The role of chlorine in tropospheric chemistry

Abstract

<p><strong>Abstract.</strong> 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 sources of tropospheric chlorine (combustion, organochlorines, transport from stratosphere) are small in comparison. Reactive gas-phase chlorine Cl*, including Cl, ClO, Cl<sub>2</sub>, BrCl, ICl, HOCl, ClNO<sub>3</sub>, ClNO<sub>2</sub>, and minor species, is produced by the HCl + OH reaction and by heterogeneous conversion of sea-salt aerosol chloride to BrCl, ClNO<sub>2</sub>, Cl<sub>2</sub>, and ICl. The model simulates successfully the observed mixing ratios of HCl in marine air (highest at northern mid-latitudes) and the associated HNO<sub>3</sub> decrease from acid displacement. It captures the high ClNO<sub>2</sub> mixing ratios observed in continental surface air at night with chlorine of sea salt origin transported inland as HCl and fine aerosol. It simulates successfully the vertical profiles of HCl measured from aircraft, where enhancements in the continental boundary layer can again be explained by transport inland of the marine source. It does not reproduce the boundary layer Cl<sub>2</sub> mixing ratios measured in the WINTER aircraft campaign (1–5 ppt in the daytime, low at night); the model is too high at night compared to WINTER observations, which could be due to uncertainty in the rate of the ClNO<sub>2</sub> + Cl<sup>−</sup> reaction, but we have no explanation for the daytime observations. The global mean tropospheric concentration of Cl atoms in the model is 620 cm<sup>−3</sup> 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<sup>−</sup> 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.</p>

Evaluating the impact of blowing snow sea salt aerosol on springtime BrO and O<sub>3</sub> in the Arctic

“…We hypothesize that including acid displacement HCl from SSA would lead to an increase in HOBr because of the competition between HOBr+Cl -(which recycles HOBr by producing BrCl) and HOBr+S(IV) (which is a sink for HOBr) in cloud droplets. Indeed, in a subsequent 265 version of GEOS-Chem, Wang et al (2018) added this source of HCl, finding an increase in BrO, especially in cloudy high latitudes. and OMI exhibit enhanced VCDtropo (3.5-5×10 13 cm -2 ) in an arc between Baffin Bay -along the west coast of Greenland -and 270 the Laptev Sea, off the Northern coast of Siberia.…”

Observational Constraints on the Formation of Cl2 From the Reactive Uptake of ClNO2 on Aerosols in the Polluted Marine Boundary Layer

“…Several recent observational studies have concluded that in summertime coastal urban areas, reactions with Cl atoms dominated the early morning oxidation of alkanes before ∼10 a.m., surpassing OH reactions and that 15–25% of the total daily alkane oxidation was driven by Cl atoms (Bannan et al, ; Riedel et al, ). On a global scale, Wang et al () estimated that Cl atoms were responsible for 1% of the global oxidation of methane, and 20% of ethane, among other alkanes. Ultimately, because of the importance of Cl atoms to the overall oxidant budget in the lower troposphere, sources of Cl atoms are important to constrain.…”

“…Although the pioneering measurements of Keene et al () and Spicer et al () showed Cl 2 * (Cl 2 * = Cl 2 + HOCl) in excess of 120 pptv peaking at night, those high concentrations of Cl 2 * are thought to be driven more by HOCl than Cl 2 at night (Pszenny et al, ). Furthermore, Wang et al () significantly overestimated nighttime aircraft observations of Cl 2 during the 2015 Wintertime Investigation of Transportation, Emissions, and Reactivity (WINTER) campaign, using the global chemistry model, GEOS‐Chem, when including Cl 2 production from ClNO 2 uptake on acidic aerosols. Therefore, observations suggest that Cl 2 production from heterogeneous ClNO 2 reaction on acidic particles either occurs with a lower efficiency in the atmosphere than was measured in the laboratory, that Cl 2 production from ClNO 2 in the atmosphere is limited by a lack of aerosol having both sufficient acidity and pCl ‐ , or no observations reported to date have been made in areas where this chemistry is relevant.…”

Effect of sea-salt aerosol on tropospheric bromine chemistry

“…The high biases of modeled surface BrO during CONTRAST and CAST may be due to the underestimate of cloud and aerosol pH. By using online aerosol pH calculation and reflecting SSA contribution to cloud pH, Wang et al (2018) found much lower BrO concentrations in tropical Pacific Ocean. Both model and observations show high values (1.9-3.0 ppt) at Tenerife Island and Cape Verde in the tropical Atlantic.…”

“…Similarly, the global mean bias is -44% in our simulation. The biases at high latitudes can be reduced by including a detail chlorine chemistry (Wang et al, 2018), since high chloride in those regions accelerate HOBr recycling by serving as a catalyst. Figure 4 also shows tropospheric BrO columns measured in in Florida, USA (Coburn et al, 2011) and derived from mean aircraft vertical profiles over the tropical Pacific from the TORERO (Volkamer et al, 2015;Wang et al, 2015;Dix et al, 2016) and CONTRAST (Koenig et al, 2017) campaigns.…”