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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. 

Abstract. We present a comprehensive simulation of tropospheric chlorinewithin the GEOS-Chem global 3-D model of oxidant–aerosol–halogen atmosphericchemistry. The simulation includes explicit accounting of chloridemobilization from sea salt aerosol by acid displacement of HCl and by otherheterogeneous 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 theHCl+OH reaction and by heterogeneous conversion of sea salt aerosolchloride to BrCl, ClNO2, Cl2, and ICl. The modelsuccessfully simulates the observed mixing ratios of HCl in marine air(highest at northern midlatitudes) and the associated HNO3decrease from acid displacement. It captures the high ClNO2 mixingratios observed in continental surface air at night and attributes thechlorine to HCl volatilized from sea salt aerosol and transported inlandfollowing uptake by fine aerosol. The model successfully simulates thevertical profiles of HCl measured from aircraft, where enhancements in thecontinental boundary layer can again be largely explained by transport inlandof the marine source. It does not reproduce the boundary layer Cl2mixing ratios measured in the WINTER aircraft campaign (1–5 ppt in thedaytime, low at night); the model is too high at night, which could be due touncertainty in the rate of the ClNO2+Cl- reaction, but we haveno explanation for the high observed Cl2 in daytime. The globalmean tropospheric concentration of Cl atoms in the model is 620 cm−3and contributes 1.0 % of the global oxidation of methane, 20 % ofethane, 14 % of propane, and 4 % of methanol. Chlorine chemistryincreases global mean tropospheric BrO by 85 %, mainly through theHOBr+Cl- reaction, and decreases global burdens of troposphericozone by 7 % and OH by 3 % through the associated bromine radicalchemistry. ClNO2 chemistry drives increases in ozone of up to8 ppb over polluted continents in winter. 

Abstract We use observations from the 2015 Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER) aircraft campaign to constrain the proposed mechanism of Cl₂production from ClNO₂reaction in acidic particles. To reproduce Cl₂concentrations observed during WINTER with a chemical box model that includes ClNO₂reactive uptake to form Cl₂, the model required the ClNO₂reaction probability, γ (ClNO₂), to range from 6 × 10⁻⁶to 7 × 10⁻⁵, with a mean value of 2.3 × 10⁻⁵(±1.8 × 10⁻⁵). These field‐determined γ (ClNO₂) are more than an order of magnitude lower than those determined in previous laboratory experiments on acidic surfaces, even when calculated particle pH is ≤2. We hypothesize this is because thick salt films in the laboratory enhanced the reactive uptake ClNO₂compared to that which would occur in submicron aerosol particles. Using the reacto‐diffusive length‐scale framework, we show that the field and laboratory observations can be reconciled if the net aqueous‐phase reaction rate constant for ClNO₂(aq) + Cl^‐(aq) in acidic particles is on the order of 10⁴s⁻¹. We show that wet particle diameter and particulate chloride mass together explain 90% of the observed variance in the box model‐derived γ (ClNO₂), implying that the availability of chloride and particle volume limit the efficiency of the reaction. Despite a much lower conversion of ClNO₂into Cl₂, this mechanism can still be responsible for the nocturnal formation of 10–20 pptv of Cl₂in polluted regions, yielding an atmospherically relevant concentration of Cl atoms the following morning.