Sea-to-air transfer of dissolved organic carbon via sea spray aerosol during phytoplankton bloom

Abstract. The formation of sea spray aerosol (SSA) is linked to wave-breaking events at the sea surface and is widely recognized as an important pathway for the transfer of marine substances to the atmosphere. Although climate change and sea eutrophication have led to the expansion and intensification of coastal phytoplankton blooms, systematic studies on the sea-to-air transfer of dissolved organic carbon (DOC) via SSA during phytoplankton blooms are still lacking, which hinders the understanding of SSA's atmospheric chemistry and climate impacts. In this study, we induced a phytoplankton bloom using coastal seawater and employed various characterization tools to investigate the sea-to-air transfer of DOC. During the phytoplankton bloom, the dynamic accumulation of DOC in coastal seawater led to fluctuations in the number concentration and mean geometric diameter of SSA by approximately 60 % and 30 %, respectively; in the meantime, the enrichment factors of DOC in the sea surface microlayer, supermicron SSA, and submicron SSA can increase up to ∼5-fold, 10-fold, and 30-fold, respectively. The sea-to-air transfer of DOC depended on its selective enrichment as well as the fractionation process at the air-water interface. Interestingly, the particulate property of operationally defined DOC still needed to be considered during SSA formation. Additionally, the sea-to-air transfer of DOC was influenced by the synergistic effects of phytoplankton production and heterotrophic microbial processing, rather than being solely dependent on chlorophyll a concentration. Compared to previous studies, this work focuses on the sea-air interface, systematically and comprehensively elucidating the relationships between DOC's transfer mechanisms, biological activity, and SSA formation. This will further improve our understanding of the ocean-atmosphere carbon cycle and provide insights into its impact on global climate change.