Formation and sink of glyoxal and methylglyoxal in a polluted subtropical environment: Observation-based photochemical analysis and impact evaluation

The dicarbonyls glyoxal (Gly) and methylglyoxal (Mgly) have been recognized as important precursors of secondary organic aerosols (SOAs) through the atmospheric heterogeneous process. In this study, field measurement was conducted at a receptor site in the Pearl River Delta (PRD) region in southern China, and an observation-based photochemical box model was subsequently applied to investigate the production and evolution of Gly and Mgly as well as their contributions to SOA formation. The model was coupled with a detailed gas-phase oxidation mechanism of volatile organic compounds (VOCs) (i.e., Master Chemical Mechanism, MCM, v3.2), heterogeneous processes of Gly and Mgly (i.e., reversible partitioning in aqueous phase, irreversible volume reactions and irreversible surface uptake processes), and the gas particle partitioning of oxidation products. The results suggested that without considering the heterogeneous processes of Gly and Mgly on aerosol surfaces, the model would overpredict the mixing ratios of Gly and Mgly by factors of 3.3 and 3.5 compared to the observed levels. The agreement between observation and simulation improved significantly when the irreversible uptake and the reversible partitioning were incorporated into the model, which in total both contributed 62% to the destruction of Gly and Mgly during daytime. Further analysis of the photochemical budget of Gly and Mgly showed that the oxidation of aromatics by the OH radical was the major pathway producing Gly and Mgly, followed by degradation of alkynes and alkenes. Furthermore, based on the improved model mechanism, the contributions of VOC oxidation to SOA formed from gas particle partitioning (SOAgp) and from heterogeneous processes of Gly and Mgly (SOAhet) were also quantified. It was found that o-xylene was the most significant contributor to SOAgp formation (29 %), while m;p-xylene and toluene made dominant contributions to SOAhet formation. Overall, the heterogeneous processes of Gly and Mgly can explain 21% of SOA mass in the PRD region. The results of this study demonstrated the important roles of heterogeneous processes of Gly and Mgly in SOA formation and highlighted the need for a better understanding of the evolution of intermediate oxidation products.