The role of chlorine in global tropospheric chemistry

Abstract

Abstract. 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.

Atmospheric Research Over the Western North Atlantic Ocean Region and North American East Coast: A Review of Past Work and Challenges Ahead

“…As noted already, WINTER results showed that the reactive uptake of ClNO 2 on aerosol particles is an important source of Cl 2 during nocturnal periods (Haskins et al, ). When including an advanced representation of tropospheric Cl chemistry into the GEOS‐Chem model, Wang, Jacob, et al () showed that the model could reproduce vertical profiles of HCl from the WINTER campaign, in addition to ClNO 2 being mainly concentrated in the MBL at night. A deficiency was that the model overpredicted Cl 2 at night, which was suggested to be linked to uncertainties in the rate of the heterogeneous reaction between ClNO 2 and Cl ‐ .…”

Potential Effect of Halogens on Atmospheric Oxidation and Air Quality in China

“…Li et al (2016) implemented the ClNO 2 production in WRF‐Chem and reported a significant increase of O 3 levels (up to 7.23 ppbv or 16.3%) in China. Wang et al (2019) updated the chlorine chemistry in GEOS‐Chem and found neutral to negative effect of chlorine chemistry and natural chlorine sources on O 3 in China. Note that Wang et al (2019) did not include the anthropogenic chlorine in their assessment because the use of a global chlorine emission inventory (RCEI, Keene et al, 1999) overestimated the observed chlorine species on a global scale in their model.…”

Is oxidation of atmospheric mercury controlled by different mechanisms in the polluted continental boundary layer vs. remote marine boundary layer?

“…Such enrichment could result, for instance, through GOM transport from the free troposphere via intrusion and boundary layer expansion. These processes may result in Br and Cl enrichment in case that their concentrations are higher in the free troposphere than in the boundary layer, as supported for reactive bromine species (Wang et al 2015, Schmidt et al 2016, Badia et al 2019 and more speculatively for reactive chlorine species (Wilmouth et al 2018, Wang et al 2019.…”