Chlorine activation — the conversion of the inert reservoir species HCl and ClONO2 into reactive chlorine that destroys ozone — proceeds heterogeneously on particle surfaces and was long thought to be confined to the cold polar winter stratosphere. Solomon et al. (2016) showed with model calculations that monsoon circulations loft cold, humid, aerosol-rich air into the tropical lowermost stratosphere, where the same reactions can proceed on liquid sulfate aerosol and deplete ozone by percent-level amounts (about 1-2 Dobson units locally, in simulations for the volcanically enhanced 2009-2011 period). Whether this modeled activation is quantitatively real remains open on three coupled fronts: the laboratory reaction probabilities for the relevant heterogeneous reactions on liquid H2SO4/H2O and H2SO4/H2O/HNO3 aerosol carry roughly 40 percent one-sigma uncertainties at the relevant sub-200 K temperatures; the effect of the nitrate-rich and organic-containing particle compositions actually observed in the tropical tropopause layer on these reactions is essentially unmeasured; and no validated ClO or ClONO2 measurements exist in the 16-18 km tropical/subtropical band to test the modeled activation signature or its ozone impact against dynamical variability. Later work has demonstrated the mechanism under extreme perturbed conditions (the 2022 Hunga Tonga eruption and Australian wildfire smoke) and found no appreciable activation in background midlatitude monsoon conditions, but the background tropical case and its ozone attribution remain unresolved.

Background

The stratospheric ozone layer is protected by the fact that inorganic chlorine released from chlorofluorocarbons and other ozone-depleting substances is normally stored in inert 'reservoir' gases, chiefly hydrogen chloride (HCl) and chlorine nitrate (ClONO2). 'Chlorine activation' means the conversion of these reservoirs into reactive forms — chlorine monoxide (ClO) and its dimer — that catalytically destroy ozone. The key activating reactions, such as ClONO2 + HCl and ClONO2 + H2O, are negligibly slow in the gas phase but proceed rapidly on the surfaces of liquid droplets and solid particles ('heterogeneous' chemistry); the efficiency of uptake is quantified by the reaction probability, gamma, the fraction of molecule-surface collisions that lead to reaction. In the polar winter lower stratosphere, this activation occurs on polar stratospheric clouds at temperatures below roughly 195 K, producing the Antarctic ozone hole. The 'lowermost' stratosphere is the lowest stratospheric layer, extending upward from the tropopause to roughly 20 km; because the meteorological tropopause in the northern tropics in summer itself lies near 90-130 hPa depending on longitude, the 16-18 km band targeted by this problem sits within this layer, which overlaps the region where the tropical tropopause layer — the transition zone between the upper troposphere and stratosphere, about 14-19 km — is crossed by deep convection. Ozone column is measured in Dobson units (DU); 1 DU corresponds to a 0.01 mm-thick layer of pure ozone at standard conditions, and a typical tropical column is about 260 DU.

Solomon, Kinnison, Garcia, Bandoro, Mills, Wilka, Neely, Schmidt, Barnes, Vernier and Hopfner (2016, Geophysical Research Letters) proposed that heterogeneous chlorine activation is not confined to the poles. During boreal summer, the Asian and North American monsoon circulations loft boundary-layer air — cold, very humid, and laden with sulfate aerosol, including volcanic material from the 2009-2011 eruptions — into the tropical lowermost stratosphere. Their model calculations indicated that the resulting wetter, aerosol-rich conditions activate chlorine on ambient liquid sulfate aerosol droplets, depleting local ozone at 16-19 km by up to a few percent and the column by about 1-2 DU at, for example, 15N in summer over 2009-2011 — a volcanically enhanced period, since the paper focuses on 2011, when tropical stratospheric aerosols were augmented by the Nabro eruption. Of the modeled ozone change, roughly three quarters is traceable to anthropogenic (industrial) emissions of chlorine and bromine rather than to natural very short-lived halogen species of biogenic origin. The paper notes that volcanic increases in liquid sulfate aerosols enhance the perturbations, but that the heterogeneous reactions are thought to be sufficiently rapid that substantial changes are also indicated for volcanically clean conditions; the modeled effect is a self-described lower limit, since chlorine from very short-lived substances, iodine chemistry and convective lofting of sea salt were not included. This matters because the tropical lower stratosphere is the region where ozone is transported into the overlying stratosphere and where lower-stratospheric ozone has been declining; a persistent chemical loss mechanism there would feed into global ozone-climate projections.

