Conditions Necessary for Chlorine Activation in the Midlatitude Summer Lower Stratosphere

[Geoengineering News]

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

[Oeste]

Hi Clive and Oswald,

The authors of the paper below state, that enhanced stratospheric sulfate aerosol levels from volcanic eruptions or artificial stratospheric sulfate injection would induce significant stratospheric ozone depletion. 

Volcanic sulfate injections are only during the volcanic maximum eruption period active. These eruption periods last very seldom more than a month. Stratospheric Aerosol Injection (SAI) periods are longer than 100 years. Hence the skin cancer inducing UV-C radiation problem induced by the stratospheric ozone depletion will also last for more than 100 years.

Because we know some much more economic and efficient climate restoration methods this should make no problems for climate restoration.

Franz

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[Douglas MacMartin]

Any statement of the form "artificial stratospheric sulfate injection would induce significant X" is obviously wrong, given that at a minimum any effect depends on how much you do.  And we need an assessment of the overall trade-offs, not mindless throwing out of options based on any one single effect.

In the case of ozone, there's lots of papers on this, and there will be more; the answer of course depends on how much you do, on what latitudes you inject at, on when you start, but there's also a lot more to it than just the heterogeneous chemistry response; when you include all of the dynamical and chemical responses then in some places total column ozone goes up (over Antarctic it consistently goes down - though if you start in 2035 to maintain temperatures at 2030 levels, then in CESM(WACCM) simulations, Antarctic ozone levels might drop to levels not seen since 2025... is that a reason to ignore SAI?)

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