Simulating Biogenic Secondary Organic Aerosol During Summertime in China

Biogenic secondary organic aerosol (BSOA) contributes significantly to summertime organic particulate matter. In this study, a simulation of BSOA for summer 2012 over China is performed using the Community Multiscale Air Quality model with an explicit representation of BSOA formation via reactive uptake of isoprene-derived intermediates, species-specific parameters for monoterpene-derived BSOA production, and a multigenerational oxidation scheme. Simulated BSOA tracks the measurements well (normalized mean bias of 1% and r(2) of 0.59). Elevated BSOA occurs in the Sichuan Basin and southeastern China (except the coastal areas) where concentrations are mostly within the range of 5-7.5 mu g/m(3). Nitrate oxidation dominates fresh BSOA originating from monoterpenes. Simulated monoterpene-formed BSOA increases by a factor of 3-11 with multigenerational oxidation. Isoprene-formed BSOA shows high concentrations in southwestern China, formed primarily by two methyltetrols (2-MT) and organosulfates (62%-83%). High concentrations in this region are driven by the abundance of IEPOX and high particle surface area of sulfate-laden aerosols. NOx emissions influence the formation pathways of isoprene-formed BSOA, as a high ratio of two-methylglyceric acid (2-MG)/2-MT is found in the North China Plain and part of the Yangtze River Delta Region. The simulated isoprene-formed BSOA is more strongly correlated with sulfate than with particle water mass concentration or particle acidity, in part because sulfate increases the surface area increasing uptake for heterogeneous reactions. Decreases in isoprene-formed BSOA were simulated with a pH increase of 2, implying that particle acidity mediates the BSOA formation. Plain Language Summary Emissions from plants contribute to particulate matter formation when they interact with emissions due to human activities. With intense anthropogenic emissions in China, how biogenic aerosols are formed as well as how the formation processes are impacted by anthropogenic emissions are not clear yet. This study applied an advanced modeling approach to simulate formation of biogenic aerosols over continental China in summer. We modeled high levels of biogenic aerosols in the Sichuan Basin and southeastern China, not only because of more forests in these areas than elsewhere but also due to particulate matter of anthropogenic origin. Based on the simulation, there is a clear evidence that anthropogenic emissions impact the chemical formation and composition of biogenic aerosols and particle acidity. Therefore, a decrease (increase) in biogenic aerosols and particle acidity would be a cobenefit (side effect) of anthropogenic emission controls.