Abstract
Abstract. 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 (< 10 ppt) up to 65 ppt 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 16:00 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 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 mid-latitude 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 sulfur chemiluminescence detector (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 calibration scales, suggesting that further investigation of the stability and/or absolute calibration of DMS standards used at sea is warranted.
Highlights
Volatile organic compounds (VOCs) are ubiquitous in the atmosphere, and they have a central role in processes affecting air quality and climate, via their role in formation of secondary organic aerosol and tropospheric ozone
This section describes a comparison of DMSa measurements from bag samples of ambient air and dimethyl sulfide (DMS) standard mixtures, as well as comparison of ambient DMSa measurements (PTR-MS and mesoCIMS)
Measurements from the proton transfer reaction mass spectrometer (PTR-MS) and mesoCIMS were interpolated to a common time stamp for comparison, and differences were examined only where data were available for both instruments
Summary
Volatile organic compounds (VOCs) are ubiquitous in the atmosphere, and they have a central role in processes affecting air quality and climate, via their role in formation of secondary organic aerosol and tropospheric ozone. The role of the ocean in the global cycle of several VOCs is becoming increasingly recognised, with recent studies showing that the ocean serves as a major source, sink, or both for many pervasive and climate-active VOCs (Law et al, 2013; Liss and Johnson, 2014; Carpenter and Nightingale, 2015). The ocean is a major source of reduced volatile sulfur gases and the most well-studied of these is dimethyl sulfide (DMS) (CH3SCH3), with a global ocean source of ∼ 28 Tg S a−1 (Lee and Brimblecombe, 2016). Methanethiol or methyl mercaptan (MeSH) (CH3SH) is another reduced volatile organic sulfur gas which originates in the ocean, with a global ocean source estimated to be ∼ 17 % of the DMS source (Lee and Brimblecombe, 2016). The importance of ocean-derived MeSH as a source of sulfur to the atmosphere, and the impact of MeSH and its oxidation products on atmospheric chemistry and climate, is not well understood
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