PNNL

Abstract

New pathways to form secondary organic aerosols (SOA) have been postulated recently. Glyoxal, the smallest dicarbonyl, is one of the proposed precursors. It has both anthropogenic and biogenic sources, and readily partitions into the aqueous-phase of cloud droplets and also deliquesce aerosols where both reversible as well as irreversible chemistry can take place. In this work we extend the regional scale chemistry transport model WRF-chem to include a more detailed glyoxal gas-phase chemistry as well as a module describing its partitioning and reactions in the aqueous-phase of aerosols. A comparison of several proposed mechanisms allows us to quantify the relative importance of different formation pathways and their regional variability. The CARES/CalNex campaigns over California in summer 2010 are used as case study to evaluate the model against observations. In all simulations the LA basin was found to be the hotspot for SOA formation from glyoxal, which contributes between 0.15% and 15% of the model SOA depending on the mechanism used. Our results indicate that a mechanism based on a simple uptake coefficient, as it has been previously employed in global modeling studies, leads to higher SOA contributions from glyoxal compared to a more detailed description that considers aerosol phase state and chemical composition. In the more detailed simulation surface uptake is found to be the main contributor to SOA mass compared to a volume process and reversible formation. We find that this is not due to restricting glyoxal SOA formation to deliquesce aerosols, but rather due to a kinetic limitation in the current formulation of the reversible pathways. If this limitation is removed, volume pathways contribute > 70% of glyoxal SOA mass, and the total mass formed is comparable to the simple uptake coefficient formulation without consideration of aerosol phase state and composition. Given that all of the model formulations are based on very limited recent field or laboratory data, we conclude that the current uncertainty on glyoxal SOA formation spans a factor of 100 in this domain and time period, from being a very minor contributor to a very substantial one.