The source paper itself identifies what would be needed to test and confirm its model result. First, the laboratory kinetics: the standard kinetic data evaluations used at the time reported one-sigma uncertainties of roughly 40 percent for the relevant heterogeneous reaction probabilities on liquid H2SO4/H2O and H2SO4/H2O/HNO3 solutions, and these are poorly constrained precisely for temperatures below about 200 K, the range relevant to the tropical lowermost stratosphere. Second, the composition question: in-situ particle measurements cited by the source study show that tropical tropopause region particles contain nonvolatile and organic components and pollution-derived constituents, not pure sulfate; whether such compositions alter heterogeneous reactivity is unknown. Later balloon measurements (Vernier et al. 2022) sharpened this gap by showing that summer particles in the Asian Tropopause Aerosol Layer — a seasonal layer of aerosol at 14-18 km over Asia during the monsoon — are dominated by nitrate and nitrite, with sulfate above detection limits mainly in winter. Third, the observational test: the source calls for validated measurements of ClO or ClONO2 in the 16-18 km region of the tropics and subtropics against which its findings could be further tested; its modeled signature for volcanically clean summers is greatly enhanced ClO and ClONO2 relative to simulations neglecting heterogeneous chemistry — of the order of 5 pptv and 25-40 pptv respectively (17 km, averaged over 14-20N), compared with only a few tenths of a pptv of ClO and 5-10 pptv of ClONO2 when these reactions are neglected, and with further increases of order 10 pptv for ClO and 5-8 pptv for ClONO2 in volcanically perturbed summers — but no validated dataset of either species exists for that band, so the modeled signature is untested. Fourth, the attribution: the modeled ozone changes are small (percent-level profiles at 16-19 km, 1-2 DU columns for the volcanically enhanced 2009-2011 period) against large dynamical variability, so isolating any chemical contribution requires improved accuracy and dynamical attribution of ozone and tracer measurements; the source itself defers detailed analysis of tropical ozone trends as outside its scope.

Problem Statement

Does heterogeneous chlorine activation proceed to a climatologically meaningful degree in the tropical lowermost stratosphere (approximately 16-18 km) under background, non-volcanically-perturbed summer monsoon conditions, and what is its contribution to ozone there?

Resolution may draw on complementary lines of evidence such as: (1) laboratory reaction probabilities for the heterogeneous reactions that convert chlorine reservoirs to reactive chlorine — ClONO2 hydrolysis, ClONO2 + HCl, and HOCl + HCl (the latter two being the net-activation channels identified in the source's supporting information) — on liquid H2SO4/H2O and H2SO4/H2O/HNO3 aerosol at temperatures below about 200 K, where the standard kinetic data evaluations report one-sigma uncertainties of roughly 40 percent, together with a determination of whether the non-sulfate components observed in tropical tropopause layer particles (nonvolatile species and pollutants including organic compounds, and the nitrate and nitrite that dominate Asian Tropopause Aerosol Layer particles in summer) alter this reactivity; (2) validated measurements of ClO or ClONO2 in the 16-18 km tropical/subtropical monsoon band against which the modeled activation signature described above can be tested, with co-measurements of water vapor, temperature and aerosol abundance/composition useful for interpreting any signal; and (3) an attribution of the resulting ozone perturbation to heterogeneous chemistry rather than dynamical variability — the source's percent-level profile changes span 16-19 km and its 1-2 DU column decrease refers to volcanically enhanced 2009-2011 calculations, so the size of the ozone effect under volcanically clean background conditions is itself part of what must be established. A negative answer — a demonstrated absence of appreciable activation across the relevant range of realistic background tropical conditions (temperature, water vapor, aerosol loading and composition, and available inorganic chlorine) — resolves the problem equally well.

References

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