Arctic Vortex Research Articles

MLS observations of Arctic ozone loss in 1996–97

The Microwave Limb Sounder (MLS) observed ozone (O3) loss in the Arctic vortex beginning in January 1997 at 585 K (∼25 hPa) and in February 1997 at 465 K (∼50 hPa). Minimum vortex‐averaged O3 mixing ratios observed in 1997 were higher than those in 1996, which were the lowest ever recorded by MLS. The vertical extent of O3 loss and maximum local O3 decreases were larger, but the decrease filled the vortex less completely, in 1997 than in 1996. Unusually low high‐latitude column O3 above 100 hPa in April 1997 resulted mainly from dynamical effects of the unusually persistent lower stratospheric vortex and winter‐like temperature patterns. Column O3 above 100 hPa averaged in comparable regions of the vortex showed a stronger decreasing trend in 1996–97 than in 1995–96, consistent with the larger vertical extent and maximum local values of lower stratospheric O3 loss. Chemical O3 loss resulted in an ∼10% observed decrease in column O3 between late January and early April 1997.

Modified Lagrangian‐mean diagnostics of the stratospheric polar vortices: 2. Nitrous oxide and seasonal barrier migration in the cryogenic limb array etalon spectrometer and SKYHI general circulation model

The Lagrangian‐mean transport and mixing properties associated with the life cycle of the stratospheric polar vortices are analyzed using nitrous oxide (N2O) as a tracer, both observed by the cryogenic limb array etalon spectrometer (CLAES) and simulated by the Geophysical Fluid Dynamics Laboratory SKYHI general circulation model. Based on the modified Lagrangian‐mean (MLM) diagnostic formalism, the area equivalent latitude‐potential temperature cross sections are constructed for the N2O mixing ratio and the squared equivalent length, a Lagrangian equivalence of the eddy diffusion coefficient Kyy. The robustness of the analysis is tested and confirmed by subsampling the SKYHI tracer field at a CLAES‐equivalent resolution and comparing the resultwith the full‐resolution analysis. The seasonal and interhemispheric variabilities of the MLM cross sections are examined in detail. Both CLAES and SKYHI identify two major barriers to horizontal mixing (minimal equivalent length) in both hemispheres: a perennial subtropical barrier and an annual polar barrier. The polar barrier splits from the subtropical barrier in fall, migrates poleward in winter, and disappears at the demise of the polar vortex. The barriers are in general collocated with a tracer edge (concentrated gradients), but larger gradients do not necessarily translate to a greater barrier. For example, at the Arctic vortex edge, where the tracer gradients are greater than at the Antarctic vortex edge, the barrier is actually weaker. A significant difference is found in the structure of the barriers between the two northern hemisphere winters examined.

CCl2F2 mixing ratio profiles in the 1995 late winter Arctic vortex from balloon‐borne spectra

The vertical profile of CCl2F2 has been retrieved in the altitude range 15 to 22 km from solar occultation spectra recorded with a balloon‐borne Fourier transform spectrometer (0.013 cm−1 resolution) launched on 22 March 1995 from Kiruna (Sweden, 67°N, 22°E). The choices of the grid of retrieval altitudes, of the spectral range, and of the absorption cross‐section data used for the retrievals are studied carefully. The vertical distribution obtained is characteristic of conditions pertaining to the late winter Arctic vortex. The measured mixing ratio profile of CCl2F2 is discussed in connection with the N2O profile also retrieved with the same instrument. These remote sensing measurements of CCl2F2 are compared with in situ results obtained from the same location and during the same period with balloon‐borne grab or cryosamplers. These data are analyzed in terms of diabatic descent in the polar vortex.

