Concentration of gaseous methanesulfonic acid measured over the Southern Ocean in the austral summer of 2016/2017, during the Antarctic Circumnavigation Expedition (ACE).

***** 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.

***** 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/).

***** 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).

Data affected by the ship exhaust are flagged as ‘0’ and good data are marked as ‘1’. The influence of the ship exhaust was identified based on the spikiness of the total particle number concentration, the CO2 and black carbon concentrations (10.5281/zenodo.2636690, 10.5281/zenodo.2636779, 10.5281/zenodo.2636763, respectively). Moreover, data showing high and spurious sulfuric acid concentrations were also flagged as influenced by the ship exhaust.

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 time series don’t show any systematic drift and the data looks consistent throughout the campaign, therefore we are confident that the calibration factor measured after the campaign can be used for this dataset.

The error of accuracy for this instrument is usually estimated to be +100%/-50% however an accurate uncertainty analysis has not been performed yet for this campaign. This work will be done in the future with the next version of the dataset and reported here as well.

A detailed analysis on the data quality will be reported in the paper about new particle formation in the Southern Ocean that should be ready by the end of 2019.

***** Data processing *****

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. The instrument was calibrated directly only for sulfuric acid but since also methanesulfonic 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.

The concentration was corrected for diffusional losses using the diffusion coefficient measured by Hanson and 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 methanesulfonic acid. Moreover, these two species have a similar molecular mass and the difference should 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 are reported as the median values of one-minute GPS data corresponding to the five-minute averaging period. Corrected cruise track data were used for the latitude and longitude (Thomas and Pina Estany, 2019). 

Data were processed in Python 2.7 and Matlab R2016B using tofTools R611 (http://www.junninen.net/tofTools/). 

***** Standards *****

We follow the best practice that we are aware of in the respective scientific community.

***** Further information for interpreting and using the dataset *****

Limit of detection: The limit of detection (LOD) is below 10^5 molecules per cubic centimetre, but was not accurately determined as such low values are not usually interesting for atmospheric application. In case someone is interested in the concentration below this limit, they should contact the data curator: eventually the integration time could be increased to reduce the LOD.

Timescales: The timescales of the main phenomena vary from hours to day (regional transport or production from dimethyl sulfide (DMS) oxidation). The lifetime of methanesulfonic acid is longer compared to sulfuric acid and generally it doesn’t show a clear diurnal cycle.

Interpolation: In theory is it possible to upsample data to a higher time resolution if needed but this requires some care. Interpolation of missing values can generally lead to generation of non-meaningful values, however an exception should be possible if only 2/3 points are missing.

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 hours to days. Averaging will eliminate shorter term signals, but could emphasize longer term features. 

See Schmale et al. (2019) “Overview of the Antarctic Circumnavigation Expedition Study of Preindustrial-like Aerosols and their Climate Effects (ACE-SPACE)” for more information. 

***** 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

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). 

***** Dataset contact *****

Andrea Baccarini, Paul Scherrer Institute, Villigen, Switzerland. ORCID: 0000-0003-4614-247X. Email: andrea.baccarini@psi.ch

***** Dataset citation *****

Please cite this dataset as:
Baccarini, A., Schmale, J., Henning, S., Tummon, F., Hartmann, M., Welti, A., Lehtipalo, K., Dommen, J. and Gysel-Beer., M. (2019). Concentration of gaseous methanesulfonic acid measured over the Southern Ocean in the austral summer of 2016/2017, during the Antarctic Circumnavigation Expedition (ACE). (Version 1.0) [Data set]. Zenodo. https://doi.org/10.5281/zenodo.2636771