Global modeling of SOA: the use of different mechanisms for aqueous phase formation

Abstract

Abstract. There is growing interest in the formation of secondary organic aerosol (SOA) through condensed aqueous phase reactions. In this study, we use a global model (IMPACT) to investigate the potential formation of SOA in the aqueous phase. We compare results from several multiphase process schemes with detailed aqueous phase reactions to schemes that use a first order gas-to-particle formation rate based on uptake coefficients. The net global SOA production rate in cloud water ranges from 19.5 Tg yr−1 to 46.8 Tg yr−1 while that in aerosol water ranges from −0.9 Tg yr−1 to 12.6 Tg yr−1. The rates using first order uptake coefficients are over two times higher than the multiphase schemes in cloud water. Using first order uptake coefficients leads to a net SOA production rate in aerosol water as high as 12.6 Tg yr−1, while the fully multiphase schemes cause a negative net production rate. These rates can be compared to the gas phase formation rate of 29.0 Tg yr−1 that results from gas-particle partitioning and the formation rate of 25.8 Tg yr−1 from the uptake of epoxide. The annual average organic acid concentrations (the major SOA products formed in cloud) peak over the tropical regions, while oligomers (the major SOA products formed in aerosol water) generally show maxima over industrialized areas in the Northern Hemisphere. A sensitivity test to investigate two representations of cloud water content from two global models shows that increasing cloud water by a factor of 2.7 can increase the net SOA production rate in cloud by a factor of 4.2 at low altitudes (below approximately 900 hPa). We also investigated the importance of including dissolved iron chemistry in cloud water aqueous reactions. Adding these reactions increases the formation rate of aqueous phase HOx by a factor of 2.2 and decreases the amount of global SOA formed by 44%. Previously, we showed that the model that uses the uptake method to simulate SOA formed in both cloud and aerosol water over-predicts observed SOA by a factor as high as 3.8 in tropical regions. The use of the multiphase reaction scheme for SOA formation in cloud water brings the model's predictions to within a factor of 2 of the observations. All simulations show reasonable agreement with aerosol mass spectrometry (AMS) measurements in the Northern Hemisphere, though using the uptake method to simulate SOA formed in aerosol water improves the results by around 10% compared to the use of the multiphase reaction scheme. All cases studied here tend to underestimate observations of oxalic acid, particularly in Europe in winter, in the Amazon, Africa, and China as well as over ocean regions. The model with iron chemistry under predicts measurements in almost all regions. Finally, the comparison of O/C ratios estimated in the model with those estimated from measurements shows that the modeled SOA has a slightly higher O/C ratio than the observed SOA for all cases.