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

Among the wide variety of VOCs emitted by the oceans, sulfur-containing compounds such as dimethyl sulfide (DMS) and methanethiol (MeSH) can be particularly important due to their prominent role in the marine sulfur cycle and their fate as secondary aerosol precursors. 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 natural DMS, MeSH and nanoparticle concentrations within the deckborne Air-Sea Interfacial Tanks (ASITs) and the effect of UV light on their fluxes and concentrations. These measurements were carried out near the Antarctic Peninsula during the PolarChange campaign in 2023. DMS dissolved concentrations showed maxima in the open Southern Ocean north of the peninsula (2.5-3 nM), minima in the Marginal Ice Zone (MIZ) (1 nM) and moderate along the western coast of the peninsula (around 1.5-2 nM). Fluxes measured inside the ASITs were always positive, i.e. degassing from seawater to air, with equivalent 2 m·s-1 wind speed fluxes averaged from 3.03 pmol·m⁻²·s⁻¹ for DMS to 0.64 pmol·m⁻²·s⁻¹ for MeSH. DMS emissions did not vary significantly between day/night conditions, however the ratio of MeSH to DMS did have a clear maximum at night and a decrease around midday. Cryptophytes, nanophytoplankton, and bacterial concentrations showed positive links with dissolved DMS and MeSH concentrations during the experiments. A clear negative impact of UV light on DMS and MeSH fluxes was observed with DMS net fluxes 24% higher and MeSH net fluxes 58% higher in UV light filtered ASIT, and on new particle formation that surprisingly occurred only in the absence of UV light. Interestingly, the highest impact of UV, especially on MeSH emissions, was seen during the night. UV light had also a negative impact on the development of nanophytoplankton especially in Open Southern Ocean waters, and a slight increase in phytoplankton stress at noon .