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

• Published in: Atmospheric Chemistry and Physics
• Main Authors: R. Müller, J.-U. Grooß, A. M. Zafar, S. Robrecht, R. Lehmann
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2018
• Subjects: Physics; QC1-999; Chemistry; QD1-999; Antarctic; The Antarctic; Antarc*; Antarctica
• Online Access: https://doi.org/10.5194/acp-18-2985-2018; https://doaj.org/article/4586f41cbb9245948143e14d5d887866

The Antarctic ozone hole arises from ozone destruction driven by elevated levels of ozone destroying (<q>active</q>) 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. Here we show that in the heart of the ozone hole (16–18 km or 85–55 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 almost complete activation of stratospheric chlorine in the lower stratosphere, the heterogeneous reaction HCl + HOCl is essential; the production of HOCl occurs via HO 2 + ClO, with the HO 2 resulting from CH 2 O photolysis. 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 <mo form="infix">⟶</mo> HCl + CH 3 ) and that extreme ozone destruction to levels below ≈ 0.1 ppm will occur until mid-century.