Photochemical dynamics of carbonyl compounds in the sea-surface microlayer (SML) based on a mesocosm study

The sea-surface microlayer (SML), the thin boundary interface between the ocean and the atmosphere, is of global relevance as oceans are largely assumed to carry an SML. Characterized by its enrichment in organic material and exposure to strong solar radiation, the SML is expected to be a photochemically active zone that plays a critical role in the cycling of organic compounds and that influences air-sea exchange processes. Carbonyl compounds are particularly important as known products of photochemical reactions at the ocean's surface, making their behavior potentially relevant for understanding abiotic reactions and exchanges with the atmosphere. This study investigates the photochemical production and degradation of aldehydes and ketones in both ambient SML and bulk seawater samples. Samples were collected during a mesocosm field campaign at the Sea-surface Facility (SURF), located at the Institute for Chemistry and Biology of the Marine Environment (ICBM) in Wilhelmshaven. To simulate natural conditions, the samples were irradiated for 5 hours using a temperature-controlled aqueous-phase photoreactor equipped with a light source that mimics actinic radiation. The formation and degradation of target carbonyl compounds were analyzed using a derivatization technique with o-(2,3,4,5,6-Pentafluorobenzyl)hydroxylamine (PFBHA), followed by solvent extraction and GC-MS analysis. The findings provide a quantitative evaluation of the formation and degradation dynamics of carbonyl compounds to understand differences between the SML and the underlying bulk seawater. First results suggest the photochemical formation of acetaldehyde and methyl vinyl ketone, and the photochemical degradation of trans-2-hexenal. For other target compounds, including acetophenone, acrolein, butyraldehyde, crotonaldehyde, glyoxal, hexanal, heptanal, hydroxyacetone, methacrolein and propionaldehyde, no consistent trend of formation or degradation was observed. The concentrations of these carbonyl compounds varied significantly depending on the sample, ranging from a few ng L^-1 to a few mg L^-1. This study contributes to a deeper understanding of the role of the SML as a reactive environment and its implications for biogeochemical cycles and air-sea interactions.