. Improving simple parameterization of secondary organic aerosol (SOA): IEPOX-SOA and urban SOA

Abstract
Secondary organic aerosol (SOA) is a major component of fine aerosols globally, but it is the least understood aerosol due to its complex chemistry. SOA schemes in global models have shown large discrepancies when compared to the observed values because of the limited capability to simulate SOA. Recent modeling studies with the full explicit chemistry showed good agreements against the observed SOA concentrations. However, the full explicit modeling is computationally expensive owing to the many species and reactions tracked, which makes it difficult to include it in chemistry climate models for long-term studies. In addition, the full explicit chemical mechanisms for SOA have not been revealed yet especially for urban SOA. Here we present two simple parameterizations for IEPOX-SOA and urban SOA, to accurately simulate SOA concentrations while being computationally efficient. We propose a new parameterization for IEPOX-SOA based on an approximate analytical / fitting solution of the SOA yield and formation timescale. The yield and timescale can be directly calculated using the global model fields of oxidants, NO, aerosol pH and other key properties, and dry deposition rates. The advantage of the parameterizations is that they do not require the simulation of the intermediates while retaining the key physico-chemical dependencies. Urban SOA is calculated using the approach proposed by Hodzic and Jimenez (2011) which assumes irreversible condensation of the oxidation products of volatile organic compounds. We use the GEOS-Chem global model to intercompare the parameterizations and the detailed full chemistry. The new parameterization captures the global tropospheric burden of IEPOX-SOA and it spatio-temporal distribution vs. those simulated by the full chemistry. Global implications on health effects are also discussed through premature death calculations for the case of regulating urban SOA precursors.