A 3-DIMENSIONAL NUMERICAL-MODEL OF CLOUD DYNAMICS, MICROPHYSICS, AND CHEMISTRY .4. CLOUD CHEMISTRY AND PRECIPITATION CHEMISTRY

A study of the cloud chemistry and precipitation chemistry of the three-dimensional cloud chemical modeling of a severe local storm case (Wang and Chang, this issue) is discussed in this paper. In our simulation the microphysical processes, especially those related to the ice phase, were found to have significant effects on influencing both the rhythm of time evolution and the amount of quantitative variation of pollutants. Most chemical species in the ice phase particles were transformed from liquid phase droplets by the riming process. The ratio of the transfer of chemical species from aqueous phase to ice phase induced by riming to that induced by freezing was about 55 to 1 for N(V) and 8 to 1 for S(VI). The average equivalent pH value had a linear correlation with the logarithm of total water content and with the logarithm of [S(VI)] + [N(V)] in the cloud. The average equivalent pH value for the whole cloud changed from 5.1 at 28 min to 4.4 at 55 min. Most low pH values appeared in the less convective region. For this particular case study, aerosol scavenging was the major source of S(VI) production, which provided a total amount 50 times that provided by sulfur oxidation in the cloud. The major contributor of sulfur oxidation changed during the lifetime of the modeled storm. The peaks of acid deposition did not correspond to the peaks of precipitation. The sub cloud dissolution and microphysics-related transformation played an important role in the acid production and deposition. The nitric acid production total in the sub cloud area doubled that in the cloud. Most of the S(VI) produced by the aqueous phase SO2 oxidation was also found to be due to sub cloud processes.