Aqueous chemistry and its role in secondary organic aerosol (SOA) formation
There is a growing understanding that secondary organic aerosol (SOA) can form through reactions in atmo-spheric waters (i.e., clouds, fogs, and aerosol water). In clouds and wet aerosols, water-soluble organic products of gas-phase photochemistry dissolve into the aqueous phase where they can react further (e.g., with OH radicals) to form low volatility products that are largely retained in the particle phase. Organic acids, oligomers and other products form via radical and non-radical reactions, including hemiacetal for-mation during droplet evaporation, acid/base catalysis, and reaction of organics with other constituents (e.g., NH + 4). This paper provides an overview of SOA formation through aqueous chemistry, including atmospheric evidence for this process and a review of radical and non-radical chem-istry, using glyoxal as a model precursor. Previously unre-ported analyses and new kinetic modeling are reported herein to support the discussion of radical chemistry. Results sug-gest that reactions with OH radicals tend to be faster and form more SOA than non-radical reactions. In clouds these reactions yield organic acids, whereas in wet aerosols they yield large multifunctional humic-like substances formed via radical-radical reactions and their O/C ratios are near 1.