https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025JD043786?af=R
*Authors: *Laila V. Howar, Ross J. Salawitch, David M. Wilmouth, Eric J. Hintsa, Jennifer S. Hare, Thomas F. Hanisco, Jason M. St. Clair, Erin R. Delaria, Elliot L. Atlas, Sue Schauffler, Kate R. Smith, Jessica B. Smith, Bradley D. Hall, Fred L. Moore, Jasna V. Pittman, Bruce Daube, T. Paul Bui, Yaowei Li, Frank N. Keutsch, David S. Sayres, Steven C. Wofsy, Jonathan Dean-Day, Stephen Donnelly, Victoria A. Treadaway, James G. Anderson, Cameron R. Homeyer, Kenneth P. Bowman *14 October 2025* https://doi.org/10.1029/2025JD043786 *Abstract* Studies have suggested that ClO could be enhanced within convectively influenced air masses in the North American Monsoon Anticyclone due to low temperature and elevated water mixing ratio conditions that are conducive to chlorine activation, potentially leading to significant loss of ozone in the midlatitude lowermost stratosphere. We analyze in situ measurements of temperature, pressure, ClO, ClONO2, H2O, NO2, aerosol surface area density (SAD), and organic chlorine species obtained by instruments aboard the NASA ER-2 over the continental US during the Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) campaign to show that chlorine activation large enough to affect loss of ozone was not observed, for the summers of 2021 and 2022. During both deployments, tropopause-overshooting convection with water vapor and temperature conditions suitable for chlorine activation were sampled. Due to their relatively young chemical age, most of these cold and wet air masses had abundances of inorganic chlorine (Cly) too low to support eventual enhancements of ClO that would lead to widespread ozone depletion. Even in the few air masses with higher levels of Cly, the abundance of nitrogen oxides was elevated and the ratio of ClONO2 to Cly was observed to be very low, limiting the availability of ClONO2 to react with HCl and sustain chlorine activation. However, we show that for the average chemical and meteorological conditions of cold, wet, overshooting air parcels observed during DCOTSS, significant chlorine activation could occur if stratospheric sulfate SAD were enhanced by major volcanic eruptions or *climate intervention efforts.* *Plain Language Summary* Stratospheric ozone shields life on Earth from the Sun's harmful ultraviolet rays. Mixed-phase chemical reactions, which typically occur only at very low temperatures, produce large amounts of chlorine monoxide (ClO), a chemical that rapidly depletes stratospheric ozone. Previous studies have hypothesized that abundances of ClO, capable of leading to substantial ozone depletion, could be found in the midlatitude stratosphere when large storms inject water vapor high into the stratosphere. During the Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) NASA ER-2 aircraft field campaign, magnitudes of ClO capable of causing significant ozone depletion were not observed within cold and wet air masses. Although large amounts of water vapor were injected into the stratosphere by these storms, the air masses lacked the chemical conditions necessary for chlorine activation to cause significant ozone loss. Additionally, our modeling analysis of data collected during DCOTSS indicates that the stratospheric injection of sulfate, in an effort to mitigate global warming, could increase the risk of chlorine activation in the midlatitude lower stratosphere. *Key Points* Observations of ClO from the Dynamics and Chemistry of the Summer Stratosphere mission show no evidence of heterogeneous chlorine activation Chlorine activation was limited by the availability of inorganic chlorine and the partitioning between the two major chlorine reservoirs Enhancement of sulfate aerosol surface area would increase the potential for chlorine activation in the midlatitude stratosphere *Source: AGU* -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion visit https://groups.google.com/d/msgid/geoengineering/CAHJsh98R7S8W9cxJ_fhJYFPfU_eU-7DBwFS7dPbFr0cgc8sPQQ%40mail.gmail.com.