Vertical profiles of N2O5, HO2NO2, and NO2 inside the Arctic vortex, retrieved from nocturnal MIPAS‐B2 infrared limb emission measurements in February 1995

Vertical profiles Of N2O5, HO2NO2, and NO2 inside the arctic vortex were retrieved from nighttime infrared limb emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding, Balloon‐borne version 2 (MIPAS‐B2) instrument from Kiruna (Sweden, 68°N) on February 11, 1995, as part of the Second European Stratospheric Arctic and Midlatitude Experiment (SESAME). Spectra were analyzed by a multiparameter nonlinear least squares fitting procedure in combination with an onionpeeling retrieval algorithm. The N2O5, HO2NO2, and NO2 results were derived from spectral features within the bands near 8.0 μm 12.5 μm, and 6.2 μm, respectively. Peak mixing ratios of 1.14 parts per billion by volume (ppbv) N2O5 and 80 parts per trillion by volume (pptv) HO2NO2 at 17.1 hPa as well as 2.79 ppbv NO2 at 12.0 hPa corresponding to 25.8 km and 28.0 km altitude were inferred from the spectra. NO2 mixing ratios measured by MIPAS fit well to the data observed by concurrent flights. A comparison with calculations performed with a three‐dimensional chemistry transport model for the time and location of the measurements shows that the best agreement of measured and calculated profiles is reached between 17 and 28 hPa corresponding to 25.8 and 22.7 km altitude, while below and above this altitude region there are some discrepancies between the modeled and observed data.

Elliptical diagnostics of stratospheric polar vortices

AbstractDiagnostics based on the spatial moments of isopleths of a long‐lived tracer, or of potential vorticity, are presented that enable the structure and evolution of stratospheric polar vortices to be concisely summarized and quantified. The area, centre, aspect ratio and orientation of the ‘equivalent ellipse’ of the vortex, on an isentropic surface, are defined using the second‐ and lower‐order spatial moments of contours within the vortex‐edge region. By examining the variations of these ‘elliptical’ diagnostics with time and altitude, the temporal evolution and vertical structure of the polar vortices can be quantified. The usefulness of the diagnostics is assessed by examining nitrous oxide data from the Geophysical Fluid Dynamics Laboratory ‘SKYHI’ general‐circulation model. The diagnostics show, and quantify, several differences between the Arctic and Antarctic vortices in the SKYHI model. The Arctic vortex moves further off the pole, is generally more elongated, and has a more complicated vertical structure than the Antarctic vortex (with larger variations of both the vortex centre and elongation with height). The elliptical diagnostics also identify the occurrence of large‐scale Rossby‐wave breaking events, both at the vortex edge and in the subtropics, in the model.

Aircraft‐borne detection of stratospheric column amounts of O3, NO2, OClO, ClNO3, HNO3, and aerosols around the arctic vortex (79°N to 39°N) during spring 1993: 1. Observational data

Using several remote sensing techniques on board a research aircraft (Transall C‐160), we measured the stratospheric column amounts of O3, NO2, OClO, ClNO3, and HNO3, and the height‐resolved aerosol backscattering ratios during March 7–10, 1993. During this period the polar vortex extended from the Arctic well into the Mediterranean (35°N). Several underflights of the vortex and its edge region were conducted, covering Arctic (79°N) to Mediterranean latitudes (39°N). During all the flights, moderate amounts of NO2 (0.7–2·1015/cm2), elevated amounts of HNO3 (1.75–2.35·1016/cm2), ClNO3 (2.4–5.4·1015/cm2), and OClO (6–13·1012/cm2) were found inside the vortex, indicating that the air masses of the vortex were still chemically disturbed. The observation is interpreted as the polar vortex going from the stage of its wintertime denoxification (conversion of NOx into NOy;) and chlorine activation (conversion of HCl and ClNO3 into chlorine oxides) to the springtime reverse processes via the formation of enhanced amounts of ClNO3. In addition, first measurements of OClO (6–1012/cm2 at solar zenith angle 90°) and high amounts of ClNO3 (4–1015/cm2) around 40°N are reported.

