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

Abstract

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.

Opinion: Challenges and needs of tropospheric chemical mechanism development

“…Detailed multiphase chemistry models have pointed to the importance of considering drop-size resolved chemical composition, in particular acidity, in affecting sulfate (Barth, 2006) and the phase partitioning of chemical compounds that affects their atmospheric residence time (Tilgner et al, 2021). Furthermore, cloud droplet size may affect the rates of uptake and chemical conversion within the aqueous phase (McVay and Ervens, 2017;Ervens, 2022). Such sensitivity studies demonstrate the challenges associated with model simulations of the atmospheric multiphase system.…”

Advances in Atmospheric Chemical and Physical Processes

“…Papers in this virtual special issue document the rich chemistry occurring in the aqueous phase�the formation of oligomers in reactions of glycolaldehyde with amines and ammonium sulfate; 39 the formation and stability of aqueous phase secondary ozonides generated in the ozonolysis of biogenic species; 40 the aqueous phase kinetics for reactions of sulfate radical ion with organics (many derived from isoprene); 41 and reactions of unsaturated isoprene byproducts with complex triplet-state organics. 42 Lastly, Ervens 43 documents the need to pay heed to cloud droplet properties in considering the veracity of aqueous phase chemical mechanisms.…”