A Case Study of Investigating Secondary Organic Aerosol Formation Pathways in Beijing using an Observation-based SOA Box Model

Current modeling studies have underestimated secondary organic aerosol (SOA) levels in China to a larger degree than over Europe and the United States. In this study, we investigated the SOA formation pathways in urban Beijing for the period of November 7-8, 2014, using an observation-constrained box model in which the multigenerational oxidation processes of volatile organic compounds (VOCs) and intermediate VOCs (IVOCs) and the chemical aging of semi-volatile primary organic aerosols (POAs) were taken into account. The results demonstrated that the SOA formation rate was 30.3 mu g m(-3) day(-1) in Beijing during the 2-day study period. The contributions of VOCs, IVOCs, and POAs to the SOA levels were 14%, 82%, and 4%, respectively. IVOC contributions were on a scale similar to the magnitude of underestimation in a previous study. The uncertainty analysis showed that SOA levels during the study period were 55.4-102.4 mu g m-3 (the 25th and 75th percentiles of the sensitivity simulations). The contribution of IVOCs to the SOA formation was dominant compared with that of VOCs and POAs. A more precise IVOC oxidation mechanism can thus improve the performance of the SOA model in China.