Satellite observations show a continuous decline of gas-phase hydrogen chloride (HCl) in the dark core of the wintertime polar vortex that no state-of-the-art stratospheric chemistry model reproduces: after the initial titration of ClONO2_2 by the heterogeneous reaction with HCl on polar stratospheric clouds, models hold HCl nearly constant while observations show ongoing removal to very low mixing ratios. First quantified as a systematic issue by Wohltmann, Lehmann and Rex (2017), and confirmed across independent models by Grooß et al. (2018), the discrepancy has acquired a strong model-level candidate — the heterogeneous reaction of chlorine peroxide with HCl forming HOOCl (Grooß et al., 2025) — but the uptake coefficient, the HOCl self-reaction kinetics, and the HOOCl absorption cross sections this mechanism requires have never been measured in the laboratory. The open problem is to identify and experimentally establish the physical process responsible for the wintertime HCl removal missing from stratospheric chemistry models, and to quantify its consequences for polar chlorine activation, ozone depletion, and ozone-hole recovery.

Background

The ozone hole arises because inert chlorine reservoir gases — mainly HCl (hydrogen chloride) and ClONO2_2 (chlorine nitrate) — are converted back to reactive chlorine on the surfaces of polar stratospheric clouds (PSCs). PSCs form at the very low temperatures of the polar winter stratosphere; they are either liquid supercooled ternary solution (STS) droplets of H2_2SO4_4/HNO3_3/H2_2O or crystalline nitric acid trihydrate (NAT) and ice particles. Heterogeneous reactions on these particles, chiefly ClONO2_2 + HCl → Cl2_2 + HNO3_3 and HOCl + HCl → Cl2_2 + H2_2O, release active chlorine (ClOx_x = Cl + ClO + 2·ClOOCl), which destroys ozone catalytically through the ClO dimer (ClOOCl) and ClO–BrO cycles when sunlight returns. Chemistry–transport models (CTMs) such as ATLAS, CLaMS, SD-WACCM, and TOMCAT/SLIMCAT, evaluated against satellite instruments such as MLS (Microwave Limb Sounder) on Aura, MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) on Envisat, and ACE-FTS (Atmospheric Chemistry Experiment Fourier Transform Spectrometer) on SCISAT, reproduce springtime ozone depletion and most reservoir species well — with one prominent exception.

That exception, quantified by Wohltmann, Lehmann and Rex (2017, Atmos. Chem. Phys. 17, 10535–10563) in their vortex-core reaction budget analysis, is HCl in early and mid winter: uncorrected model runs overestimate HCl by amounts that grow through the polar night in both hemispheres. Since early-winter ClONO2_2 is the limiting reactant of the first activation step, the models' HCl stays roughly constant after this titration, whereas MLS observations show continued HCl decline in the dark vortex core. Wohltmann et al. listed candidate causes — underestimated initial ClONO2_2, too little NOx_x transport across the vortex edge, sedimenting HCl-containing PSC particles, an unknown heterogeneous HCl sink, reanalysis temperature or water-vapor bias, and underestimated HCl solubility in STS droplets — and concluded the available evidence could not narrow the list; they forced agreement with an empirical −5 K temperature offset in the HCl solubility (Henry constant) parameterization. The dedicated follow-up study of Grooß et al. (2018, Atmos. Chem. Phys. 18, 8647–8666) demonstrated the discrepancy in three independent models (clearest in the Lagrangian CLaMS, partly masked by numerical diffusion in Eulerian models), excluded PSC HCl uptake and particulate HNO3_3 photolysis, and found galactic-cosmic-ray ionization explains only about 20 % of it, leaving the responsible process unidentified. A review of PSC science (Tritscher et al., 2021) meanwhile judged heterogeneous chlorine activation on liquid droplets to be well understood, and Zhang-Liu et al. (2024) showed that HCl null cycles maintain the observed very low HCl once established — but neither explains the anomalous wintertime removal itself. Grooß, Müller, Crowley and Hegglin (2025, Communications Earth & Environment 6, 496) then showed in CLaMS simulations that the heterogeneous reaction ClOOCl + HCl → Cl2_2 + HOOCl on PSCs, followed by HOOCl photolysis and an additional HOCl loss pathway — a reaction first modeled by De Haan, Fløisand and Stordal (1997) — brings simulated HCl, HOCl, ClO, and ClONO2_2 much closer to observations and increases simulated Antarctic ozone column depletion by over 15 %; however, neither the uptake coefficient of this reaction nor the HOCl self-reaction kinetics nor the HOOCl absorption cross sections have ever been measured, so the mechanism remains a model-level hypothesis awaiting laboratory confirmation.

