Methanethiol, Dimethyl Sulfide and Acetone Over Biologically Productive Waters in the Southwest Pacific Ocean

Authors

Sarah J. Lawson, ommonwealth Scientific and Industrial Research Organisation (CSIRO), Australia
Cliff S. Law, National Institute of Water and Atmospheric Research (NIWA), New Zealand
Mike J. Harvey, National Institute of Water and Atmospheric Research (NIWA), New Zealand
Thomas G. Bell, Plymouth Marine Laboratory, United Kingdom
Carolyn F. Walker, National Institute of Water and Atmospheric Research (NIWA), New Zealand
Warren J. De Bruyn, Chapman UniversityFollow
Eric S. Saltzman, University of California, Irvine

Document Type

Article

Publication Date

3-16-2020

Abstract

Atmospheric methanethiol (MeSH_a), dimethyl sulfide (DMS_a) and acetone (acetone_a) were measured over biologically productive frontal waters in the remote southwest Pacific Ocean in summertime 2012 during the Surface Ocean Aerosol Production (SOAP) voyage. MeSH_a mixing ratios varied from below the detection limit (< 10 ppt) up to 65 ppt and were 3 %–36 % of parallel DMS_a mixing ratios. MeSH_a and DMS_a were correlated over the voyage (R²=0.3, slope = 0.07) with a stronger correlation over a coccolithophore-dominated phytoplankton bloom (R²=0.5, slope 0.13). The diurnal cycle for MeSH_a shows similar behaviour to DMS_a with mixing ratios varying by a factor of ∼ 2 according to time of day with the minimum levels of both MeSH_a and DMS_a occurring at around 16:00 LT (local time, all times in this paper are in local time). A positive flux of MeSH out of the ocean was calculated for three different nights and ranged from 3.5 to 5.8 µmol m⁻² d⁻¹, corresponding to 14 %–24 % of the DMS flux (MeSH ∕ (MeSH + DMS)). Spearman rank correlations with ocean biogeochemical parameters showed a moderate-to-strong positive, highly significant relationship between both MeSH_a and DMS_a with seawater DMS (DMS_sw) and a moderate correlation with total dimethylsulfoniopropionate (total DMSP). A positive correlation of acetone_a with water temperature and negative correlation with nutrient concentrations are consistent with reports of acetone production in warmer subtropical waters. Positive correlations of acetone_a with cryptophyte and eukaryotic phytoplankton numbers, and high-molecular-weight sugars and chromophoric dissolved organic matter (CDOM), suggest an organic source. This work points to a significant ocean source of MeSH, highlighting the need for further studies into the distribution and fate of MeSH, and it suggests links between atmospheric acetone levels and biogeochemistry over the mid-latitude ocean.

In addition, an intercalibration of DMS_a at ambient levels using three independently calibrated instruments showed ∼ 15 %–25 % higher mixing ratios from an atmospheric pressure ionisation chemical ionisation mass spectrometer (mesoCIMS) compared to a gas chromatograph with a sulfur chemiluminescence detector (GC-SCD) and proton transfer reaction mass spectrometer (PTR-MS). Some differences were attributed to the DMS_a gradient above the sea surface and differing approaches of integrated versus discrete measurements. Remaining discrepancies were likely due to different calibration scales, suggesting that further investigation of the stability and/or absolute calibration of DMS standards used at sea is warranted.

Comments

This article was originally published in Atmospheric Chemistry and Physics, volume 20, in 2020. https://doi.org/10.5194/acp-20-3061-2020

Recommended Citation

Lawson, S. J., Law, C. S., Harvey, M. J., Bell, T. G., Walker, C. F., de Bruyn, W. J., and Saltzman, E. S.: Methanethiol, dimethyl sulfide and acetone over biologically productive waters in the southwest Pacific Ocean, Atmos. Chem. Phys., 20, 3061–3078, https://doi.org/10.5194/acp-20-3061-2020, 2020.

Copyright

The authors

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This work is licensed under a Creative Commons Attribution 4.0 License.