Method to calculate the aerosol asymmetry factor based on measurements from the humidified nephelometer system

Abstract. The aerosol asymmetry factor (g) is one of the most important factors for assessing direct aerosol radiative forcing. So far, few studies have focused on the measurements and parameterization of g. The characteristics of g are studied based on field measurements over the North China Plain by using the Mie scattering theory. The results show that calculated g values can vary over a wide range (between 0.54 and 0.67). When ambient relative humidity (RH) reaches 90 %, g is significantly enhanced by a factor of 1.2 due to aerosol hygroscopic growth. Direct aerosol radiative forcing can be reduced by 40 % when g increases by 20 %. For the first time, a novel method to calculate g based on measurements from the humidified nephelometer system is proposed. This method can constrain the uncertainty of g within 2 % for dry aerosol populations and 4 % for ambient aerosols, taking into account aerosol hygroscopic growth. Sensitivity studies show that ambient RH and aerosol hygroscopicity are the most important factors that influence the accuracy of predicting g.