Summary of the cloud chemistry modeling intercomparison: Photochemical box model simulation

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 CH₂O, CH₃OOH, HNO₃, and O₃, while H₂O₂ (in the absence of sulfur chemistry), NO, and NO₂ 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 CH₂O and HCOOH concentrations are decreased and increased, respectively, because of the timing of the CH₂O 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.