| Sumario: | Global emissions of methanethiol (MeSH) are highly uncertain and the drivers influencing its seawater concentrations are quasi unexplored. Here we try to address this gap in our understanding by contributing new seawater concentration measurements from the Southern Ocean. The POLAR-CHANGE cruise was conducted in February and March 2023 on board the BIO RV Hesperides A33 in waters of the Antarctic Peninsula and the Weddell Sea. A Vocus PTR-ToF (TOFWERK AG, Thun, Switzerland, Vocus Scout) coupled to a segmented flow coil equilibrator (SFCE) was used to measure DMS and MeSH in seawater from the underway seawater inlet. This dataset contains MeSH and DMS surface seawater concentrations and relevant auxiliary data (chlorophyll a, sea surface temperature (SST) and sea surface salinity (SSS)). Sea surface temperature (SST) and sea surface salitity (SSS) were measured in seawater from the underway water inlet located at 4-5 m depth. There was good agreement between the underway SST, SSS and the CTD sensors, confirming that the underway sensors were well calibrated. Underway seawater fluorescence measurements were not calibrated for total chlorophyll a (Chl a) concentration by frequent discrete measurements of Chl a from the on-board 10AU Turner fluorometer. For calibrated Chl a data, please get in touch with the autors. From the beginning of the POLAR-CHANGE cruise we observed extremely high MeSH concentrations which immediately dropped down to within previously observed ranges, after cleaning the equilibrator daily with 10 % HCl. We decided to exclude all MeSH measurements before regular cleaning commenced from 26/02/2023 onwards as these datapoints are likely affected by biofouling. MeSH seawater concentrations prior to this date are replaced by -999 in the dataset shared here. Other gaps in the timestamp of data acquisition are due to calibrations or measuring other samples. Details on the measurement methodology can be found here: Wohl, C.; Güell-Bujons, Q.; Castillo, Y.M.; Calbet, A.; Simó, R. Volatile Organic Compounds Released by Oxyrrhis marina Grazing on Isochrysis galbana. Oceans 2023, 4, 151-169. https://doi.org/10.3390/oceans4020011
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