Conditions Necessary for Chlorine Activation in the Midlatitude Summer Lower Stratosphere

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, ClONO 2 , H 2 O, NO 2 , 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 (Cl y ) 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 Cl y , the abundance of nitrogen oxides was elevated and the ratio of ClONO 2 to Cl y was observed to be very low, limiting the availability of ClONO 2 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.

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Publication Details

Journal
Journal of Geophysical Research Atmospheres
Published
2025-10-14
DOI
https://doi.org/10.1029/2025jd043786
Primary Topic
Atmospheric Ozone and Climate
Type
article
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article

Conditions Necessary for Chlorine Activation in the Midlatitude Summer Lower Stratosphere

Cameron R. Homeyer, Frank N. Keutsch, S. Schauffler, Victoria Treadaway et al.
Journal of Geophysical Research Atmospheres
Atmospheric Ozone and Climate
article

Conditions Necessary for Chlorine Activation in the Midlatitude Summer Lower Stratosphere

Cameron R. Homeyer, Frank N. Keutsch, S. Schauffler, Victoria Treadaway, E. Atlas, D. S. Sayres, Katie Smith, F. L. Moore, Jason M. St. Clair, Yaowei Li, T. P. Bui, Kenneth P. Bowman, Eric J. Hintsa, Laila V. Howar, Jonathan M. Dean‐Day, B. D. Hall, J. B. Smith, Steven C. Wofsy, Erin R. Delaria, R. J. Salawitch, Bruce C. Daube, J. Hare, David M. Wilmouth, Thomas Frost Hanisco, James G. Anderson, J. V. Pittman, Stephen Donnelly
article en

Abstract

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, ClONO 2 , H 2 O, NO 2 , 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 (Cl y ) 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 Cl y , the abundance of nitrogen oxides was elevated and the ratio of ClONO 2 to Cl y was observed to be very low, limiting the availability of ClONO 2 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.

Journal of Geophysical Research AtmospheresVol. 130(20)
NSF National Center for Atmospheric Research (US), Ames Research Center (US), Goddard Space Flight Center (US), University of Miami (US), Cooperative Institute for Research in Environmental Sciences (US), University of Colorado Boulder (US), Harvard University Press (US), Bay Area Environmental Research Institute (US), Oldham Council (GB), University of Maryland, College Park (US), University of Maryland, Baltimore County (US), University of Oklahoma (US), Texas A&M University (US)
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Atmospheric Ozone and Climate
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