Abstract
Abstract. 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 ∼ 11 000 km long transect at approximately 60∘ 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 (±1σ) surface water concentrations of dimethyl sulfide, isoprene, methanol, acetone, and acetaldehyde were 2.60 (±3.94), 0.0133 (±0.0063), 67 (±35), 5.5 (±2.5), and 2.6 (±2.7) nmol dm−3 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 (±1σ) air mixing ratios of dimethyl sulfide, isoprene, methanol, acetone, and acetaldehyde were 0.17 (±0.09), 0.053 (±0.034), 0.17 (±0.08), 0.081 (±0.031), and 0.049 (±0.040) 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 (±1σ) fluxes of 4.3 (±7.4) µmol m−2 d−1 DMS and 0.028 (±0.021) µmol m−2 d−1 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 (±1σ) net flux of −2.4 (±4.7) µmol m−2 d−1, 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 −0.55 (±1.14) µmol m−2 d−1 for acetone and −0.28 (±1.22) µmol m−2 d−1 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.
Highlights
Dimethyl sulfide is a key source of secondary aerosol in the global atmosphere, likely influencing cloud formation and the albedo of the planet (Charlson et al, 1987; Lana et al, 2011)
This paper presents underway seawater and ambient air measurements of simultaneously measured DMS, isoprene, methanol, acetone, and acetaldehyde
The measurements were taken in the Atlantic sector of the Southern Ocean along a 60◦ S transect during the transition from late austral summer to early autumn
Summary
Dimethyl sulfide is a key source of secondary aerosol in the global atmosphere, likely influencing cloud formation and the albedo of the planet (Charlson et al, 1987; Lana et al, 2011). Global oceanic isoprene emissions have been estimated to be 0.31 ± 0.08 Tg yr−1 using seawater concentration data (bottom-up approach) and 1.9 Tg yr−1 using marine air mixing ratios and an atmospheric inversion model (top-down approach) (Arnold et al, 2009). Yang et al (2014b) observed that the acetone flux can be either in or out of the ocean, depending on location This leads to highly uncertain global extrapolations as these authors predict the ocean to be a net sink of −1 Tg yr−1 with a propagated uncertainty of ±19 Tg yr−1. Direct flux measurements from a transatlantic transect suggest that the oceans are both a source and a sink of acetaldehyde (Yang et al, 2014b) These authors estimate the net oceanic emission of acetaldehyde to be much lower, around 3 Tg yr−1 with a propagated uncertainty of ±14 Tg yr−1 (Yang et al, 2014b). These observations represent a valuable dataset of a broad range of gases in a climatically important but under-sampled region
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