Effect of composition variations in cloud droplet populations on aqueous-phase chemistry

We prove that the use of a cloud or fog droplet population's volume weighted average pH results in the underestimation of the actual rate of sulfate production for most atmospheric conditions. To quantify the magnitude of this error, we have developed two aqueous-phase chemistry models: a droplet size-resolved model and a bulk chemistry model. The discrepancy between the results of these two models indicates the magnitude of the error introduced by using bulk aqueous-phase properties. This error depends mainly on the availability of gas-phase species (SO2, O-3, H2O2, and NH3), the aerosol size/composition distribution, and the residence time of the air parcel in cloud containing air. The ratio of predicted sulfate production between the two models for the cases studied here varies from as low as unity to as high as 30. The largest ratios occur during the first few minutes of cloud formation. After this peak the difference in sulfate production rates between the two models decreases rapidly. For the scenarios simulated, the largest error introduced by the bulk modeling approach at the end of a cloud event was underprediction of the sulfate production by a factor of 2. The magnitude of the sulfate underprediction by the bulk model decreases with increasing initial levels of gas-phase NH3 and H2O2 and is rather insensitive to the gas-phase O-3 and SO2 concentrations.