The importance of cloud drop representation on cloud photochemistry

Because the acidity of the drop varies with cloud drop size, the concentration of species whose dissolution or whose aqueous reactions depend on the pH of the drop may also depend on the manner the cloud drop size distribution is represented in the cloud model. Cloud parcel model simulations coupled with gas and aqueous-phase chemistry are performed to assess the importance of the cloud drop size distribution on aqueous photochemical species. This study focuses on sulfate, sulfur dioxide, hydrogen peroxide, ozone, formaldehyde, and formic acid because these species concentrations can be dependent on the pH of the cloud drops. In one set of simulations the cloud drop size distribution is represented explicitly, that is, sectioned into sizes using the bin method. In a second set of simulations, the cloud drop size distribution is represented by a single-sized drop with radius = 10 μm, which is a common assumption in cloud physics models. The cloud parcel model simulations are computed with and without the coalescence process. Simulations without coalescence show that predicted formaldehyde and formic acid mixing ratios are affected by the cloud drop size distribution particularly in average background aerosol concentrations. Effects on other species are small. Results from simulations that included the coalescence process (resulting in a broader cloud drop size distribution) have similar results as the no coalescence simulations. Due to entrainment and turbulence processes, cloud water size distributions measured in clouds are generally broader than those modeled in this study. Thus, the effect of these processes on cloud photochemistry needs to still be assessed.