DMS, MeSH and nanoparticles in semi-controlled deck-borne experiments using Antarctic seawater: on the effect of UV light

Abstract. Sulfur-containing volatile organic compounds (VOCs) such as dimethyl sulfide (DMS) and methanethiol (MeSH) are are of particular interest among oceanic VOCs emitted by the ocean, both for their central role in the marine sulfur cycle and as potential precursors to secondary aerosol formation. However, the quantification of DMS and MeSH emissions as a function of biological components of the ocean under variable environmental factors are still too scarce for reliable future predictions. In this study we report on measurements of DMS, MeSH and nanoparticle concentrations in the headspace of two on-deck Air-Sea Interface Tanks (ASITs). The cover of one ASIT prevented the transmission of UV light below 380 nm in wavelength and we report on the effect of UV light on fluxes and concentrations. These measurements were carried out near the Antarctic Peninsula during the POLAR-CHANGE campaign in summer 2023. Air-sea fluxes inside the ASITs were always positive, i.e. degassing from seawater to air, with DMS and MeSH fluxes averaging 3.03 pmol·m⁻²·s⁻¹ (FASIT-DMS) and 0.64 pmol·m⁻²·s⁻¹ (FASIT-MeSH), respectively. DMS emission did not vary significantly between day and night, but the ratio FASIT-MeSH/(FASIT-DMS + FASIT-MeSH) showed a clear maximum at night and a decrease over daytime. Calculated aqueous DMS concentrations showed maxima in the open Southern Ocean north of the Antarctic Peninsula (2.5–3 nM), minima in the Marginal Ice Zone (MIZ) in the Weddell Sea (1 nM) and moderate values along the western coast of the peninsula (around 1.5–2 nM). Cryptophytes, nanophytoplankton, and bacterial concentrations showed positive correlations with calculated aqueous DMS and MeSH concentrations during two experiments when seawater was held in the ASITs for two days. Removal of UV light increased DMS fluxes by 24 % and MeSH fluxes by 58 %. New particle formation occurred only in the absence of UV-light. Interestingly, the highest impact of UV removal, especially on increased MeSH emission, was seen during the night suggesting a lag period between the exposure and the physiological response of the cells. UV light caused slight phytoplankton light stress at noon, which negatively affected the short-term growth of nanophytoplankton in the ASIT, especially in open Southern Ocean waters.