Underway seawater and atmospheric measurements of volatile organic compounds in the Southern Ocean

• Main Authors: C. Wohl, I. Brown, V. Kitidis, A. E. Jones, W. T. Sturges, P. D. Nightingale, M. Yang
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-05-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/17/2593/2020/bg-17-2593-2020.pdf

<p>Dimethyl sulfide and volatile organic compounds (VOCs) are important for atmospheric chemistry. The emissions of biogenically derived organic gases, including dimethyl sulfide and especially isoprene, are not well constrained in the Southern Ocean. Due to a paucity of measurements, the role of the ocean in the atmospheric budgets of atmospheric methanol, acetone, and acetaldehyde is even more poorly known. In order to quantify the air–sea fluxes of these gases, we measured their seawater concentrations and air mixing ratios in the Atlantic sector of the Southern Ocean, along a <span class="inline-formula">∼</span>&thinsp;11&thinsp;000&thinsp;km long transect at approximately 60<span class="inline-formula">^∘</span>&thinsp;S in February–April 2019. Concentrations, oceanic saturations, and estimated fluxes of five simultaneously sampled gases (dimethyl sulfide, isoprene, methanol, acetone, and acetaldehyde) are presented here. Campaign mean (<span class="inline-formula">±1<i>σ</i></span>) surface water concentrations of dimethyl sulfide, isoprene, methanol, acetone, and acetaldehyde were 2.60 (<span class="inline-formula">±3.94</span>), 0.0133 (<span class="inline-formula">±0.0063</span>), 67 (<span class="inline-formula">±35</span>), 5.5 (<span class="inline-formula">±2.5</span>), and 2.6 (<span class="inline-formula">±2.7</span>)&thinsp;nmol&thinsp;dm<span class="inline-formula">⁻³</span> respectively. In this dataset, seawater isoprene and methanol concentrations correlated positively. Furthermore, seawater acetone, methanol, and isoprene concentrations were found to correlate negatively with the fugacity of carbon dioxide, possibly due to a common biological origin. Campaign mean (<span class="inline-formula">±1<i>σ</i></span>) air mixing ratios of dimethyl sulfide, isoprene, methanol, acetone, and acetaldehyde were 0.17 (<span class="inline-formula">±0.09</span>), 0.053 (<span class="inline-formula">±0.034</span>), 0.17 (<span class="inline-formula">±0.08</span>), 0.081 (<span class="inline-formula">±0.031</span>), and 0.049 (<span class="inline-formula">±0.040</span>) ppbv. We observed diel changes in averaged acetaldehyde concentrations in seawater and ambient air (and to a lesser degree also for acetone and isoprene), which suggest light-driven production. Campaign mean (<span class="inline-formula">±1<i>σ</i></span>) fluxes of 4.3 (<span class="inline-formula">±7.4</span>)&thinsp;<span class="inline-formula">µ</span>mol&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;d<span class="inline-formula">⁻¹</span> DMS and 0.028 (<span class="inline-formula">±0.021</span>)&thinsp;<span class="inline-formula">µ</span>mol&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;d<span class="inline-formula">⁻¹</span> isoprene are determined where a positive flux indicates from the ocean to the atmosphere. Methanol was largely undersaturated in the surface ocean with a mean (<span class="inline-formula">±1<i>σ</i></span>) net flux of <span class="inline-formula">−2.4</span> (<span class="inline-formula">±4.7</span>)&thinsp;<span class="inline-formula">µ</span>mol&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;d<span class="inline-formula">⁻¹</span>, but it also had a few occasional episodes of outgassing. This section of the Southern Ocean was found to be a source and a sink for acetone and acetaldehyde this time of the year, depending on location, resulting in a mean net flux of <span class="inline-formula">−0.55</span> (<span class="inline-formula">±1.14</span>)&thinsp;<span class="inline-formula">µ</span>mol&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;d<span class="inline-formula">⁻¹</span> for acetone and <span class="inline-formula">−0.28</span> (<span class="inline-formula">±1.22</span>)&thinsp;<span class="inline-formula">µ</span>mol&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;d<span class="inline-formula">⁻¹</span> for acetaldehyde. The data collected here will be important for constraining the air–sea exchange, cycling, and atmospheric impact of these gases, especially over the Southern Ocean.</p>