A wintertime in situ profile of BrO between 17 and 27 km in the Arctic vortex

The vertical profile of bromine oxide (BrO) was measured in situ from a balloon launched near Kiruna, Sweden (68°N, 21°E) at sunrise on February 3, 1995. BrO mixing ratios of 10±2 parts per trillion by volume (pptv) were observed at high solar zenith angles (90–92°) between 20 and 23 km in a layer that coincided with significantly enhanced abundances of ClO. Above 23 km, where ClO mixing ratios typical of midlatitudes were observed, BrO mixing ratios ranged from 5 to 11 pptv, increasing nearly monotonically with increasing altitude and decreasing solar zenith angle. The measurements indicate that 30 to 60% of the total bromine (assuming 18 pptv total bromine) is in the form of BrO between 23 and 27 km at these latitudes in winter. These results are consistent with lower altitude measurements of BrO from the NASA ER‐2 at similar latitudes and season. Observations of higher fractions of BrO (50 to 95% of total bromine) in the chemically perturbed region at sunrise imply the rapid release of bromine from a photolabile reservoir species, such as BrCl.

What role do type I polar stratospheric cloud and aerosol parameterizations play in modelled lower stratospheric chlorine activation and ozone loss?

The chlorine activation and subsequent ozone loss of the northern winter lower stratosphere have been modelled using different schemes for type I polar stratospheric clouds (PSCs) and sulphate aerosols. Type I PSCs were assumed to consist of either nitric acid trihydrate (NAT) at equilibrium, supercooled ternary solutions (STS) at equilibrium, or to follow a hysteresis cycle between frozen and liquid particles depending on the temperature history. The sulphate aerosol was assumed to be present as either liquid binary H2SO4/H2O aerosol (LBA) or as solid sulphuric acid tetrahydrate (SAT). Our box model integrations show that NAT and STS, representing the upper and lower limits of lower stratospheric chlorine activation, respectively, appear to destroy ozone equally efficiently after a cold PSC event (Tmin ≤ 190K at 50 mbar). For higher minimum temperatures, up to the equilibrium NAT point, there is significantly more ozone loss in the NAT scheme than in the STS scheme. On NAT, chlorine is activated directly by ClONO2 + HCl → 2Cl + HNO3, whereas on STS, indirect activation by ClONO2 + H2O → HOCl + HNO3 followed by HOCl + HCl → 2Cl + H2O, dominates. During the processing period, the indirect activation on STS will produce a temporary peak in HOCl. Box model integrations also show that direct chlorine activation is faster on SAT than on LBA, yielding significantly more ozone loss in air parcels which remain below the SAT melting point (215–220 K). Our single‐layer chemical transport model simulations (θ = 465K) of the lower stratospheric chlorine activation during Arctic winter 1994/1995 show that chlorine is activated more quickly on NAT than on STS. However, in mid December 1994, when temperatures are low enough for substantial STS particle growth, maximum active chlorine becomes similar in both schemes and remains similar until the end of January 1995. A model integration which includes SAT produces up to 200 parts per trillion by volume more ClOx, inside the polar vortex during Arctic winter 1994/1995, than a model integration which includes LBA. The high melting point of SAT means that it may contribute to midlatitude ozone loss when filaments of processed air are shed by the vortex. For example, a model integration shows that air peeling off the Arctic vortex on February 14, 1995, contains 10% more ClOx at middle latitudes in an integration that includes SAT formation in a hysteresis scheme, than in an integration that includes LBA. The major differences in ozone loss predicted by the model PSC schemes occur inside the polar vortex. The largest differences in ozone at 465 K inside the vortex at the end of March 1995, up to 500 parts per billion by volume, are found between the equilibrium NAT and STS schemes. Ozone values in the hysteresis schemes are intermediate between those of the NAT and STS schemes. The inclusion of SAT in a hysteresis scheme des not have a major global effect.

Trace gas transport in the Arctic Vortex inferred from ATMOS ATLAS‐2 observations during April 1993

Measurements of the long‐lived tracers CH4, N2O, and HF from the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument during the Atmospheric Laboratory for Science and Applications‐2 (ATLAS‐2) Space Shuttle mission in April 1993 are used to infer average winter descent rates ranging from 0.8 km/month at 20 km to 3.2 km/month at 40 km in the Arctic polar vortex during the 1992–93 winter. Descent rates in the mid‐stratosphere are similar to those deduced for the Antarctic vortex using ATMOS/ATLAS‐3 measurements in November 1994, but the shorter time period of descent in the Arctic leads to smaller total distances of descent. Strong horizontal gradients observed along the vortex edge indicate that the Arctic vortex remains a significant barrier to transport at least until mid‐April in the lower to middle stratosphere.

