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

• Main Authors: X. Wang, D. J. Jacob, W. Downs, S. Zhai, L. Zhu, V. Shah, C. D. Holmes, T. Sherwen, B. Alexander, M. J. Evans, S. D. Eastham, J. A. Neuman, P. R. Veres, T. K. Koenig, R. Volkamer, L. G. Huey, T. J. Bannan, C. J. Percival, B. H. Lee, J. A. Thornton
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-09-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/21/13973/2021/acp-21-13973-2021.pdf
• ISSN: 1680-7316; 1680-7324

<p>We present an updated mechanism for tropospheric halogen (Cl <span class="inline-formula">+</span> Br <span class="inline-formula">+</span> I) chemistry in the GEOS-Chem global atmospheric chemical transport model and apply it to investigate halogen radical cycling and implications for tropospheric oxidants. Improved representation of <span class="inline-formula">HOBr</span> heterogeneous chemistry and its pH dependence in our simulation leads to less efficient recycling and mobilization of bromine radicals and enables the model to include mechanistic sea salt aerosol debromination without generating excessive <span class="inline-formula">BrO</span>. The resulting global mean tropospheric <span class="inline-formula">BrO</span> mixing ratio is 0.19 <span class="inline-formula">ppt</span> (parts per trillion), lower than previous versions of GEOS-Chem. Model <span class="inline-formula">BrO</span> shows variable consistency and biases in comparison to surface and aircraft observations in marine air, which are often near or below the detection limit. The model underestimates the daytime measurements of <span class="inline-formula">Cl₂</span> and BrCl from the ATom aircraft campaign over the Pacific and Atlantic, which if correct would imply a very large missing primary source of chlorine radicals. Model IO is highest in the marine boundary layer and uniform in the free troposphere, with a global mean tropospheric mixing ratio of 0.08 <span class="inline-formula">ppt</span>, and shows consistency with surface and aircraft observations. The modeled global mean tropospheric concentration of Cl atoms is 630 <span class="inline-formula">cm⁻³</span>, 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 %, <span class="inline-formula">NO_<i>x</i></span> by 6 %, and OH by 4 %. Most of the ozone decrease is driven by iodine-catalyzed loss. The resulting GEOS-Chem ozone simulation is unbiased in the Southern Hemisphere but too low in the Northern Hemisphere.</p>