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Dataset abstract The authors would highly appreciate being contacted if the data is used for any purpose. We measured methanesulfonic acid concentration in the gas phase with a nitrate chemical ionization mass spectrometer (we used an APi-HTOF mass spectrometer produced by Tofwerk AG coupled with a Chemical ionization inlet A70 produced by Airmodus). Methanesulfonic acid is detected in the mass spectrometer either as a deprotonated ion or as a cluster with the reagent ion (NO3-). The concentration is calculated as the area of these two peaks normalized to the concentration of the reagent ions (monomer, dimer and trimer) and multiplied by a calibration factor that was experimentally derived at Paul Scherrer Institute in the summer 2017, after the campaign. In the atmosphere, methanesulfonic acid is produced by the oxidation of dimethyl sulfide that is mostly emitted in the atmosphere by phytoplankton. Methanesulfonic acid can contribute to the growth of aerosol particles by condensation or aqueous phase processing in clouds. The data have been cleaned from the influence of the exhaust of the research vessel. Temporal coverage is from 22 January 2017 to 19 March 2017. There are no data for the first leg of the expedition because the instrument was not on the ship. The instrument was operated during leg 4 but data has not been processed yet as they require additional work due to some instrumental issues that affects their quality. Data were collected with one-second time resolution but integrated to five minutes to increase the signal-to-noise ratio. Concentrations are reported as molecules per cubic centimeter. The lower limit of detection is in the ppq (part per quadrillion) range but was not determined exactly as it varies with the integration time and background noise. Dataset contents gas_methanesulfonic_acid_concentration_data.csv, data file, comma-separated values data_file_header.txt, metadata, text format README.txt, metadata, text format change_log.txt, metadata, text format There are no null values in the file. Data that were missing or bad because of instrumental problems were simply removed from the file (no entry). Change log v1.1 - data files updated re-analysis of the raw data with an improved mass spectra peak fitting routine, a different calibration factor was also applied to the data following a more detailed analysis of the calibration experiment use updated, corrected GPS data v1.0 - initial release of dataset

The Antarctic Circumnavigation Expedition was made possible by funding from the Swiss Polar Institute and Ferring Pharmaceuticals. We acknowledge the Swiss National Sciences Foundation grant number 200021_169090 and the German Research Foundation (DFG) within SSP 1158 (Grant no. STR 453/12-1).