Average Cloud Droplet Size and Composition: Good Assumptions for Predicting Oxidants in the Atmospheric Aqueous Phase?

Chemical models that describe the atmospheric multiphase (gas/aqueous) system often include detailed kinetic and mechanistic schemes describing chemical reactions in both phases. The present study explores the importance of properties including the chemical composition of droplet populations, such as pH value and iron present in only a few droplets, as well as droplet size and their distribution. It is found that the assumption of evenly distributed iron in all cloud droplets leads to an underestimate by up to 1 order of magnitude of OH concentrations in the aqueous phase, whereas the predicted iron(II)/iron(total) ratio is overestimated by up to a factor of 2. While the sulfate mass formed in cloud droplets is not largely affected by any of the assumptions, the predicted secondary organic aerosol mass varies by an order of magnitude. This sensitivity study reveals that multiphase chemistry model studies should focus not only on chemical mechanism development but also on careful considerations of droplet properties to comprehensively describe the atmospheric multiphase chemical system.