Numerical simulation of the dynamical response of the Arctic Vortex to Aerosol‐associated chemical perturbations in the lower stratosphere

A general circulation model has been coupled interactively to a stratospheric photochemistry model to study the dynamical response of the Arctic polar vortex to ozone depletion catalyzed by volcanic aerosols. It is found that temperatures may be depressed by as much as 3–7 K in late March in the lower stratosphere at northern mid‐ and high‐latitudes owing to ozone‐radiation‐dynamics feedback effects. A corresponding delay in the breakdown of the stratospheric vortex (that is, the final warming) is predicted. Further, a dipole‐like anomaly in the zonal‐mean zonal wind, extending downward from the stratosphere into the troposphere, is found. In the model, the reduced solar heating in late winter and early spring associated with aerosol ozone depletion acts as a trigger for the net cooling that is observed. However, a major part of the cooling is related to anomalies induced in the circulation of the lower stratosphere. Our simulations demonstrate the close coupling between the dynamics and chemistry of the lower stratosphere, and the complexity in analyzing cause and effect related to perturbations in this region.

Chlorine activation and ozone depletion in the Arctic vortex: Observations by the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite

Chlorine‐catalyzed ozone destruction is clearly observed during austral spring in the Antarctic lower stratosphere. While high concentrations of ozone‐destroying ClO radicals have likewise been measured during winter in the Arctic stratosphere, the chemical ozone depletion there is more difficult to quantify. Here we present observations of the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite in the vortex region of the Arctic lower stratosphere during the winter and spring months of 1991/1992, 1992/1993, 1993/1994, and 1994/1995. All February measurements indicate an almost complete conversion of the otherwise main chlorine reservoir species HCl to chemically more reactive forms. Using CH4 as a chemically conserved tracer, we show that significant chemical ozone loss occurred in the Arctic vortex region during all four winters. The deficit in column ozone was about 60 and 50 Dobson units (DU) in the winters 1991/1992 and 1993/1994, respectively. During the two winters of 1992/1993 and 1994/1995 a severe chemical loss in lower‐stratospheric ozone took place, with local reductions of the mixing ratios by over 50% and a loss in the column ozone of the order of 100 DU.

Model calculations of ozone depletion in the Arctic Polar Vortex for 1991/92 to 1994/95

We have used a 3D chemical transport model to investigate the potential for chemical ozone depletion in the Arctic polar vortex for the past 4 winters. By using the same chemical initial conditions for each winter we have isolated the effects of meteorological variability on polar ozone loss. These calculations show that during 1994/95 conditions in the Arctic polar vortex were the most conducive to ozone depletion in recent years. Our numerical experiments show that at 475 K (around 18 km) in the lower stratosphere the chemical ozone destruction between late December and mid March was greater than 30% over a large area of the polar vortex. Larger percentage losses, reaching 50%, occur at slightly lower altitudes. The average model depletion in the entire region poleward of 60°N over the same period was around 22% at 475 K. Similar calculations for the preceding three Arctic winters show less depletion. The large depletion for 1994/95 is a consequence of the extreme meteorological conditions during this winter.

Evidence for the removal of gaseous HNO3 inside the arctic polar vortex in January 1992

Unexpectedly low column amounts of HNO3 (1.3×1016 cm−2) inside the Arctic polar vortex were obtained on January 9, 1992 by airborne FTIR emission measurements. After correction for dynamic variations it is concluded that about 40% of the column HNO3 had been removed from the gas phase. The observed depletion is consistent with a nearly equilibrium saturation of HNO3 over nitric acid trihydrate. It cannot be explained by the uptake of HNO3 in ternary H2SO4/H2O/HNO3 mixtures.

