The maintenance of elevated active chlorine levels in the Antarctic lower stratosphere through HCl null-cycles

Abstract

Abstract. The Antarctic ozone hole arises from ozone destruction driven by elevated levels of ozone destroying ("active") chlorine in Antarctic spring. These elevated levels of active chlorine have to be formed first and then maintained throughout the period of ozone destruction. It is a matter of debate, how this maintenance of active chlorine is brought about in Antarctic spring, when the rate of formation of HCl (considered to be the main chlorine deactivation mechanism in Antarctica) is extremely high. 5Here we show that in the heart of the ozone hole (16-18 km or 100-70 hPa, in the core of the vortex), high levels of active chlorine are maintained by effective chemical cycles (referred to as HCl null-cycles hereafter). In these cycles, the formation of HCl is balanced by immediate reactivation, i.e. by immediate reformation of active chlorine. Under these conditions, polar stratospheric clouds sequester HNO 3 and thereby cause NO 2 concentrations to be low. These HCl null-cycles allow active chlorine levels to be maintained in the Antarctic lower stratosphere and thus rapid ozone destruction to occur. For the observed 10 almost complete activation of stratospheric chlorine in the lower stratosphere, the heterogeneous reaction HCl + HOCl, the production of HOCl via HO 2 + ClO, with the HO 2 resulting from CH 2 O photolysis, is essential. These results are important for assessing the impact of changes of the future stratospheric composition on the recovery of the ozone hole. Our simulations indicate that, in the lower stratosphere, future increased methane concentrations will not lead to enhanced chlorine deactivation (through the reaction CH 4 + Cl −→ HCl + CH 3 ) and that extreme ozone destruction to levels below ≈ 0.1 ppm will occur until 15 mid-century.

Simulation of the record Arctic stratospheric ozone depletion in 2020

“…This is also the case for winter/ spring 2020 (Manney et al, 2020). The CLaMS simulation similarly shows more typical Antarctic behavior as low ozone mixing ratios-almost comparable to Antarctic values-are reached, allowing deactivation into HCl to occur (Douglass et al, 1995;Grooß et al, 2011;Müller et al, 2018).…”

Simulation of Record Arctic Stratospheric Ozone Depletion in 2020

“…This is also the case for winter/spring 2020 (Manney et al., 2020). The CLaMS simulation similarly shows more typical Antarctic behavior as low ozone mixing ratios—almost comparable to Antarctic values—are reached, allowing deactivation into HCl to occur (Douglass et al., 1995; Grooß et al., 2011; Müller et al., 2018).…”

Chemical Evolution of the Exceptional Arctic Stratospheric Winter 2019/2020 Compared to Previous Arctic and Antarctic Winters

“…There are both activation and deactivation reactions that depend on sunlight and their rates increase with increasing illumination. Note that under certain conditions, activation and deactivation reactions may not be independent of each other and may exactly cancel out (Müller et al., 2018). Activation by the reactions HCl + ClONO 2 and HOCl + HCl was about equally important in this phase.…”