The distributions and direct radiative effects of marine aerosols over East Asia in springtime

The characteristics, distributions, and direct radiative effects (DRE) of marine aerosols in the western Pacific Ocean over East Asia during the period from 17 March to 22 April 2014 were investigated by an online-coupled regional atmospheric chemistry/aerosol-climate model RIEMS-Chem (Regional Integrated Environmental Model System with Chemistry). The emissions and relevant processes of sea salt, marine primary organic aerosol (MPOA), sulfate and Methyl sulfonic acid (MSA) produced from dimethylsulfide (DMS) were parameterized and coupled with RIEMS-Chem. The model results for total aerosol masses (PM₁₀ and PM_2.5), inorganic and carbonaceous aerosols, gas precursors, and aerosol optical depth (AOD) were compared with various observational data sets including a research cruise Dongfanghong II from the Yellow Sea to the open oceans, near-surface aerosol and gas concentrations from the Acid Deposition Monitoring Network in East Asia (EANET) and China National Environmental Monitoring Center (CNEMC), and AOD from the Aerosol Robotic Network (AERONET). Model comparisons demonstrated a generally good skill of the RIEMS-Chem in representing the temporal and spatial variations of these variables. The distributions of marine aerosols were characterized by the maximum sea salt concentration up to 70 μg m^-3 in the ocean northeast of Japan, the maximum concentration of MPOA >2 μg m^-3 in the East China Sea and in portions of the northwest Pacific (NWP) region, and the maximum DMS-produced aerosol concentration >0.3 μg m^-3 in the southern parts of the ocean. It was noteworthy that marine aerosols can be easily transported to the inland areas of south China. The clear-sky DREs by sea salt ranging from -9 to -17 W/m² occurred in the open oceans northeast of Japan, comparable to the DREs of -10 ~ -20 W/m² by anthropogenic aerosols, whereas the DREs by MPOA were strongest (up to -1.3 Wm^-2) in the East China Sea and the oceans northeast of Japan due to active phytoplankton blooms there and comparable in magnitude to the DREs by sea salt (around -3 Wm^-2) in the East China Sea. The maximum DRE by the DMS-produced aerosols was -0.4 Wm^-2 mainly in the northern parts of the South China Sea. Sea salt exhibited an increasing radiative importance from the China marginal seas to the open oceans, accounting for 10% and 33% of the DREs by all aerosols, respectively. Under all-sky conditions, the sum of DREs by all the marine aerosols were estimated to be -2.2 W/m², -3.5 W/m², -2.3 W/m², and -4.3 W/m² averaged over the entire domain, ocean, East China Sea, and the NWP region, accounting for 20%, 27%, 13%, and 36% of the DREs by all aerosols, respectively, which demonstrated the important role of marine aerosols in modulating shortwave radiation in springtime in the western Pacific Ocean which was just downwind of the Asian continent with large amounts of anthropogenic and dust emissions.