Photochemistry of the sea-surface microlayer (SML) influenced by a phytoplankton bloom: A mesocosm study

Abstract. The sea-surface microlayer (SML) is the thin boundary interface between the ocean and the atmosphere, and it is expected to play a crucial role in atmospheric chemistry on a global scale. Being a biologically-enriched environment exposed to strong actinic radiation, the SML is potentially a hotspot for photochemical reactions that have relevance in the transformation and cycling of organic compounds. The present study explores the photochemical production and degradation of carbonyl compounds, as well as the photochemical oxidation capacity in both ambient SML and underlying water (ULW) samples. Natural seawater samples were collected during a mesocosm study where a phytoplankton bloom was induced through the controlled addition of inorganic nutrients. To assess the photochemistry of carbonyl compounds, collected SML and ULW samples were irradiated for 5 hours. The photochemical formation and degradation of 17 carbonyl compounds were quantified by monitoring compound-specific changes in concentrations, which varied significantly across the samples. Before irradiation, values in the SML ranged from 201 to 762 nmol L^-1 in the pre-bloom phase, 984 to 4591 nmol L^-1 in the bloom phase, and 647 to 4894 nmol L^-1 in the post-bloom phase; while in the ULW they were significantly lower (e.g., 136 to 366 nmol L^-1 in the bloom phase). After 5 hours of irradiation, the concentrations of carbonyl compounds increased further, reaching up to 6026 nmol L^-1 in the SML during the bloom phase and 419 nmol L^-1 in the ULW. Experimental evidence suggests an enhanced photochemical activity in the SML during the bloom phase for glyoxal, methylglyoxal, methyl vinyl ketone, methacrolein, acrolein, crotonaldehyde, heptanal, biacetyl, hexanal and trans-2-hexenal. The observed photooxidation capacity of the seawater samples indicate a dominant influence of redox active species like metal ions, rather than of the phytoplankton bloom phases. The overall photochemical oxidation capacity was similar for both SML and ULW samples, with average values of 34 μM s^-1. Our findings show an influence of biological activity in the photochemistry of carbonyl compounds in the SML and its implications for the emission of volatile organic compounds (VOCs) to the marine atmosphere, pointing to the complex interaction of biotic and abiotic factors in the air-sea boundary and underscoring the relevance of marine photochemistry in biogeochemical processes.