Concentration of gaseous iodic acid measured over the Southern Ocean in the austral summer of 2016/2017, during the Antarctic Circumnavigation Expedition (ACE). ***** Dataset abstract ***** Measurements of iodic acid concentration in the gas phase obtained with a nitrate chemical ionization mass spectrometer (we used an APi-TOF mass spectrometer produced by Tofwerk AG coupled with a Chemical ionization inlet A70 produced by Airmodus). Iodic 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 equal to 6.9E9 molecules cm-3 that was experimentally derived at Paul Scherrer Institute in the summer 2017, after the campaign. Iodic acid can participate in both new particle formation and growth, affecting the Earth radiative balance and cloud properties. Iodic acid is produced from the iodine radical but the exact formation pathways is still unknown. Measurements were performed on the upper deck of the icebreaker Akademik Tryoshnikov along the track of the Antarctic Circumnavigation expedition. Temporal coverage is from January 22, 2017 to March 19, 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. 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 in five minutes averages. The lower limit of detection was estimated to be lower than 6E3 molecules cm-3. Data below the detection limit were replaced by the detection limit divided by the square root of 2. ***** Original data collection ***** We operated a Tofwerk APi-HTOF mass spectrometer coupled with a Chemical ionization inlet A70 produced by Airmodus. We used nitric acid as reagent ion. Description of the instrument and the inlet are provided by Junninen et al., 2010 and by Jokinen et al., 2012. The instrument was core-sampling from a non-heated inlet that was composed of a vertical straight stainless steel tube (5 cm internal diameter and 180 cm long) with a U-shape bend at the top to prevent rain and water from entering the line. The total flow rate in the line was about 70 liters per minute. The instrument was placed in an aerosol container constructed by the Paul Scherrer Institute situated on the foredeck of the R/V Akademik Tryoshnikov. More about this set-up can be found in the cruise report (Walton and Thomas, 2018). Mass spectra are collected summing up the current pulses generated by the micro channel plate detector of the instrument. These high frequency pulses are averaged directly on the Analog to Digital Converter (ADC) board and sent to the computer as one-second time resolution data. Raw spectra were stored as hdf5 files in real-time on the instrument computer situated in the PSI laboratory container. Data were collected using TofDAQ software, version 1.96 (https://www.tofwerk.com/software/tofdaq/). ***** Data processing ***** Iodic 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. The instrument was calibrated directly only for sulfuric acid but since also iodic acid has a proton affinity that is lower than nitric acid we can assume that the ionization proceeds at the kinetic limit and the calibration factor should be the same. A detailed description of the calibration procedure and results is provided in Baccarini, 2021 and in Baccarini et al., 2021. The concentration was corrected for diffusional losses using the diffusion coefficient measured by Hanson & Eisele, 2000 for sulfuric acid. This choice was made because to the best of our knowledge there are no experimental studies on the diffusion coefficient of iodic acid and the difference between these two species is expected to be small. The original spectra were integrated to a five-minute resolution to improve the signal-to-noise ratio. The date and time are reported as for the start time of each five-minute averaging period. The latitude and longitude data are obtained from Landwehr, Thomas & Schmale 2020. Data were processed in Python 3.6 and Matlab R2016B using tofTools R611 (http://www.junninen.net/tofTools/). ***** Quality checking ***** Data were inspected to insure good quality. All the periods affected by instrumental issues were removed (e.g. issues with the mass spectrometer or the inlet flow rate). Pollution from the ship exhaust and other human activities (e.g. helicopter flights) was identified as described in Schmale et al. 2019. Data affected by the ship exhaust are flagged as ‘0’ and good data are marked as ‘1’. However, an analysis of the campaign data (Baccarini et al, 2021) revealed that human activities on the ship did not affect iodic acid concentration implying that data during polluted periods can also be used. The instrument experienced some issues during the campaign (we had to replace the TOF power supply during leg 2) and it was operated without the electric field on the nitrate chemical ionization inlet from the 4th of February until the 10th of March 2017. Due to the fact that we couldn’t calibrate the instrument on board we are not able to accurately estimate the effect of these changes on the instrument sensitivity. However, the different instrumental settings were tested during the calibration experiments and produced results within the experimental error, therefore we are confident that the same calibration can be used for the entire dataset. The error of accuracy for this instrument is estimated to be +100%/-50%. The lower limit of detection is estimated to be lower than 6E3 molecules cm-3. Data below the detection limit were replaced by the detection limit divided by the square root of 2. ***** Standards ***** We follow the best practice that we are aware of in the respective scientific community. ***** Further information for interpreting the data and using the dataset ***** Timescales: Iodic acid has a generally short lifetime (minutes to hours), it is produced from the photolysis of iodine precursor molecules (e.g. I2 or CH2I2) and lost by condensation to aerosol particles or other surfaces. Iodic acid has a clear diurnal cycle with low concentrations during night. Interpolation: It is possible to upsample data to a higher time resolution if needed but the result should be carefully evaluated. Interpolation of missing values for more than 2/3 values should be avoided. Aggregation to a lower temporal resolution: This depends on the research question. The data set features a number of environmental processes that happen on timescales from minutes to days. Averaging will eliminate shorter term signals, but could emphasize longer term features. See Baccarini et al., 2021 and Schmale et al., 2019 for more information. ***** Dataset contents ***** - 01_gas_iodic_acid_concentration_data.csv, data file, comma-separated values - 02_iodic_acid_file_header.txt, metadata, text - README.txt, metadata, text ***** Dataset contact ***** Andrea Baccarini, École Polytechnique Fédérale de Lausanne, Sion, Switzerland. ORCID: 0000-0003-4614-247X. Email: andrea.baccarini@epfl.ch Julia Schmale, École Polytechnique Fédérale de Lausanne, Sion, Switzerland. ORCID: 0000-0002-1048-7962. Email: Julia.schmale@epfl.ch ***** Dataset license***** Creative Commons Attribution 4.0 International.