Investigation of natural and anthropogenic effects on aerosols optical properties over the Western Pacific ocean by the research vessel KEXUE

In winter 2018, an aerosol physicochemical experiment was conducted in the Western Pacific Ocean (WPO) aboard the Research Vessel KEXUE of Chinese Academy of Sciences. This study systematically investigated both natural and anthropogenic effects on marine aerosols optical properties, as well as the applicability of multi-satellite products and IMPROVE equation. The averaged aerosol optical depth (AOD_{500 nm}) was 0.31 ± 0.16 and Ångström exponent_{440–675 nm} was 0.29 ± 0.30. In offshore China, significant anthropogenic emissions affected the marine environment. In remote WPO, dust aerosols transported from northern China, Siberia, Central Asia, and those settling from the upper troposphere originating from north Africa, Arabian peninsula, and western India, were dominant. The spatial trends of AOD were opposite in the mid-latitude and southern seas of WPO. The highest AOD, 0.32 ± 0.23, appeared along the coast of South Asia at mid-latitude, decreasing from offshore seas to remote oceans. In low-latitude and equatorial seas, AOD significantly increased from coast to remote oceans. Ångström exponent dropped significantly from the coast to remote oceans as anthropogenic influence diminished across the entire WPO. Correlation analysis showed that both MODIS-C6 and Himawari AOD products showed similar applicability in coastal urban areas, while Himawari AOD is highly recommended for coastal background and marine environment due to its finer resolution. The extinction coefficient derived from PM_2.5 chemical compositions using IMPROVE algorithm exhibited a significant correlation (R² = 0.58) with the concurrently measured AOD in the absence of long-distance transport, suggesting that the IMPROVE is a reasonable proxy of the columnar average of marine aerosol extinctions free from transport influences.