An alternate explanation is that the calculated water contents are incorrect due to the equilibrium thermodynamics of inorganics being altered in these complex mixtures.

On the other hand, the opposite effect is observed for particles at the more remote site near the Grand Canyon. Figure 9.59 shows that these particles appear to contain more water than expected based on the inorganics. Figure 9.60 shows the relationship between the excess water in the particles and the organics in 0.1-pm particles. Clearly, water appears to be associated with the organics; i.e., the organics are behaving as if they are hydrophilic. Saxena and co-workers estimate that for RH of 80-88%, about 25-40% of the total water uptake is associated with organics and conclude that this behavior is consistent with the organics being primarily due to formation in the atmosphere and hence highly oxidized and hydrophilic in nature.

Clearly, this is a complex area that requires much more research.

(4) Elemental vs organic carbon Because the organics found in particles in urban areas are largely associated with human activities, and particularly combustion

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