Natural and Continental Influence on Aerosol Distributions over the Northwestern Pacific Ocean in the Late Winter and Early Spring of 2021

Aerosols have substantial impacts on climate change and marine ecosystems over the northwestern Pacific Ocean (NWPO); however, their spatiotemporal variability and characteristics have not been sufficiently characterized especially in the East Asian winter monsoon. The compositions and physical parameters of fine-mode aerosols over the NWPO were investigated during a cruise on the research vessel Mirai in the late winter and early spring of 2021. Ship-based high-temporal resolution observations and laboratory analyses of aerosol number concentrations and chemical and elemental compositions revealed the diverse sources and transport processes significantly affecting the behavior of fine-mode aerosol particles in the marine boundary layer (MBL) over the NWPO. The number concentrations of aerosol particles with mobility diameters greater than 100 nm, which were used as a proxy for cloud condensation nuclei (CCN), were generally positively correlated with black carbon and sulfate concentrations throughout the cruise, indicating that even in remote oceans, higher CCN number concentrations were significantly associated with non-natural sources. Latitudinal contrasts in fine-mode aerosol compositions were observed between the subarctic (> 35°N) and subtropical (< 30°N) regions. In the subarctic region, sea-salt (SS) aerosols were dominant in fine-mode mass under windy conditions (up to approximately 30 m s−1) and their mass concentration variations were well explained by the MBL structure and wind–wave conditions. In the subtropical regions, in contrast, the CCN proxy number concentrations and mass concentrations of non-SS aerosols (sulfate, carbonaceous, and metallic components) were elevated due to synoptic-scale transport of continental air masses. As ammonium sulfate was a dominant component in the subtropical regions over the NWPO, the observed enhancement of aerosol concentrations substantially impacted CCN concentrations in this region where is far (1000–1500 km) from continental source regions in Southeast and East Asia. Additionally, volcanic emissions of sulfur compounds contributed significantly to the enhancement of sulfate aerosol mass and particle number concentrations near Nishinoshima, which was not in an eruptive state during the cruise. These results highlight the significances and further requirements of in situ ship-based characterizations to better understand aerosol physicochemical properties and processes, such as sea-spraying emission, in the MBL over the NWPO.