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 predicted net global SOA production rate in cloud water ranges from 13.1 Tg yr−1 to 46.8 Tg yr−1 while that in aerosol water ranges from −0.4 Tg yr−1 to 12.6 Tg yr−1. The predicted global burden of SOA formed in the aqueous phase ranges from 0.09 Tg to 0.51 Tg. A sensitivity test to investigate two representations of cloud water content from two global models shows that increasing cloud water by an average factor of 2.7 can increase the net SOA production rate in cloud water by a factor of 4 at low altitudes (below approximately 900 hPa). We also investigated the importance of including dissolved Fe chemistry in cloud water aqueous reactions. Adding these reactions increases the formation rate of aqueous-phase OH by a factor of 2.6 and decreases the amount of global aqueous SOA formed by 31%. None of the mechanisms discussed here is able to provide a best fit for all observations. Rather, the use of an uptake coefficient method for aerosol water and a multi-phase scheme for cloud water provides the best fit in the Northern Hemisphere and the use of multiphase process scheme for aerosol and cloud water provides the best fit in the tropics. The model with Fe chemistry underpredicts oxalate measurements in all regions. Finally, the comparison of oxygen-to-carbon (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.
Highlights
Secondary organic aerosol (SOA) has been shown to be an important component of non-refractory submicron aerosol in the atmosphere (Zhang et al, 2007; Jimenez et al, 2009)
For Case 1, the net global aqSOA production rate totals 20.1 Tg yr−1, over 95 % of which is removed by wet deposition while the rest is removed by dry deposition
This rate is comparable to the SOA production rate of 28.0 Tg yr−1 formed from gas–particle partitioning and the rate of 26.0 Tg yr−1 formed from epoxide predicted in the model
Summary
Secondary organic aerosol (SOA) has been shown to be an important component of non-refractory submicron aerosol in the atmosphere (Zhang et al, 2007; Jimenez et al, 2009). The observed O / C ratios in aged ambient organic aerosol (OA) cannot be explained using measured O / C ratios in dry smoke chamber experiments (Aiken et al, 2008; Ng et al, 2010). One method that has been used to help close the gap between measured and modeled SOA is a refined treatment for primary organic aerosol (POA) that allows them to evaporate and further oxidize (Robinson et al, 2007; Pye and Seinfeld, 2010; Hodzic et al, 2010; LeeTaylor et al, 2011). There are large uncertainties in how to treat the evaporation rate as well as the oxidation mechanism for POA and the SOA yield from this source (Pye and Seinfeld, 2010; Spracklen et al, 2011)
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