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[1] We report results of a cloud chemistry numerical modeling intercomparison, which shows good agreement among gas-aqueous photochemistry box models that are being used in the community. For the case studied, cloud chemistry depleted concentrations of CH 2 O, CH 3 OOH, HNO 3 , and O 3 , while H 2 O 2 (in the absence of sulfur chemistry), NO, and NO 2 increased. Because parcels of air usually flow in and out of cloud in a matter of minutes rather than remain in cloud for an hour, an optional simulation was performed in which frequent brief cloud encounters were represented. Representing a cloud intermittently rather than continuously does not alter the total concentration of many of the species. However CH 2 O and HCOOH concentrations are decreased and increased, respectively, because of the timing of the CH 2 O production during clear-sky intervals and its destruction during cloudy intervals. Further differences between a continuous cloud simulation and an intermittent cloud simulation are expected if pH is allowed to vary during the cloud periods. Simulating an intermittent cloud brought out the importance of using a chemistry time step that is a multiple of the cloud time step because deviations of results from a simulation in which the chemistry time step did not coincide with the appearance and disappearance of cloud were quite large. To better quantify the effect of cloud on HO x photochemistry, future investigations should include nonmethane hydrocarbon and sulfur chemistry. Future cloud chemistry modeling intercomparisons should bring in cloud physical and chemical measurements so that the models can be evaluated with observations.

Abstract. We examine factors controlling the photochemical oxidation of SO2 in tropospheric aerosols using a gas-aqueous photochemical model. Over a range of liquid water contents (3x10 -4g H20 m -3 to 9 g H20 m -3) and pH values (0 to 8), we find that H202(aq) and O3(aq) provide the major sinks for SO2 in the aqueous phase when pH is held constant at below 5 and larger than 6, respectively. OH(aq) may be an important oxidant of SO2 in the aqueous phase when pH is held constant between 5 and 6 and H2 02 is depleted in an air parcel. When pH is allowed to vary during the integration, H202(aq) is the most important oxidant in the aqueous phase. O3(aq) is important primarily when the liquid water content is large (> 1 g m -3) and the solution pH is above 4.0 3 (aq) is also important when the pH is initially high (> 6) for quickly oxidizing SO2 and, thereby, reducing the pH into the pH region where H202(aq) is the most important oxidant. OH(aq) may be important when H20 2 is depleted and the liquid water content is large. When aerosols are present during noncloudy. days in summer, the aqueous-phase oxidation of SO2 is insignificant compared with the gas-phase oxidation of SO2. We find, however, that the SO2 oxidation in wet aerosols may be enhanced in winter or when the temperature is low (273 K) and the relative humidity is high. Uncertainties in the reaction rate coefficients may significantly affect the concentrations of oxidants and other compounds of photochemical origin. Using a relatively stringent criterion, a compressed gas-aqueous phase chemical mechanism for photochemical oxidation of SO2 is proposed for global tropospheric modeling.

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Abstract. The sensitivity of tropospheric 03 to aqueous HOx (OH + HO2) phase chemistry in clouds is examined using photochemical model calculations of the 03 production efficiency per unit NO, and the chemical lifetime of 03, combined with estimates for the residence time of air in clouds. It is found that the maximum perturbation to 03 from cloud chemistry in the tropics and midlatitudes summer is less than 3%. This result is supported by calculations using a threedimensional, continental-scale model for North America.

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