A three‐dimensional numerical model of cloud dynamics, microphysics, and chemistry: 3. Redistribution of pollutants

The evolution of chemical species during the life cycle of a severe local storm has been simulated using our three‐dimensional cloud chemistry model (Wang and Chang, 1993 a, b ). The importance of dynamical transport was found to be different for different gases, depending on their solubility. For insoluble gases with a comparatively long chemical lifetime, dynamical transport was a major factor in the evolution of in‐cloud gas phase concentration. The three‐dimensional divergence (convergence) and eddy mixing accounted for nearly 100% of the total variation of the in‐cloud O 3 amount. For the highly soluble gases such as HNO 3 and H 2 O 2 the dominant sources and sinks were dissolution and evaporation. The dissolution of HNO 3 and H 2 O 2 accounted for more than 44% of the total change. The extensive condensation occurring in selected parts of the storm caused sharp decreases in the HNO 3 and H 2 O 2 mixing ratios. The dissolution of SO 2 can exceed the influence of dynamical processes for a short time period, corresponding to a newly born convective cell. For other times, dynamics was the major factor influencing the variation of the SO 2 in‐cloud amount. The in‐cloud amounts of the gas phase pollutants were reduced significantly during the development of the storm. The net decrease of SO 2 is approximately equal to 4 ppt on the average throughout the entire cloud volume during the simulation, or approximately −3 ppt/h in rate. For O 3 the total change is approximately equal to −1.1 ppb/h on the average. The net decrease of HNO 3 is equal to 20 ppt on the average, or −15 ppt/h in rate. The H 2 O 2 decreasing rate is approximately equal to −5.9 ppb/h on the average, while the average net change is about 7.8 ppb in the cloud volume and averaged through the duration of the simulation.