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Fig. 16

Variation of the ratio of molar concentration of sulfate to that of S02 as a function of temperature ( 7) and solar radiation intensity (/) during summer daylight (E. Mészâros, 1973) (By courtesy of J. of Aerosol

Science)

One possible alternative process has already been mentioned: the oxidation of SO2 in cloud and fog drops by different absorbed oxygen species. Since this oxidation plajs an important role also in the wash-out of sulfur dioxide lrom the troposphere, we will discuss it in more detail in Chapter 5. We will note here only that this oxidation is promoted by the presence of metal (iron, manganese etc.) ions (Junge and Ryan, 1958). Moreover, considering that the rate of this process depends upon the pH of the solution, the presence of NH3 also increases the rate of this conversion. Thus, Georgii (1970) found by calculation with the model of Scott and Hobbs (1967) that the quantity of sulfate ions formed depends more on the NH3 concentration than on the S02 level, both measured in the air. The aircraft measurements of the same author and his associate (Georgii and Miiller, 1974) show, however, that the concentration of ammonia at dilferent altitudes is sufficient for this process only during summertime (see Fig. 14). It was also proposed (see Subsection 3.4.5) that ozone absorbed by atmospheric liquid water is important as a reaction partner for the oxidation. The present author believes that 63 plays an essential role in these transformation processes, feeling that it is very improbable that the concentration of the metal catalysts mentioned is significant in the clean troposphere (see Chapter 5).

The amount of observational support for the formation of sulfate particles by the evaporation of cloud water is also very limited. Thus, according to Jost (1974) the concentration of larger sulfate particles below the clouds might be high even if their number is low at the cloud levels. Hobbs (1971) showed that downwind from dissipating clouds, a great number of condensation nuclei active at 0.5 % supersaturation can be detected. This observational fact, of course, supports the foregoing hypothesis only on the reasonable assumption (see Twomey, 1971), that active cloud nuclei are composed of sulfates. However, Hobbs (1971) did not make any chemical analyses.

The oxidation of SOa can also take place on the surface of existing aerosol particles. According to the laboratory work of Urone et al. (1968) the rate of the S02 oxidation can be rapid in the presence of aerosol particles, even without illumination. In case of Fe203 particles the S02 loss in the laboratory chamber air is as much as 100 % hr~\ while high loss rates were found in the presence of aluminum, calcium, chromium and other metal oxides as well. It is questionable, however, whether the S02 loss is due to chemical conversion or to simple physical adsorption (Corn and Cheng, 1972). Furthermore, in Urone's experiments the mass concentration of aerosol particles was 100-200 times greater than that of S02 which is unrealistic under atmospheric conditions. Finally, the experimental results of h2s+

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