Marine gas-phase sulfur emissions during an induced phytoplankton bloom

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 nine days 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.

Oceanic emissions of dimethyl sulfide and methanethiol and their contribution to sulfur dioxide production in the marine atmosphere

“…However, this result highlights that biological activity can drive variations in dissolved ratios of DMS and MeSH resulting in variability in ambient FDMS:FMeSH emission ratios, and that further study is needed to elucidate this mechanism and its spatiotemporal variability. Measurements during an induced mesocosm phytoplankton bloom experiment using seawater collected as SIO pier immediately before this study showed that the ratio of gas phase DMS to MeSH varied by more than a factor of five over the course of a phytoplankton bloom and decay (Kilgour et al, 2021). DMS to MeSH ratios 350 in that study were strongly linked to changes in bacterial sulfur demand and changes in the available pool of dissolved sulfur…”

“…Heterogeneous chemistry of the DMS oxidation product HPMTF is not included in our base model case. HPMTF 415 heterogeneous chemistry has been proposed to be a potentially large sink for HPMTF which would reduce SO2 production from DMS (Novak et al, 2021;Veres et al, 2020;Vermeuel et al, 2020). These details of HPMTF heterogeneous chemistry do not impact the yield of SO2 from MeSH described previously but do impact the calculated relative production of SO2 from MeSH compared to DMS.…”

“…HPMTF has been shown to be globally ubiquitous in the marine boundary layer from airborne observations (Veres et al, 2020) and at a coastal ocean ground site (Vermeuel et al, 2020). Global chemical transport modelling shows that 46% of emitted DMS goes on to form HPMTF (Novak et al, 2021). The atmospheric fate of HPMTF is an active topic of research but ambient observations show that dry deposition to the ocean surface is a significant loss term (lifetime ~30 hours, (Vermeuel et al, 2020)) and that HPMTF is efficiently lost to clouds (Novak et al, 2021;Veres et al, 2020;Vermeuel et al, 2020), 95 resulting in a 35% decrease in global SO2 production from DMS oxidation (Novak et al, 2021).…”

“…Global chemical transport modelling shows that 46% of emitted DMS goes on to form HPMTF (Novak et al, 2021). The atmospheric fate of HPMTF is an active topic of research but ambient observations show that dry deposition to the ocean surface is a significant loss term (lifetime ~30 hours, (Vermeuel et al, 2020)) and that HPMTF is efficiently lost to clouds (Novak et al, 2021;Veres et al, 2020;Vermeuel et al, 2020), 95 resulting in a 35% decrease in global SO2 production from DMS oxidation (Novak et al, 2021). These previously unconsidered https://doi.org/10.5194/acp-2021-891 Preprint.…”

Exploring DMS oxidation and implications for global aerosol radiative forcing

“…Marine dimethyl sulfide (DMS: CH3SCH3) accounts for >50% of natural gas-phase sulfur emissions (Chin et al, 1996;Andreae, 1990;Kilgour et al, 2021). Once emitted into the troposphere, oxidation of DMS takes place within 1-2 days, forming other sulfur-containing products such as sulfuric acid (H2SO4) and methane sulfonic acid (MSA: CH3SO3H) (Boucher et al, 2003;Breider et al, 2010).…”