Vertical profiles of N2O5 along with CH4, N2O, and H2O in the late Arctic winter retrieved from MIPAS‐B infrared limb emission measurements

Vertical profiles of N2O5, CH4, N2O, and H2O inside the arctic vortex were retrieved from nighttime infrared limb emission spectra obtained during a flight of the Michelson interferometer for passive atmospheric sounding, balloonborne version (MIPAS‐B) Fourier spectrometer from Kiruna (Sweden, 68°N) on March 14/15, 1992, as part of the European Arctic Stratospheric Ozone Experiment. Spectra were analyzed by a nonlinear multiparameter least squares fitting procedure in combination with an onion‐peeling retrieval algorithm. The N2O5 results were derived from the intensity of the v12 band near 8 μm. These data represent the first ever reported N2O5 profile inside the polar vortex. Between 21.5 and 31.7 km altitude, N2O5 mixing ratios from 0.38 to 0.74 parts per billion by volume (ppbv) were inferred. Below 21.5 km there is a steep decrease in the mixing ratio toward values lower than 0.07 ppbv at 18.9 and 16.1 km. This discontinuity in the vertical profile correlates in altitude with the bulk of the Pinatubo aerosol layer inside the arctic vortex. N2O5 concentrations are calculated as a function of time since local sunset by using initial NO2 concentrations, O3 concentrations, aerosol surface area densities, and reaction rate coefficients, as found in the literature; calculated N2O5 concentrations are consistent with the MIPAS results. These suggest efficient heterogeneous hydrolysis of N2O5 having taken place on sulphate aerosol particles. Retrieved CH4 and N2O profiles reflect the subsided polar vortex air.

Three‐dimensional simulation of the influence of a cutoff low on the distribution of northern hemisphere processed air in late January 1992

The NASA Goddard three‐dimensional chemistry and transport model (3D CTM), utilizing winds and temperatures from a stratospheric data assimilation, is used to investigate the influence of a large cutoff low (COL) system on the transport of chemically processed stratospheric air. The conversion of hydrogen chloride to reactive chlorine is simulated for the early winter of 1991/1992. This heterogeneously converted air is defined as being processed. Throughout December and early January, simulated processed air remained nearly isolated within the Arctic vortex. In late January there were intrusions of midlatitude air across the boundary of the vortex over the North Atlantic and northwestern Europe. These intrusions, which are associated with horizontal transport associated with the COL system, dilute regions of highly processed air within several days. The simulation demonstrates the importance of synoptic scale events on processed air distribution within the vortex as well as transport out of the vortex base into the upper troposphere and midlatitudes.

Ozone loss in middle latitudes and the role of the Arctic polar vortex

There have been a number of different suggestions as to the cause of the observed ozone decline over middle latitudes. Here, the particular impact of polar processes on the middle latitude lower stratosphere is discussed. Recent studies suggest that air, recently activated and then torn from the edge of the polar vortex, contributes to the observed ozone decrease. For example, observational and modelling studies both indicate that there is an important role for filaments of vortex air being stripped away from the vortex edge. However, there appears to be little support for the idea of the vortex as a massive ‘flowing processor’ through which large quantities of air, primed for ozone destruction, are transported.

Interhemispheric differences in springtime production of HCl and ClONO2 in the polar vortices

UARS observations of O3 and ClO (Microwave Limb Sounder), ClONO2 and HNO3 (Cryogenic Array Etalon Spectrometer), NO, NO2, and HCl (Halogen Occultation Experiment), and model calculations are used to produce an exposition of the different processes through which the reservoir gases ClONO2 and HCl are reformed at the end of the polar winter. Comparison of the observations within the polar vortices shows that HCl increases more rapidly in the Antarctic vortex in spring than in the Arctic vortex. Model analysis shows that this occurs because the O3 concentrations in the southern vortex fall well below those in the northern vortex. The Cl/ClO fraction calculated for the southern hemisphere is therefore up to 30 times higher, leading to rapid HCl formation by Cl + CH4. The concentrations of NO observed by HALOE are substantially lower for the northern hemisphere than for the southern hemisphere, even for similar values of the concentration of HNO3 and the production of NOX from HNO3 through photolysis and reaction with OH. This is consistent with the dependence of the NO/NOX ratio on the O3 concentration, i.e., the daytime production rate of NO2 via NO + O3 is reduced, leading to higher NO in the southern hemisphere. This higher concentration of NO also contributes to the rapid HCl increase as Cl production from ClO + NO is enhanced.

