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1. The role of chlorine in global tropospheric chemistry

   Wang, X. (January 2019, Atmospheric chemistry and physics)

   We present a comprehensive simulation of tropospheric chlorine within the GEOS-Chem global 3-D model of oxidant–aerosol–halogen atmospheric chemistry. The simulation includes explicit accounting of chloride mobilization from sea salt aerosol by acid displacement of HCl and by other heterogeneous processes. Additional small sources of tropospheric chlorine (combustion, organochlorines, transport from stratosphere) are also included. Reactive gas-phase chlorine Cl*, including Cl, ClO, Cl2, BrCl, ICl, HOCl, ClNO3, ClNO2, and minor species, is produced by the HCl+OH reaction and by heterogeneous conversion of sea salt aerosol chloride to BrCl, ClNO2, Cl2, and ICl. The model successfully simulates the observed mixing ratios of HCl in marine air (highest at northern midlatitudes) and the associated HNO3 decrease from acid displacement. It captures the high ClNO2 mixing ratios observed in continental surface air at night and attributes the chlorine to HCl volatilized from sea salt aerosol and transported inland following uptake by fine aerosol. The model successfully simulates the vertical profiles of HCl measured from aircraft, where enhancements in the continental boundary layer can again be largely explained by transport inland of the marine source. It does not reproduce the boundary layer Cl2 mixing ratios measured in the WINTER aircraft campaign (1–5 ppt in the daytime, low at night); the model is too high at night, which could be due to uncertainty in the rate of the ClNO2+Cl− reaction, but we have no explanation for the high observed Cl2 in daytime. The global mean tropospheric concentration of Cl atoms in the model is 620 cm−3 and contributes 1.0 % of the global oxidation of methane, 20 % of ethane, 14 % of propane, and 4 % of methanol. Chlorine chemistry increases global mean tropospheric BrO by 85 %, mainly through the HOBr+Cl− reaction, and decreases global burdens of tropospheric ozone by 7 % and OH by 3 % through the associated bromine radical chemistry. ClNO2 chemistry drives increases in ozone of up to 8 ppb over polluted continents in winter. 

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2. Global tropospheric halogen (Cl, Br, I) chemistry and its impact on oxidants

   https://doi.org/10.5194/acp-21-13973-2021  

   Wang, Xuan; Jacob, Daniel J.; Downs, William; Zhai, Shuting; Zhu, Lei; Shah, Viral; Holmes, Christopher D.; Sherwen, Tomás; Alexander, Becky; Evans, Mathew J.; et al (January 2021, Atmospheric Chemistry and Physics)

   Abstract. We present an updated mechanism for tropospheric halogen (Cl + Br + I) chemistry in the GEOS-Chem global atmospheric chemical transportmodel and apply it to investigate halogen radical cycling and implications for tropospheric oxidants. Improved representation of HOBr heterogeneouschemistry and its pH dependence in our simulation leads to less efficient recycling and mobilization of bromine radicals and enables the model toinclude mechanistic sea salt aerosol debromination without generating excessive BrO. The resulting global mean tropospheric BrO mixingratio is 0.19 ppt (parts per trillion), lower than previous versions of GEOS-Chem. Model BrO shows variable consistency and biases in comparison tosurface and aircraft observations in marine air, which are often near or below the detection limit. The model underestimates the daytimemeasurements of Cl2 and BrCl from the ATom aircraft campaign over the Pacific and Atlantic, which if correct would imply a very largemissing primary source of chlorine radicals. Model IO is highest in the marine boundary layer and uniform in the free troposphere, with a globalmean tropospheric mixing ratio of 0.08 ppt, and shows consistency with surface and aircraft observations. The modeled global meantropospheric concentration of Cl atoms is 630 cm−3, contributing 0.8 % of the global oxidation of methane, 14 % of ethane,8 % of propane, and 7 % of higher alkanes. Halogen chemistry decreases the global tropospheric burden of ozone by 11 %,NOx by 6 %, and OH by 4 %. Most of the ozone decrease is driven by iodine-catalyzed loss. The resulting GEOS-Chem ozonesimulation is unbiased in the Southern Hemisphere but too low in the Northern Hemisphere. 

