EGUsphere EGUsphere EGUsphere EGUsphere Copernicus Publications Göttingen, Germany 10.5194/egusphere-2026-3480 Analysis of inorganic chlorine chemistry in the midlatitude summer stratosphere using aircraft and satellite observations Wilmouth David https://orcid.org/0000-0002-8832-1479 1 Hare Jennifer 1 Howar Laila https://orcid.org/0000-0002-4154-7999 2 Salawitch Ross 2 Hintsa Eric https://orcid.org/0000-0002-5289-630X 3 4 Smith Jessica 1 Sayres David 1 Anderson James 1 St. Clair Jason https://orcid.org/0000-0002-9367-5749 5 6 Delaria Erin 2 5 Hannun Reem https://orcid.org/0000-0001-5195-5307 7 Hanisco Thomas https://orcid.org/0000-0001-9434-8507 5 Czizco Daniel 8 9 Shen Xiaoli https://orcid.org/0000-0001-7174-024X 8 10 Homeyer Cameron https://orcid.org/0000-0002-4883-6670 11 Santee Michelle https://orcid.org/0000-0002-9466-7257 12 Bui T. Paul 7 13 Newman Paul https://orcid.org/0000-0003-1139-2508 5 6 Keutsch Frank 1 Bowman Kenneth 14 Harvard University, Cambridge, MA, USA University of Maryland, College Park, MD, USA Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO, USA NOAA Global Monitoring Laboratory, Boulder, CO, USA NASA Goddard Space Flight Center, Greenbelt, MD, USA University of Maryland, Baltimore County, Baltimore, MD, USA NASA Ames Research Center, Moffett Field, CA, USA Purdue University, West Lafayette, IN, USA University of Chicago, Chicago, IL, USA Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, Germany University of Oklahoma, Norman, OK, USA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA Bay Area Environmental Research Institute, Moffett Field, CA, USA Texas A&M University, College Station, TX, USA 25 06 2026 2026 1 29 Copyright: © 2026 David Wilmouth et al. 2026 This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/ This article is available from https://egusphere.copernicus.org/preprints/2026/egusphere-2026-3480/ The full text article is available as a PDF file from https://egusphere.copernicus.org/preprints/2026/egusphere-2026-3480/egusphere-2026-3480.pdf

In recent years, significant perturbations to stratospheric inorganic chlorine have been observed following wildfires or volcanic eruptions, as well as modeled in simulations with elevated water vapor from overshooting convection or organic species in biomass burning aerosol. A detailed evaluation of inorganic chlorine in the stratosphere sampled under various conditions is presented here using high precision in situ aircraft measurements of chlorine monoxide (ClO) and chlorine nitrate (ClONO<sub>2</sub>). Data were obtained over North America during the NASA Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) mission. The vertical distributions of ClO and ClONO<sub>2</sub> from all 29 research flights of the 2-year mission are found to be relatively compact and lacking substantial outliers. The peak mixing ratios are approximately 30 ppt and 400 ppt for ClO and ClONO<sub>2</sub>, respectively. No chlorine activation was observed in the lower stratosphere in the presence of low temperatures and elevated water vapor from convective injection. Steady-state calculated ClONO<sub>2</sub> is found to be in good agreement with measured values, suggesting that a reduction in the uncertainty of the recommended rate coefficient for ClONO<sub>2</sub> production is possible. HCl is calculated at high time resolution throughout the mission using satellite data, and the resulting evaluation of the inorganic chlorine budget shows excellent agreement, with the ratio of ClO + ClONO<sub>2</sub> + HCl to total inorganic chlorine equal to 0.95 for mixing ratios greater than 500 ppt. No evidence of inorganic chlorine activation was observed during DCOTSS when aerosol organic mass fraction and biomass burning fraction were elevated.

 National Aeronautics and Space Administration 80NSSC19K0326 80NSSC19K1399 80NSSC19K1058 80NSSC19K0347 80NSSC19K0341 80NM0018D0004 National Science Foundation 2426145