Photochemical oxidative processing of volatile organic compounds (VOCs) in the troposphere is coupled to the formation of ozone and secondary organic aerosol (SOA), important secondary pollutants. However, the chemistry that forms them is complex and not completely understood. In this work, field measurements are applied to a zero-dimensional box model to investigate the validity of current chemical mechanisms and the fate of reactive carbon in aqueous aerosol. First, field measurements of gas-phase species, dominant oxidants, and aerosol composition were used to model the fate of reactive carbon during local aqueous aerosol processing of aldehydes and epoxides. Data from the Pan-European Gas-AeroSOls-climate interaction Study (PEGASOS 2012) showed that glyoxal was the dominant contributor to locally-formed aqueous SOA (aqSOA) and that modeled aqueous aerosol processing converted small gas-phase aldehydes into higher–volatility carboxylic acids. Data from the Southern Oxidant and Aerosol Study (SOAS 2013) showed that non-radical-mediated aqueous chemistry dominates the formation of aqueous aerosol. Sensitivity tests reveal that aqSOA composition, aqSOA mass, and product degassing were controlled by reactant through limiting chemistry, by product volatility through degassing, and by liquid water through medium availability. Second, a number of studies have suggested that observations of glyoxal and formaldehyde, and the ratio between the two (RGF) can help define the composition of an airmass on a local or global scale in terms of volatile organic compound (VOC) speciation, and HOx and NOx concentration. Glyoxal and formaldehyde measurements taken as part of SOAS correlated well (R2=0.71), resulting in an RGF value of 1.4% despite larger variation in glyoxal and formaldehyde, consistent with previous campaigns and satellite retrievals. Ambient variation in glyoxal, formaldehyde, and RGF stemmed mainly from diel variability and afternoon variability stemmed almost entirely from RO2 fate, which determines the chemical pathway through which a VOC is processed and therefore the product distribution. However, the model indicated OH concentration and then RO2 fate controlled RGF values, indicating that current chemical mechanisms misrepresent either the formation or loss of glyoxal and formaldehyde with respect to OH. Since NOx concentrations at this site were low, this highlights a need for updated low-NOx isoprene chemistry.
