Methanethiol, dimethyl sulfide and acetone over biologically productive waters in the southwest Pacific Ocean
<p>Atmospheric methanethiol (MeSH<span class="inline-formula"><sub>a</sub></span>), dimethyl sulfide (DMS<span class="inline-formula"><sub>a</sub></span>) and acetone (acetone<span class="inline-formula"><su...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-03-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/20/3061/2020/acp-20-3061-2020.pdf |
| Summary: | <p>Atmospheric methanethiol (MeSH<span class="inline-formula"><sub>a</sub></span>), dimethyl sulfide (DMS<span class="inline-formula"><sub>a</sub></span>) and
acetone (acetone<span class="inline-formula"><sub>a</sub></span>) 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<span class="inline-formula"><sub>a</sub></span> mixing ratios
varied from below the detection limit (<span class="inline-formula"><</span> 10 ppt) up to 65 ppt and were 3 %–36 % of parallel DMS<span class="inline-formula"><sub>a</sub></span> mixing ratios. MeSH<span class="inline-formula"><sub>a</sub></span> and DMS<span class="inline-formula"><sub>a</sub></span> were
correlated over the voyage (<span class="inline-formula"><i>R</i><sup>2</sup>=0.3</span>, slope <span class="inline-formula">=</span> 0.07) with a stronger
correlation over a coccolithophore-dominated phytoplankton bloom (<span class="inline-formula"><i>R</i><sup>2</sup>=0.5</span>, slope 0.13). The diurnal cycle for MeSH<span class="inline-formula"><sub>a</sub></span> shows similar
behaviour to DMS<span class="inline-formula"><sub>a</sub></span> with mixing ratios varying by a factor of
<span class="inline-formula">∼</span> 2 according to time of day with the minimum levels of both
MeSH<span class="inline-formula"><sub>a</sub></span> and DMS<span class="inline-formula"><sub>a</sub></span> 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 <span class="inline-formula">µ</span>mol m<span class="inline-formula"><sup>−2</sup></span> d<span class="inline-formula"><sup>−1</sup></span>, corresponding to 14 %–24 % of the DMS flux (MeSH <span class="inline-formula">∕</span> (MeSH <span class="inline-formula">+</span> DMS)). Spearman rank correlations
with ocean biogeochemical parameters showed a moderate-to-strong positive, highly significant relationship between both MeSH<span class="inline-formula"><sub>a</sub></span> and DMS<span class="inline-formula"><sub>a</sub></span>
with seawater DMS (DMS<span class="inline-formula"><sub>sw</sub></span>) and a moderate correlation with total
dimethylsulfoniopropionate (total DMSP). A positive correlation of
acetone<span class="inline-formula"><sub>a</sub></span> with water temperature and negative correlation with nutrient
concentrations are consistent with reports of acetone production in warmer
subtropical waters. Positive correlations of acetone<span class="inline-formula"><sub>a</sub></span> 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.</p>
<p>In addition, an intercalibration of DMS<span class="inline-formula"><sub>a</sub></span> at ambient levels using three
independently calibrated instruments showed <span class="inline-formula">∼</span> 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<span class="inline-formula"><sub>a</sub></span> 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.</p> |
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| ISSN: | 1680-7316 1680-7324 |