Seasonal characteristics of emission, distribution and radiative effect of marine organic aerosols over the western Pacific Ocean: an analysis combining observations with regional modeling

Abstract. Organic aerosols from marine sources over the western Pacific Ocean of East Asia were investigated by using an online-coupled regional chemistry-climate model RIEMS-Chem for the entire year 2014. Model evaluation against a wide variety of observations from research cruises and in-situ measurements demonstrated a good skill of the model in simulating temporal variation and spatial distribution of particulate matter with aerodynamic diameter less than 2.5 μm and 10 μm (PM_2.5 and PM₁₀), black carbon (BC), organic carbon (OC), and aerosol optical depth (AOD) in marine atmosphere. The inclusion of marine organic aerosols apparently improved model performance on OC aerosol concentration, reducing the normalized mean biases from −19 % to −13 % (KEXUE-1 cruise) and −21 % to −3 % (Huaniao Island) over the marginal seas of east China, and from 33 % to 5 % (Dongfanghong II cruise) and from −13 % to 3 % (Chichijima Island) over remote oceans of the western Pacific. It was found that marine primary organic aerosol (MPOA) accounted for majority of marine organic aerosol (MOA) mass in the western Pacific. High MPOA emission mainly occurred over the marginal seas of China and remote oceans of the western Pacific northeast of Japan. The seasonality of MPOA emission is determined by the combined effect of Chlorophyll-a (Chl-a) concentration and sea salt emission flux, exhibiting the maximum in autumn and the minimum in summer in terms of domain average over the western Pacific. The annual mean MPOA emission rate was estimated to be 0.16×10⁻² μg m⁻² s⁻¹, yielding an annual MPOA emission of 0.78 Tg yr⁻¹ over the western Pacific, which potentially accounted for approximately 8~12 % of global annual MPOA emission. The regional and annual mean near surface MOA concentration was estimated to be 0.27 μg m⁻³ over the western Pacific, with the maximum in spring and the minimum in winter, resulting from the combined effect of MPOA emission, dry and wet depositions. Marine secondary organic aerosol (MSOA) produced by marine biogenic VOCs (isoprene and monoterpene) was approximately 1~2 orders of magnitude lower than MPOA. The simulated annual and regional mean MSOA was 2.2 ng m⁻³, with the maximum daily mean value up to 28 ng m⁻³ over the western Pacific in summer. MSOA had a distinct summer maximum and winter minimum in the western Pacific, generally consistent with the seasonality of marine isoprene emission flux. In terms of annual mean, 26 % of the total organic aerosol concentration was contributed by MOA over the western Pacific, with an increasing importance of MOA from the marginal seas of China (13 %) to remote oceans of the western Pacific (42 %). MOA induced a minor direct radiative effect (DRE), with a domain and annual mean of −0.21 W m⁻² at the top of the atmosphere (TOA) under all-sky condition over the western Pacific, whereas the mean indirect radiative effect (IRE) due to MOA at TOA (IREMOA) was estimated to be −4.2 W m⁻². MSOA contributed approximately 6 % of the annual and regional mean IREMOA over the western Pacific, with the maximum seasonal mean contribution up to 14 % in summer, which meant MPOA dominated the IREMOA. It was noteworthy that the IREMOA accounted for approximately 32 % of that due to all aerosols over the western Pacific of East Asia, indicating an important role of MOA in perturbing cloud properties and shortwave radiation in this region.