Conditions Necessary for Chlorine Activation in the Midlatitude Summer Lower Stratosphere

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 (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 ClONO₂ to Cl_y 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.