Methanethiol, dimethyl sulfide and acetone over biologically productive waters in the southwest Pacific Ocean

Lawson, SJ, Law, CS, Harvey, MJ, Bell, TG, Walker, CF, de Bruyn, WJ and Saltzman, ES 2020 Methanethiol, dimethyl sulfide and acetone over biologically productive waters in the southwest Pacific Ocean. Atmospheric Chemistry and Physics, 20 (5). 3061-3078.

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Atmospheric methanethiol (MeSHa), dimethyl sulfide (DMSa) and acetone (acetonea) were measured over biologically productive frontal waters in the remote southwest Pacific Ocean in summertime 2012 during the Surface Ocean Aerosol Production (SOAP) voyage. MeSHa mixing ratios varied from below the detection limit (<10ppt) up to 65ppt and were 3%–36% of parallel DMSa mixing ratios. MeSHa and DMSa 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 MeSHa shows similar behaviour to DMSa with mixing ratios varying by a factor of ∼2 according to time of day with the minimum levels of both MeSHa and DMSa occurring at around 16:00LT (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 5.8µmolm−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 MeSHa and DMSa with seawater DMS (DMSsw) and a moderate correlation with total dimethylsulfoniopropionate (total DMSP). A positive correlation of acetonea with water temperature and negative correlation with nutrient concentrations are consistent with reports of acetone production in warmer subtropical waters. Positive correlations of acetonea 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 midlatitude ocean. In addition, an intercalibration of DMSa 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 sulfurchemiluminescencedetector(GC-SCD)and proton transfer reaction mass spectrometer (PTR-MS). Some differences were attributed to the DMSa gradient above the sea surface and differing approaches of integrated versus discrete measurements. Remaining discrepancies were likely due to different calibrationscales,suggesting that further investigation of the stability and/or absolute calibration of DMSstandards used at sea is warranted.

Item Type: Publication - Article
Divisions: Plymouth Marine Laboratory > Science Areas > Marine Biochemistry and Observations
Depositing User: S Hawkins
Date made live: 03 Jun 2020 13:00
Last Modified: 03 Jun 2020 13:02

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