Assessment of dicarbonyl contributions to secondary organic aerosols over China using RAMS-CMAQ

The concentration of secondary organic aerosol (SOA) is underestimated in current model studies. Recent research suggests that the reactive uptake of dicarbonyls contributes to the production of SOA, although few models have included this pathway. Glyoxal, an important representative component of dicarbonyls in models, is significantly underestimated. We therefore incorporated the reactive uptake of dicarbonyls into the regional air quality modeling system RAMS-CMAQ (the Regional Atmospheric Modeling System-Community Multiscale Air Quality) to evaluate the contribution of dicarbonyls to SOA, and we then assess the impact of the underestimation of glyoxal on the production of SOA in China during two time periods: 3 June to 11 July 2014 (episode 1) and 14 October to 14 November 2014 (episode 2). When the reactive uptake process was added, the modeled mean concentration of SOA in episode 1 increased by 3.65 mu g m(-3), which explained 34.8% of the unaccounted-for source of SOA. Meanwhile the increase in the concentration of SOA in episode 2 was 1.82 mu gm(-3) as a result of the lower liquid water content and the lower amount of dicarbonyls produced from biogenic precursors in the fall. On this basis, when the glyoxal simulation was improved, the modeled mean dicarbonyl-derived SOA (AAQ) increased by more than a factor of 2 in both episodes relative to case 1. AAQ in episode 1 contributed, on average, 60.6% of the total concentration of SOA and the increase in this contribution represented 69.1% of the unaccounted-for concentration of SOA, whereas the mean AAQ in episode 2 accounted for 64.5% of total concentration of SOA. Based on the results, the mean AAQ over China was generally higher in the east than in the west during the two episodes. The highest value (10-15 mu g m(-3)) of episode 1 appeared in the areas around the lower reaches of the Yellow River, whereas the highest value of 5-10 mu g m(-3) in episode 2 was concentrated over regions from south of the lower reaches of the Yellow River to the south of Guangzhou Province as well as the Sichuan Basin. The contribution of AAQ to the concentration of SOA in episode 1 varied from 10% to 90% throughout China, with the highest contributions (70%-90%) in the coastal regions and offshore along the East China Sea to the South China Sea and in the southwestern regions. The fraction of AAQ to SOA in episode 2 was in the range of 10%-80% over China, with the fraction up to 80% in a small portion of northeastern China.