║ SWP-A
╙ Air-Sea Exchange Science Questions
Background
Surface seawater DMS concentration, and thus sea-to-air DMS flux, vary substantially as a function of space and time and are particularly influenced by biological activity1. Due to patchy productions from phytoplankton and rapid biological turnover rates, North Atlantic seawater DMS (and its precursor DMSP) concentrations at a given time can be substantially different from current climatological or parameterized best-estimates2.
Biological degradation of DMSP primarily leads to CH3SH, with DMS being only a minor product. Previously, CH3SH has only been considered a small source of sulfur to the atmosphere as it is biologically consumed within just a few hours in surface waters. However, recent studies3,4,5 suggest that a significant amount of CH3SH can be emitted from the ocean to the atmosphere in biologically productive waters with intense blooms of sulfur-producing phytoplankton. These studies show that the magnitude of CH3SH emissions may be as much as a third of the DMS emissions, and that the CH3SH:DMS ratio varies depending on the stage of the phytoplankton bloom.
A major atmospheric oxidation product of natural marine sulfur emissions is SO2, yet the atmospheric loss terms of SO2 are poorly constrained. A few direct flux measurements by the eddy covariance (EC) method have been made from an aircraft6 and from the coast7,8. These direct flux measurements show that SO2 deposition is substantially lower than model predictions by as much as ~30%. However, SO2 dry deposition flux to the ocean has never been measured directly over the open ocean from a ship. Ship-based EC measurements of SO2 deposition flux to the open ocean offer the opportunity to significantly improve our understanding of this key process.
Air-sea fluxes of DMS and CH3SH may be calculated with a bulk method by using atmospheric and seawater concentrations and the gas transfer velocity. However, recent studies also show that the gas transfer velocity may be impacted by waves8,9,10 and biological surfactants11,12. The impact of waves and surfactants may be very different for the transfer of SO2 (a surface reactive and largely airside controlled gas) compared to the transfer of DMS and CH3SH (both sparingly soluble and waterside controlled).
Hypotheses
H1.1: Marine biological activity has a substantial influence on the emission and deposition of sulfur gases by (i) determining the concentrations of these gases in seawater and (ii) causing the release of surfactants, which alter the gas transfer velocity vs. wind speed relationship.
H1.2: The CH3SH:DMS ratios (in seawater concentration and in sea-to-air flux) vary inside/outside of phytoplankton blooms and depend on the stage of bloom.
H1.3: Impacts of waves and surfactants on gas exchange are stronger for waterside controlled gases (DMS & CH3SH) than for airside-controlled gases (SO2).
Workplan
CARES will address H1.1 and H1.2 by quantifying in situ seawater DMS and CH3SH concentrations and emissions, using these to constrain the input terms to the atmosphere in the region of interest. We will assess the variations in the DMS:CH3SH ratio (in concentrations and fluxes) both inside and outside of different phytoplankton blooms, as well as at different stages of the bloom.
To address H1.3, our team will directly measure DMS and SO2 fluxes with the eddy covariance (EC) method and compare the measurements against the bulk fluxes of these gasses to assess whether local processes (such as wave conditions, phytoplankton blooms) have a significant impact on air-sea gas exchange.
To clarify the impact of biological activity on the chemical reactions of SO2 near the sea surface, we will take simultaneous EC measurements of air-sea SO2 and water vapour exchange inside/outside of blooms. Finally, the sulfur fluxes will be compared with emission estimates existing climatologies and model concentration fields.
Associated Deliverables
D1.1: DMS and CH3SH emissions into the atmosphere within the study area.
D1.2: Paper(s) on SO2 deposition to the sea surface within the study area.
D1.3: Parameterization of the air-sea transfer velocity of DMS (applicable also to CH3SH) and the deposition velocity of SO2 as a function of wind/wave parameters.
D1.4: Simple parameterization of DMS:CH3SH emission ratio inside/outside of blooms.
References
- Lana et al., Global Biogeochemical Cycles, 2011 ↩︎
- Bell et al., Frontiers in Marine Science, 7, 2021 ↩︎
- Lawson et al., Atmos. Chem. Phys, 20(5), 2020 ↩︎
- Kilgour et al., Atmos. Chem. Phys. Discuss., 2021 ↩︎
- Novak, et al., Atmos. Chem. Phys. Discuss., 2021 ↩︎
- Faloona, Atmos. Env., 43(18), 2009 ↩︎
- Porter et al., Atmos. Chem. Phys., 2018 ↩︎
- Porter et al., GRL, 47, 2020 ↩︎
- Bell et al., Atmos. Chem. Phys., 13, 2013 ↩︎
- Brumer et al., GRL., 44, 2017 ↩︎
- Pereira et al., Nature Geosci, 11,2018 ↩︎
- Yang, et al., Sci Rep, 11, 2021 ↩︎
SWP-A is led by Dr Mingxi Yang and Prof Tom Bell
CARES 