SWP-B

║ SWP-B

╙ Atmospheric Processes

Background

The atmospheric oxidation of reduced sulfur gases, like DMS and CH₃SH, ultimately leads to the formation of sulfate. However, the specific pathways and their rates determine the aerosol yield and the climate impacts. There remains uncertainty in the role of different oxidants in DMS oxidation. This uncertainty in oxidising agents leads to uncertainty over the products of DMS oxidation¹^,².

There is even greater uncertainty in the formation and fate of HPMTF (which may help reconcile uncertainty in the DMS-to-SO₂ yield)³, as well as a poor understanding of the products from HPMTF–OH oxidation, the role of other oxidants of HPMTF (Cl, NO₃, etc.) and its photolysis, and the products of in clouds HPMTF processing.

Similarly, CH₃SH is a potentially important – and as yet poorly quantified – source of sulfur to the marine atmosphere⁴^,⁵. Little is known about the contribution of CH₃SH to aerosol formation or growth. Understanding the CH₃SH contribution to SO₂ production, as well as the (currently unknown) fate of HPMTF are key focus points of CARES.

Hypotheses

H2.1: CH₃SH makes a significant contribution to the reactive sulfur burden in the remote marine atmosphere.4

H2.2: Autoxidation of DMS to produce HPMTF is a major pathway for DMS oxidation in the marine boundary layer.

H2.3: HPMTF and CH₃SH enhance SO₂ production and may stimulate new particle formation in the clean, cloud-free marine boundary layer.

H2.4: HPMTF leads to rapid increase in sulfate mass on existing particles in the cloudy marine boundary layer.

H2.5: The presence of NO_x (e.g., from ship emissions) leads to a reduction in HPMTF during the day but an increase at night via the nitrate radical.

Workplan

To address H2.1-2.5, CARES will make measurements of the main sulfur species on board the research ship and FAAM aircraft, alongside a wide range of other important gas phase and aerosol measurements. Targeting air travel over regions of high biological activity and using PTR-ToF-MS to detect CH₃SH will provide a model constraint to allow the determination of CH₃SH contribution to SO₂.

Ship-based measurements will enable the diel cycle of key species to be explored over a range of environmental conditions to test the temperature dependence and effects of NO_x on the oxidation mechanisms.

The combination of ship-based and aircraft measurements allows the role of boundary layer and free troposphere processes to be investigated through constrained box modelling. Airborne fluxes (above, in, and below cloud) of SO₂ and HPMTF will be made simultaneously for the first time and will act as further constraints in modelling of boundary layer chemical processes.

Actinic flux observations, coupled with absorption cross sections for the functional groups present in HPMTF, will enable us to determine the photolysis rates for HPMTF and derive the photolytic products. HCHO observations will enable the loss of HPMTF via OH oxidation to be derived as well as the DMS oxidation rate via OH (H2.2).

CIMS measurements, calculations of the Cl atom concentration, box model calculations, and continuous measurements of H₂O₂ will allow us to assess the loss of DMS due to BrO, the rate of DMS oxidation via Cl atom reaction, the contribution of NO₃, as well as estimate the heterogeneous oxidation rate of SO₂, and probe whether H₂O₂ is also a key oxidant for HPMTF.

We will use aerosol surface area and composition, coupled with laboratory studies, to provide the loss rate of HPMTF by aerosol uptake, which will be used to constrain box modelling of the loss rate of HPMTF to clouds. This will also allow us to determine the products deriving from HPMTF aqueous phase processing.

LES modelling will be used to predict cloud structure and entrainment rates Trajectories derived from the LES will drive the environmental conditions of a cloud box model to assess cloud processing. Ensembles of these trajectories will be averaged to produce a vertical profile for comparison with measured aircraft profiles and fluxes to examine the role of cloud losses (H2.3-2.4).

Flights will target both polluted and pristine regions and analysis of the correlations of HPMTF (or HPMTF:DMS ratio) with NO_x levels and temperature will provide model constraints to investigate the prevalence of the autoxidation channel under a range of atmospheric conditions (H2.5).

Associated Deliverables

SWP-B delivers paper(s) on:

D2.1: Improved constraints on the products of HPMTF oxidation from gaseous and aqueous/heterogeneous processes.

D2.2: Estimate of the OCS indirect source from HPMTF photolysis.

D2.3: First quantification of the importance of CH₃SH in marine sulfur chemistry during the spring time phytoplankton bloom in the North Atlantic.

References

Faloona, Atmos. Env., 43(18), 2009 ↩︎
Hodshire et al., Atmos. Chem. Phys., 19, 2019 ↩︎
Vermeuel et al., Env. Sci. & Tech., 54(19), 2020 ↩︎
Atkinson et al., Atmos. Chem. Phys., 4, 2004 ↩︎
Novak, et al., Atmos. Chem. Phys. Discuss., 2021 ↩︎

SWP-B is led by Dr Lisa Whalley and Dr Pete Edwards

SWP-A

SWP-C