Optimization of the sulfate aerosol hygroscopicity parameter in WRF-Chem

Abstract. A new sulfate aerosol hygroscopicity parameter ( κ SO 4 ) parameterization is suggested that is capable of considering the two major sulfate aerosols, H2SO4 and (NH4)2SO4 , using the molar ratio of ammonium to sulfate ( R ). An alternative κ SO 4 parameterization method is also suggested that utilizes typical geographical distribution patterns of sulfate and ammonium, which can be used when ammonium data are not available for model calculation. Using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), the impacts of different κ SO 4 parameterizations on cloud microphysical properties and cloud radiative effects in East Asia are examined. Comparisons with the observational data obtained from an aircraft field campaign suggest that the new κ SO 4 parameterizations simulate more reliable aerosol and cloud condensation nuclei concentrations, especially over the sea in East Asia, than the original κ SO 4 parameterization in WRF-Chem that assumes sulfate aerosols as (NH4)2SO4 only. With the new κ SO 4 parameterizations, the simulated cloud microphysical properties and precipitation became significantly different, resulting in a greater cloud albedo effect of about −1.5 W m−2 in East Asia than that with the original κ SO 4 parameterization. The new κ SO 4 parameterizations are simple and readily applicable to numerical studies investigating the impact of sulfate aerosols in aerosol–cloud interactions without additional computational expense.