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<p>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<span class="inline-formula">^*</span>, including Cl, ClO, <span class="inline-formula">Cl₂</span>, BrCl, ICl, HOCl, <span class="inline-formula">ClNO₃</span>, <span class="inline-formula">ClNO₂</span>, and minor species, is produced by the <span class="inline-formula">HCl+OH</span> reaction and by heterogeneous conversion of sea salt aerosol chloride to BrCl, <span class="inline-formula">ClNO₂</span>, <span class="inline-formula">Cl₂</span>, and ICl. The model successfully simulates the observed mixing ratios of HCl in marine air (highest at northern midlatitudes) and the associated <span class="inline-formula">HNO₃</span> decrease from acid displacement. It captures the high <span class="inline-formula">ClNO₂</span> 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 <span class="inline-formula">Cl₂</span> mixing ratios measured in the WINTER aircraft campaign (1–5&thinsp;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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">ClNO</mi><mn mathvariant="normal">2</mn></msub><mo>+</mo><msup><mi mathvariant="normal">Cl</mi><mo>-</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="62pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="df983f5d64d41ca8dd3748b9c08f7ae2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-3981-2019-ie00001.svg" width="62pt" height="14pt" src="acp-19-3981-2019-ie00001.png"/></svg:svg></span></span> reaction, but we have no explanation for the high observed <span class="inline-formula">Cl₂</span> in daytime. The global mean tropospheric concentration of Cl atoms in the model is 620&thinsp;cm<span class="inline-formula">⁻³</span> and contributes 1.0&thinsp;% of the global oxidation of methane, 20&thinsp;% of ethane, 14&thinsp;% of propane, and 4&thinsp;% of methanol. Chlorine chemistry increases global mean tropospheric BrO by 85&thinsp;%, mainly through the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M14" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">HOBr</mi><mo>+</mo><msup><mi mathvariant="normal">Cl</mi><mo>-</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="58pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="813fc4a530245e7624a6bb6e43294191"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-19-3981-2019-ie00002.svg" width="58pt" height="12pt" src="acp-19-3981-2019-ie00002.png"/></svg:svg></span></span> reaction, and decreases global burdens of tropospheric ozone by 7&thinsp;% and OH by 3&thinsp;% through the associated bromine radical chemistry. <span class="inline-formula">ClNO₂</span> chemistry drives increases in ozone of up to 8&thinsp;ppb over polluted continents in winter.</p>