Problem Statement

Identify and experimentally establish the process responsible for the observed removal of gas-phase HCl in the core of the wintertime polar vortex (in both the Antarctic and, to a smaller degree, the Arctic) that state-of-the-art stratospheric chemistry models do not reproduce: after the initial ClONO2_2 + HCl titration step, models hold HCl nearly constant through the polar night while satellite observations (MLS, MIPAS, ACE-FTS) show a continuous decline to very low mixing ratios. The currently leading candidate is the heterogeneous reaction ClOOCl+HCl→Cl2+HOOClClOOCl + HCl \rightarrow Cl_2 + HOOCl on polar stratospheric cloud particles, followed by HOOCl photolysis and an additional HOCl loss pathway (Grooß et al., 2025); neither the uptake coefficient of this reaction (the probability that a gas-phase ClOOCl molecule striking the particle surface reacts) under polar-stratospheric conditions (approximately 180–200 K, on STS/NAT/ice substrates), nor the HOCl self-reaction kinetics, nor the HOOCl absorption cross sections have ever been measured in the laboratory. The identified entity may be the leading candidate, a different physical, chemical, or transport process (or a combination of such processes), or the finding that the observed decline is an observational artifact. The leading candidate would be established by the uptake coefficient of ClOOCl+HClClOOCl + HCl on the relevant PSC substrates, the HOCl self-reaction kinetics, and the HOOCl absorption cross sections, together with a model simulation that reconciles simulated wintertime HCl with observations; a different physical, chemical, or transport process by the measurements that establish that process; and a combination of such processes additionally by a test that discriminates among its components. An observational artifact would be established by the retrieval and validation analyses that establish the bias, with the model reproducing the corrected observations in place of the disputed ones. Because the data required by some of these measurements do not yet exist, a resolution may establish them by specifying, from the literature and the model, the quantities to be measured, their expected magnitudes, and the experimental or observational approach that would determine them; reporting the laboratory measurements is the physical realization of such a resolution rather than a precondition for it. Quantify the consequences of the identified entity for the timing and extent of chlorine activation, for vortex ozone depletion, and for assessments of polar ozone recovery.

References

  1. Wohltmann, I., Lehmann, R., and Rex, M.: A quantitative analysis of the reactions involved in stratospheric ozone depletion in the polar vortex core, Atmospheric Chemistry and Physics, 17, 10535–10563, 2017, doi:10.5194/acp-17-10535-2017, https://acp.copernicus.org/articles/17/10535/2017/
  2. Grooß, J.-U., Müller, R., Spang, R., Tritscher, I., Wegner, T., Chipperfield, M. P., Feng, W., Kinnison, D. E., and Madronich, S.: On the discrepancy of HCl processing in the core of the wintertime polar vortices, Atmospheric Chemistry and Physics, 18, 8647–8666, 2018, doi:10.5194/acp-18-8647-2018, https://acp.copernicus.org/articles/18/8647/2018/
  3. Grooß, J.-U., Müller, R., Crowley, J. N., and Hegglin, M. I.: Chlorine peroxide reaction explains observed wintertime hydrogen chloride in the Antarctic vortex, Communications Earth & Environment, 6, 496, 2025, doi:10.1038/s43247-025-02499-4, https://www.nature.com/articles/s43247-025-02499-4
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