Understanding and improving model representation of aerosol optical properties for a Chinese haze event measured by KORUS-AQ

KORUS-AQ was an International Cooperative Air Quality Field Study in Korea that measured local and remote sources of air pollution affecting the Korean peninsula during May-June 2016. Some of the largest aerosol concentrations were measured during a Chinese haze transport event. Air quality forecasts using the WRF-Chem model assimilating satellite data captured aerosol optical depth (AOD) during the period but overpredicted surface particulate matter concentrations, especially PM2.5 often by factor of 2 or larger. Analysis showed good model representation of vertical distribution of the haze layer against airborne lidar data but revealed multiple sources of uncertainties related to conversion of aerosol mass to optical properties. Driving the optical properties calculations with in-situ observations of aerosol size and composition showed that using a coarse size bin representation underestimates the efficiency at which aerosols produce extinction. Besides using finer size bins, it's also necessary to increase refractive indexes and hygroscopicity of select aerosol species (within values reported in the literature) to achieve consistency with measured values of mass/volume extinction efficiencies and light scattering enhancement factor (f(RH)) due to aerosol hygroscopic growth. Furthermore, evaluation of optical properties obtained using modeled aerosol properties revealed the inability of sectional and modal aerosol representations in WRF-Chem to properly reproduce the observed size distribution. Other model uncertainties will be discussed. These results represent a step into improving model representation of optical properties that could improve air quality predictions and aerosol radiative forcing estimates. It also shows future applications that a platform like A-CCP could facilitate by providing optical and microphysical vertically-resolved aerosol properties.