[A-CG41] Biogeochemical linkages between the surface ocean and atmosphere

[ACG41-03] Origin and formation process of marine atmospheric organic aerosols during the spring phytoplankton bloom in the southern Sea of Okhotsk

*Yuzo Miyazaki¹, Yunhan Wang^1,2, Eri Tachibana¹, Youhei Yamashita^2,3, Koji Suzuki^2,3, Jun Nishioka¹ (1.Institute of Low Temperature Science, Hokkaido University, 2.Graduate School of Environmental Science., Hokkaido University, 3. Faculty of Environmental Earth Science)

Keywords:Marine atmospheric organic aerosol, The Sea of Okhotsk, Sea ice melting, Biogeochemical linkage between the atmosphere and the ocean

Marine atmospheric aerosols act as cloud condensation nuclei and ice nuclei, controlling the atmospheric radiative budget through cloud formation, and thus play a vital role in the climate system. The ocean surface is a major source of submicrometer aerosols in both number and mass concentrations, which contain a complex mixture of organic matter (OM) and inorganic components. The Sea of Okhotsk is one of the most biologically productive regions in the world’s oceans, which lies at the lowest latitude among the seasonal sea ice areas. After sea ice melting, the primary production during spring phytoplankton blooms is expected to contribute to the atmospheric emissions of OM from the sea surface. It is essential to elucidate the effects of oceanic biological activity in the blooms on the amount and composition of sea spray aerosol (SSA) as well as secondary organic aerosols (OAs). However, there is a large uncertainty in our understanding of the effects of phytoplankton blooms on the formation of primary SSA and marine secondary OAs, particularly over the seasonal sea ice areas. The objective of this study is to elucidate how oceanic biological activity during the bloom in the southern Sea of Okhotsk affects the amount and formation process of atmospheric OAs in terms of water-solubility which is an important parameter for cloud formation.
Ambient submicrometer aerosol and surface seawater (SSW) samples were collected in April 2021, on board the R/V Shinsei Maru in the Sea of Okhotsk. The entire cruise was characterized by two periods, which were defined as the bloom and bloom-decay periods. The chemical analysis of aerosol samples showed that OM was dominant (51±14%) in the submicrometer aerosol mass, where OM was found to be more water-soluble. Stable carbon isotope ratios of organic carbon (OC) (δ¹³C_OC) showed that 71% and 64% of the observed aerosols were of marine origin during the bloom and bloom-decay periods, respectively. Correlations between concentrations of water-soluble OC (WSOC) and those of molecular tracers suggested that the majority of ocean-derived WSOC was affected by secondary formation from precursors, such as α-pinene and dimethylsulfide (DMS)-relevant compounds, instead of primary emission as SSA. Furthermore, lower C:N ratios in water-soluble OAs (WSOAs) during the bloom period indicated the preferential formation of nitrogen-containing WSOAs in aerosols associated with marine microbial activity. Indeed, the average dissolved OC (DOC): dissolved organic nitrogen (DON) ratio in SSW during the bloom (12.8) was lower than that during the bloom-decay (14.7) period, where DOC and DON during the bloom period were likely associated with the predominated diatoms, such as Fragilariopsis cylindrus (Watanabe, 2022), in the Sea of Okhotsk during the study period. This study points to the importance of the secondary formation of OAs associated with ice algae in the bloom, which should be taken into account in evaluating the effect of marine OAs on the regional cloud formation.