Modeling and observational constraints on tropospheric sulfur-halogen interactions
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Sulfur and reactive halogens are key components in tropospheric chemistry and the radiation budget of the Earth. Tropospheric sulfur and halogen budgets are still not well constrained. Laboratory and modeling studies have revealed the important role of halogen species in the tropospheric sulfur cycle, such as the oxidation of dimethyl sulfide (DMS) by XO and the oxidation of S(IV) by HOX (X=Cl, Br and I). However, the feedbacks of sulfur-halogen interactions on the halogen cycle have raised little attention. My Ph.D. research involves laboratory measurements and model developments to understand the interaction between sulfur and reactive halogens (especially bromine) in the troposphere, and implications for the global sulfur and reactive bromine budgets. In the first part my Ph.D. project (Chapter 2), I measured oxygen isotopes of sulfate (Δ17O) collected in the remote marine boundary layer (MBL), which reveal the signature of sulfate formed via aqueous-phase oxidation of S(IV) by HOX. Assisted by the GEOS-Chem chemical transport model, we calculated that HOX are responsible for 33-50% of sulfate formation in the remote Southern Hemisphere MBL during spring and summer. This provides the first observational constraint on the contribution of reactive halogens to sulfate aerosol formation. In the second part of my Ph.D. project (Chapter 3), I implemented HOBr+S(IV) reaction into GEOS-Chem to evaluate the contribution of this reaction to both sulfate production and reactive bromine (Bry) removal in the troposphere for the first time. This reaction reduces the global reactive bromine burden by 50% and contributes to significant sulfate formation even at sub-ppt levels of HOBr. The reduction in Bry resulting from HOBr+S(IV) has implication for the oxidative capacity of the atmosphere through the increase in O3 and OH abundances. In the third part of my Ph.D. project (Chapter 4), I explored another aspect of tropospheric sulfur-halogen interactions: oxidation of DMS by BrO and Cl radicals. I added 2 new sulfur intermediates dimethyl sulfoxide (DMSO) and methane sulphinic acid (MSIA) as well as 12 new reactions in GEOS-Chem involving the formation of sulfate and methane sulfonic acid (MSA) from the oxidation of DMS in both the gas- and aqueous-phase. BrO and Cl account for 12% and 4% of global DMS oxidation, respectively, with large uncertainties stemming from uncertainties in tropospheric BrO and Cl abundances. Multiphase chemistry in cloud droplets and aerosols is critical for the production and removal of MSA in the troposphere. Our updates decrease the conversion yield of DMS to SO2 from 91% to 78% and increase the conversion yield of DMS to MSA from 9% to 13%. We suggest that previous climate models with simplified DMS oxidation scheme (gas-phase oxidation by OH and NO3 only) may overestimate SO2 and sulfate production in the pre-industrial environment, potentially leading to underestimates in sulfate aerosol radiative forcing calculations in climate models, and that MSA may not be a good ice-core proxy for DMS emissions from sea ice melt. The large uncertainties on reactive halogen abundances are the largest impediment for our understanding of tropospheric sulfur-halogen interactions. To improve our understanding of MBL sulfur chemistry, we need to prioritize measurements of reactive halogen abundances, especially in the marine boundary layer.
- Atmospheric sciences