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.
A nitrogenase-like enzyme system catalyzes methionine, ethylene, and methane biogenesis