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3. Effects of Sea Salt Aerosol Emissions for Marine Cloud Brightening on Atmospheric Chemistry: Implications for Radiative Forcing

   https://doi.org/10.1029/2019GL085838  

   Horowitz, Hannah_M; Holmes, Christopher; Wright, Alicia; Sherwen, Tomás; Wang, Xuan; Evans, Mat; Huang, Jiayue; Jaeglé, Lyatt; Chen, Qianjie; Zhai, Shuting; et al (February 2020, Geophysical Research Letters)

   Abstract Marine cloud brightening (MCB) is proposed to offset global warming by emitting sea salt aerosols to the tropical marine boundary layer, which increases aerosol and cloud albedo. Sea salt aerosol is the main source of tropospheric reactive chlorine (Cl_y) and bromine (Br_y). The effects of additional sea salt on atmospheric chemistry have not been explored. We simulate sea salt aerosol injections for MCB under two scenarios (212–569 Tg/a) in the GEOS‐Chem global chemical transport model, only considering their impacts as a halogen source. Globally, tropospheric Cl_yand Br_yincrease (20–40%), leading to decreased ozone (−3 to −6%). Consequently, OH decreases (−3 to −5%), which increases the methane lifetime (3–6%). Our results suggest that the chemistry of the additional sea salt leads to minor total radiative forcing compared to that of the sea salt aerosol itself (~2%) but may have potential implications for surface ozone pollution in tropical coastal regions. 

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4. Significant Decrease in Wet Deposition of Anthropogenic Chloride Across the Eastern United States, 1998–2018

   https://doi.org/10.1029/2020GL090195  

   Haskins, Jessica D.; Jaeglé, Lyatt; Thornton, Joel A. (November 2020, Geophysical Research Letters)

   Abstract Using deposition observations from precipitation samples collected by the National Atmospheric Deposition Program at 125 sites across the United States, we show that the mean wet deposition flux of non‐sea‐salt chloride (NSS Cl⁻) has decreased by 83% throughout the eastern United States between 1998 and 2018. We find that 30% of the sites switch from having excess Cl⁻to being depleted in Cl⁻. We attribute the observed decreases in NSS Cl⁻deposition to a 95% decrease in U.S. anthropogenic HCl emissions since 1998. We propose that industry emission controls that remove HCl as a cobenefit of NO_xand SO₂have caused significant decreases in NSS Cl⁻deposition throughout the eastern United States, in addition to shifts from coal to natural gas and to coal with lower Cl⁻content. Our analysis implies that the lower tropospheric reactive inorganic chlorine burden was larger over the United States in the past than it is today. 

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5. Measurements and Modeling of the Interhemispheric Differences of Atmospheric Chlorinated Very Short‐Lived Substances

   https://doi.org/10.1029/2023JD039518  

   Roozitalab, Behrooz; Emmons, Louisa K; Hornbrook, Rebecca S; Kinnison, Douglas E; Fernandez, Rafael P; Li, Qinyi; Saiz‐Lopez, Alfonso; Hossaini, Ryan; Cuevas, Carlos A; Hills, Alan J; et al (January 2024, Journal of Geophysical Research: Atmospheres)

   Abstract Chlorinated very short‐lived substances (Cl‐VSLS) are ubiquitous in the troposphere and can contribute to the stratospheric chlorine budget. In this study, we present measurements of atmospheric dichloromethane (CH₂Cl₂), tetrachloroethene (C₂Cl₄), chloroform (CHCl₃), and 1,2‐dichloroethane (1,2‐DCA) obtained during the National Aeronautics and Space Administration (NASA) Atmospheric Tomography (ATom) global‐scale aircraft mission (2016–2018), and use the Community Earth System Model (CESM) updated with recent chlorine chemistry to further investigate their global tropospheric distribution. The measured global average Cl‐VSLS mixing ratios, from 0.2 to 13 km altitude, were 46.6 ppt (CH₂Cl₂), 9.6 ppt (CHCl₃), 7.8 ppt (1,2‐DCA), and 0.84 ppt (C₂Cl₄) measured by the NSF NCAR Trace Organic Analyzer (TOGA) during ATom. Both measurements and model show distinct hemispheric gradients with the mean measured Northern to Southern Hemisphere (NH/SH) ratio of 2 or greater for all four Cl‐VSLS. In addition, the TOGA profiles over the NH mid‐latitudes showed general enhancements in the Pacific basin compared to the Atlantic basin, with up to ∼18 ppt difference for CH₂Cl₂in the mid troposphere. We tagged regional source emissions of CH₂Cl₂and C₂Cl₄in the model and found that Asian emissions dominate the global distributions of these species both at the surface (950 hPa) and at high altitudes (150 hPa). Overall, our results confirm relatively high mixing ratios of Cl‐VSLS in the UTLS region and show that the CESM model does a reasonable job of simulating their global abundance but we also note the uncertainties with Cl‐VSLS emissions and active chlorine sources in the model. These findings will be used to validate future emission inventories and to investigate the fast convective transport of Cl‐VSLS to the UTLS region and their impact on stratospheric ozone. 

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