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. Atmospheric methanethiol (MeSH a ), dimethyl sulfide (DMS a ) andacetone (acetone a ) were measured over biologically productive frontalwaters in the remote southwest Pacific Ocean in summertime 2012 during theSurface Ocean Aerosol Production (SOAP) voyage. MeSH a mixing ratiosvaried 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 werecorrelated over the voyage ( R2=0.3 , slope  =  0.07) with a strongercorrelation over a coccolithophore-dominated phytoplankton bloom ( R2=0.5 , slope 0.13). The diurnal cycle for MeSH a shows similarbehaviour to DMS a with mixing ratios varying by a factor of ∼  2 according to time of day with the minimum levels of bothMeSH 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  µ mol m −2  d −1 , corresponding to 14 %–24 % of the DMS flux (MeSH  ∕  (MeSH  +  DMS)). Spearman rank correlationswith 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 totaldimethylsulfoniopropionate (total DMSP). A positive correlation ofacetone a with water temperature and negative correlation with nutrientconcentrations are consistent with reports of acetone production in warmersubtropical waters. Positive correlations of acetone a with cryptophyteand eukaryotic phytoplankton numbers, and high-molecular-weight sugars andchromophoric dissolved organic matter (CDOM), suggest an organic source.This work points to a significant ocean source of MeSH, highlighting theneed for further studies into the distribution and fate of MeSH, andit suggests links between atmospheric acetone levels and biogeochemistry overthe mid-latitude ocean. In addition, an intercalibration of DMS a at ambient levels using threeindependently calibrated instruments showed ∼  15 %–25 % highermixing ratios from an atmospheric pressure ionisation chemical ionisationmass spectrometer (mesoCIMS) compared to a gas chromatograph with a sulfurchemiluminescence detector (GC-SCD) and proton transfer reaction massspectrometer (PTR-MS). Some differences were attributed to the DMS a gradient above the sea surface and differing approaches of integratedversus discrete measurements. Remaining discrepancies were likely due todifferent calibration scales, suggesting that further investigation of thestability and/or absolute calibration of DMS standards used at sea iswarranted.

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