SP1.1 Dynamic enrichment processes of organic matter in the SML

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The sea-surface microlayer (SML) is the uppermost thin layer of the ocean and part of any interaction between the air and sea, such as gas exchange, atmospheric deposition, and aerosol emission. Organic matter (OM) enrichment in the SML modifies air-sea exchange processes, but which OM components become selectively enriched, as well as why and when they do, has remained largely unknown (Engel et al., 2017). Our past research showed that biopolymers derived from photoautotrophic production are important components of the SML and influence air-sea exchange by acting as biosurfactants (Galgani et al., 2016; Engel et al., 2018) and as a source of primary organic aerosols (Trueblood et al., 2021). Therefore, our project’s motivation is to elucidate and describe the dynamic enrichment processes of OM in the SML, with a particular emphasis on resolving OM sources. With our modelling approach the goal is to consolidate our mechanistic understanding of the linkages between plankton growth conditions, production, and the release of biochemicals, including potential surfactants, and the accumulation of OM within the SML. Such model development will greatly benefit from outcomes and findings of diverse subprojects of the BASS consortium. Conversely, it is our motivation to establish a model that becomes applicable as a synthesis tool for the interpretation and integration of field, mesocosm and laboratory measurements of OM enrichment in the SML. Relevance to the research unit BASS – SP1.1 will decipher the sources, amount, and biochemical composition of OM in the SML and thus, provide important information for all BASS subprojects. The primary origin of OM in the surface ocean is photosynthetic production and the major biochemical components of freshly produced OM; i.e., carbohydrates, amino acids, and lipids are subject to microbial processing (SP1.2) and photoreactions within the SML (SP1.3, SP1.4) and also replenish the pool of dissolved organic matter (DOM) (SP1.5). The model development in SP1.1 establishes a linkage between the production of OM and its enrichment within SML, and aims to resolve the corresponding effects on air-sea gas exchange (SP2.1) by accounting for changes in momentum flux on ocean surface layers (SP2.2) as well as in buoyancy (SP2.3). The proposed SML sub-model will be refined based on results from SP1.4 and SP2.3. Results from model sensitivity analyses will provide complementary information about surface-active properties of various OM and their impacts on air-sea exchange processes, which will be evaluated within BASS.