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

Lawson, Sarah J.; Law, Cliff S.; Harvey, Mike J.; Bell, Thomas G.; Walker, Carolyn F.; de Bruyn, Warren J.; Saltzman, Eric S.

Atmospheric methanethiol (MeSHinline-formulaa), dimethyl sulfide (DMSinline-formulaa) and acetone (acetoneinline-formulaa) were measured over biologically productive frontal waters in the remote southwest Pacific Ocean in summertime 2012 during the Surface Ocean Aerosol Production (SOAP) voyage. MeSHinline-formulaa mixing ratios varied from below the detection limit (inline-formula< 10 ppt) up to 65 ppt and were 3 %–36 % of parallel DMSinline-formulaa mixing ratios. MeSHinline-formulaa and DMSinline-formulaa were correlated over the voyage (inline-formulaR2=0.3, slope inline-formula= 0.07) with a stronger correlation over a coccolithophore-dominated phytoplankton bloom (inline-formulaR2=0.5, slope 0.13). The diurnal cycle for MeSHinline-formulaa shows similar behaviour to DMSinline-formulaa with mixing ratios varying by a factor of inline-formula∼ 2 according to time of day with the minimum levels of both MeSHinline-formulaa and DMSinline-formulaa 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 inline-formulaµmol minline-formula−2 dinline-formula−1, corresponding to 14 %–24 % of the DMS flux (MeSH inline-formula∕ (MeSH inline-formula+ DMS)). Spearman rank correlations with ocean biogeochemical parameters showed a moderate-to-strong positive, highly significant relationship between both MeSHinline-formulaa and DMSinline-formulaa with seawater DMS (DMSinline-formulasw) and a moderate correlation with total dimethylsulfoniopropionate (total DMSP). A positive correlation of acetoneinline-formulaa with water temperature and negative correlation with nutrient concentrations are consistent with reports of acetone production in warmer subtropical waters. Positive correlations of acetoneinline-formulaa 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 DMSinline-formulaa at ambient levels using three independently calibrated instruments showed inline-formula∼ 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 DMSinline-formulaa 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.

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Lawson, Sarah J. / Law, Cliff S. / Harvey, Mike J. / et al: Methanethiol, dimethyl sulfide and acetone over biologically productive waters in the southwest Pacific Ocean. 2020. Copernicus Publications.

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