Particles In The Troposphere

a Los Angeles

100%

Total fine particle mass 24.5 ng m"3

Others

Ammonium

Nitrate

Sulfate

Elemental carbon

Organics

Resolvable elutable organics 910 ng m"3

Unidentified organics

Aromatic polycarboxylic acids

Aliphatic dicarboxylic acids n-Alkanoic acids n-Alkanes

• Diterpenoid Acids n-Alkenoic Acids b Rubidoux (downwind)

Total Fine Particle Mass 42.1 ng m"3

Others

Ammonium

Nitrate

Sulfate

Elemental carbon

Organics

Resolvable Elutable Organics 1070 ng m"3

Unidentified organics

Aromatic polycarboxylic acids

Aliphatic dicarboxylic acids n-Alkanoic acids n-Alkanes

Other PAHs

Diterpenoid acids n-Alkenoic acids

FIGURE 9.54 Composition of particles in Los Angeles, at west end of air basin, and in Rubidoux, at east end (adapted from Rogge et al., 1993d).

degrees of compression to a liquid-like state to finally a highly ordered, solid condensed state (Gaines, 1966; MacRitchie, 1990). The compressed films would be expected to be less permeable. There is some evidence for this. For example, Rubel and Gentry (1985) measured the accommodation coefficient (see Chapter 5.E.1) for water as well as ammonia on acid droplets coated with hexadecanol. The water accommodation coefficient decreased from 8 X 10 3 as the alcohol coverage increased, i.e., as the degree of compression of the organic film increased, to 4 X 10 4, with a sharp change at the point that the film underwent a phase transition from the liquid to the solid condensed state. Similarly, Daumer et al. (1992) showed that coating an H2S04 aerosol with straight-chain organics retarded the rate of neutralization by ammonia, whereas

"cn

"cn

Hexad Canol Mol Cule

FIGURE 9.55 Typical negative ion mass spectra for a single particle in the (a) upper troposphere (14.6 km, 22°N) and (b) stratosphere (19 km, 31°N) (adapted from Murphy et al., 1998).

TABLE 9.23 Structures of Some Classes of Surface-Active Molecules Found in the Atmosphere"

Class of compound

Structure

FIGURE 9.55 Typical negative ion mass spectra for a single particle in the (a) upper troposphere (14.6 km, 22°N) and (b) stratosphere (19 km, 31°N) (adapted from Murphy et al., 1998).

branched molecules did not, presumably because the permeability of the films was much larger.

The representation in Fig. 9.56 is simplified in that such an orderly arrangement applies to surfactants having saturated hydrophobic chains that can compress to an ordered solid condensed phase. However, this is not the case for all potential surface-active compounds in the atmosphere; for example, the presence of a double bond as in the case of oleic acid, CH3(CH2)7CH=CH(CH2)7COOH, gives the molecule a "crooked" shape, which requires more surface area per molecule and which does not lead to a well-ordered solid condensed phase at the interface. Thus, Xiong et al. (1998) measured effects of organic films on the uptake of water into particles of H2S04. The saturated straight-chain lauric and stearic acids significantly reduced the uptake of water when present at amounts equivalent to one monolayer, whereas oleic acid had no effect at this concentration.

The possibility of such organic films being formed on aerosol particles in the atmosphere as well as on fog, cloud, and rain droplets and snowflakes has been discussed in detail by Gill and co-workers (1983). As seen from our earlier discussions on the types of organics that have been observed in both urban and nonurban aerosols, there is no question that surface-active species

TABLE 9.23 Structures of Some Classes of Surface-Active Molecules Found in the Atmosphere"

Class of compound

Structure

Alcohol

R—CH2OH

Acid

R-Cf

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