Comment on acp-2022-151

Abstract

Chlorofluorocarbon (CFC) emissions in the latter part of the 20th century reduced stratospheric ozone abundance substantially, especially in the Antarctic region. Simultaneously, polar stratospheric ozone is also destroyed catalytically by nitrogen oxides (NOx = NO + NO2) descending from the mesosphere and the lower thermosphere during winter. These are produced by energetic particle precipitation (EPP) linked to solar activity and space weather. NOx and active chlorine (ClOx = Cl + ClO) also react mutually and transform both reactive agents into reservoir gas chlorine nitrate, which buffers ozone destruction by both NOx and ClOx. We study the interaction between EPP produced NOx, ClO and ozone over the 20th century by using free running climate simulations of the chemistry-climate model SOCOL3-MPIOM. Substantial increase of NOx descending to polar stratosphere is found during winter, which causes ozone depletion in the upper and mid-stratosphere. However, in the Antarctic mid-stratosphere the EPP induced ozone depletion becomes less efficient after 1960s, especially during springtime. Simultaneously, significant decrease in stratospheric ClO and increase in chlorine nitrate between 10–30 hPa can be ascribed to EPP forcing. Hence, interaction between NOx and ClO likely suppressed the ozone depletion due to both EPP-NOx and ClO at these altitudes. Furthermore, at the end of the century significant ClO increase and ozone decrease is obtained at 100 hPa altitude during winter and spring. This lower stratosphere response is likely due to activation of chlorine from reservoir gases on polar stratospheric clouds. Our results show that EPP has been a significant modulator of reactive chlorine in the Antarctic stratosphere during the CFC era. With the implementation of the Montreal Protocol, stratospheric chlorine is estimated to return to pre-CFC era levels after 2050. Thus, we expect increased efficiency of chemical ozone destruction by EPP-NOx in the Antarctic upper and mid-stratosphere over coming decades. The future lower stratosphere ozone response by EPP is more uncertain.