Abstract
Abstract. The oxidation of dimethyl sulfide (DMS; CH3SCH3), emitted from the surface ocean, contributes to the formation of Aitken mode particles and their growth to cloud condensation nuclei (CCN) sizes in remote marine environments. It is not clear whether other less commonly measured marine-derived, sulfur-containing gases share similar dynamics to DMS and contribute to secondary marine aerosol formation. Here, we present measurements of gas-phase volatile organosulfur molecules taken with a Vocus proton-transfer-reaction high-resolution time-of-flight mass spectrometer during a mesocosm phytoplankton bloom experiment using coastal seawater. We show that DMS, methanethiol (MeSH; CH3SH), and benzothiazole (C7H5NS) account for on average over 90 % of total gas-phase sulfur emissions, with non-DMS sulfur sources representing 36.8 ± 7.7 % of sulfur emissions during the first 9 d of the experiment in the pre-bloom phase prior to major biological growth, before declining to 14.5 ± 6.0 % in the latter half of the experiment when DMS dominates during the bloom and decay phases. The molar ratio of DMS to MeSH during the pre-bloom phase (DMS : MeSH = 4.60 ± 0.93) was consistent with the range of previously calculated ambient DMS-to-MeSH sea-to-air flux ratios. As the experiment progressed, the DMS to MeSH emission ratio increased significantly, reaching 31.8 ± 18.7 during the bloom and decay. Measurements of dimethylsulfoniopropionate (DMSP), heterotrophic bacteria, and enzyme activity in the seawater suggest the DMS : MeSH ratio is a sensitive indicator of the bacterial sulfur demand and the composition and magnitude of available sulfur sources in seawater. The evolving DMS : MeSH ratio and the emission of a new aerosol precursor gas, benzothiazole, have important implications for secondary sulfate formation pathways in coastal marine environments.
Highlights
The ocean accounts for the largest natural source of sulfur to the atmosphere, primarily as dimethyl sulfide (DMS; CH3SCH3) (Andreae, 1990; Simó, 2001)
DMSP is present in both particulate (DMSPp) and dissolved (DMSPd) forms, where DMSPp consists of phytoplankton intracellular DMSP, and DMSPd consists of the dissolved pool in the seawater (Kiene and Linn, 2000a)
10 % of the dissolved DMS ventilates to the atmosphere, where it is oxidized by the hydroxyl radical (OH), halogen radicals (Cl and BrO), and nitrate radical (NO3) to form lower-volatility products, including sulfur dioxide (SO2), with yields ranging between 30 % and 100 %, and methanesulfonic acid (MSA) (Carpenter et al, 2012; Chen et al, 2018; Faloona, 2009; Lawson et al, 2020)
Summary
The ocean accounts for the largest natural source of sulfur to the atmosphere, primarily as dimethyl sulfide (DMS; CH3SCH3) (Andreae, 1990; Simó, 2001). Isotopic labeling experiments using the 35S isotope show wide variability in the DMS yield from DMSPd (3 %–50 %) (Carpenter et al, 2012), but the yield is commonly estimated as 10 % (Kiene and Linn, 2000a). This results in waterside DMS concentrations in the range of 1– 7 nM globally, with higher values in the summer and in bloom conditions (Kiene et al, 2000; Kiene and Linn, 2000b; Lana et al, 2011). Atmospheric SO2 is further oxidized to sulfuric acid (H2SO4) and sulfate (SO24−), which can lead to new particle formation, while MSA primarily contributes to particle growth (Carpenter et al, 2012)
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