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. 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 −→ HCl+CH 3 ) and that extreme ozone destruction to levels below ≈ 0.1 ppm will occur until mid-century.

Chlorine partitioning near the polar vortex edge observed with ground-based FTIR and satellites at Syowa Station, Antarctica, in 2007 and 2011

“…Spiky increases in HOCl at 76.7 and 87.9 • S and a simultaneous decrease in HCl occurred on around 7 July (day 188) and 20 July (day 201) in Fig. 15e and f. The continuous loss of HCl at the core of the polar vortex in August and September was recently proposed by Müller et al (2018), who proposed that HCl destruction cycles C 3 and C 4 (see Appendix C) are responsible for the decline of HCl in the vortex core. These chemical cycles also require sunlight to occur, which may not be available in June and July at the vortex core.…”

“…Here, HO 2 was needed to yield HOCl. One possibility to yield HO 2 in August is either one of the HCl null cycles C 1 or C 2 (see Appendix C) or one of the HCl destruction cycles C 3 or C 4 (see Appendix C), which was described in Müller et al (2018). If the air mass at 87.9 • S was located equatorward due to the obliqueness of the polar vortex a few days earlier, then sunlight may have been available and such reactions could yield HOCl at 87.9 • S. The continuous loss of HCl was seen at 87.9 • S between 9 June (day 160) and 19 July (day 200) even after the disappearance of the counterpart of the heterogeneous Reaction (R1) (Fig.…”

“…Therefore, observed day-to-day variations of HCl were small and did not show any correlation with . A possible explanation to keep a near zero HCl value close to the vortex edge is due to so-called "HCl null cycles", which were started with Reaction (R13), proposed by Müller et al (2018). This cycle is discussed later.…”

“…We installed a Bruker IFS-120M highresolution Fourier transform infrared spectrometer (FTIR) in the observation hut at Syowa Station in March 2007. This was the third high-resolution FTIR site in Antarctica in operation after the USA's South Pole Station (90.0 • S) (Goldman et al, 1983Murcray et al, 1987), the USA's Mc-Murdo Station, and New Zealand's Arrival Heights facility at the Scott Base station (77.8 • S, 166.7 • E) (Farmer et al, 1987;Murcray et al, 1989;Kreher et al, 1996;Wood et al, 2002. The IFS-120M FTIR has a wavenumber resolution of 0.0035 cm −1 , with two liquid-nitrogen-cooled detectors (with InSb and HgCdTe covering the frequency ranges 2000-5000 and 700-1300 cm −1 , respectively) with six optical filters, and is fed by an external solar tracking system.…”

“…When the stratospheric temperatures get warmer than NAT saturation temperature (about 195 K at 50 hPa) and no PSCs are present, deactivation of chlorine starts to occur. Reformation of ClONO 2 and HCl mainly occurs through reactions (Santee et al, 2008;Grooß et al, 2011;Müller et al, 2018) ClO…”