April 1993 Arctic profiles of stratospheric HCl, ClONO2, and CCl2F2 from atmospheric trace molecule spectroscopy/ATLAS 2 infrared solar occultation spectra

Partitioning among the major components of the stratospheric odd chlorine family inside and outside of the remanent Arctic vortex has been studied on the basis of infrared solar occultation measurements obtained by the atmospheric trace molecule spectroscopy (ATMOS) Fourier transform spectrometer during the ATLAS 2 shuttle mission (April 8–17, 1993). Profiles of hydrogen chloride (HCl) and simultaneous profiles of chlorine nitrate (ClONO2) and CFC‐12 (CCl2F2) are reported for examples of in‐vortex and out‐of‐vortex conditions. Increased ClONO2 volume‐mixing ratios (VMRs) are measured in the vortex below 20 mbar (∼25 km altitude) with a peak ClONO2 VMR of 2.05±0.45 ppbv (10−9 per volume) at 56 mbar (∼19 km altitude). The reported error correspond to 1σ uncertainties. Simultaneous CCl2F2 and N2O measurements, combined with published empirical relations, indicate that only 0.34±0.15 ppbv, about 10% of total chlorine, was bound in organic species at the ClONO2 VMR peak in the vortex. A colocated vortex profile of HCl, referenced to simultaneous N2O VMR measurements, has been used to derive a HCl mixing ratio of 1.21±0.12 ppbv corresponding to the ClONO2 VMR peak. The internal consistency of the ATMOS measurements is demonstrated by the agreement between the total chlorine mixing ratio of 3.60±0.72 ppbv derived at the ClONO2 VMR peak in the vortex and HCl measurements of 3.37±0.37 and 3.76±0.41 ppbv at 0.56 mbar, where HCl is the only significant chlorine‐bearing molecule. Outside the vortex the mixing ratio of HCl exceeds the mixing ratio of ClONO2 throughout the stratosphere.

Determination of the stratospheric organic chlorine budget in the spring arctic vortex from MIPAS B limb emission spectra and air sampling experiments

Vertical profiles of halogenated source gases, CF2Cl2, CFCl3, CHF2Cl, Cl4, and CF4, were retrieved from limb emission spectra recorded by the Michelson Interferometer for Passive Atmospheric Sounding, Balloonborne version (MIPAS B) during a balloon flight launched from Esrange near Kiruna, northern Sweden (68°N) on March 14, 1992. This flight was a contribution to the balloon launch program of the European Arctic Stratospheric Ozone Experiment (EASOE) campaign. All problems encountered during the analysis of the recorded spectra are discussed in detail. These are primarily the lack of spectral data for HNO3 which interferes with the CF2Cl2 v6 band, and the strong effects attributed to the Pinatubo aerosol. As the air mass sounded by MIPAS was polar vortex air, these data supplement the results of in situ air sampling experiments, which investigated air masses outside or at the edge of the polar vortex at altitudes below 18 km during the last phase of EASOE. An analysis is made of the vertical profiles of the seven most abundant organic chlorine species (CF2Cl2, CFCl3, CHF2Cl, CCl4, CH3Cl, CH3CCl3, and C2F3Cl3) during that phase of the EASOE campaign. Mixing ratios of those organic chlorine compounds which had not been measured by MIPAS are inferred from profiles provided by air sampling experiments performed between November 30, 1991, and March 12, 1992. These profiles were adjusted to the dynamic conditions during the MIPAS observations, namely the effect of subsidence, using CF2Cl2 as a tracer. This allowed to derive the relative contributions of the organic chlorine species to the total chlorine budget of the air mass sounded by MIPAS. The results are consistent with the high ClONO2 mixing ratio of 2.6 parts per billion by volume (ppbv) observed at 18.9‐km altitude during this flight of MIPAS B.

Spatial and temporal variability of ClONO2, HNO3, and O3 in the Arctic winter of 1992/1993 as obtained by airborne infrared emission spectroscopy

In the winter of 1992/1993 the airborne Michelson interferometer for passive atmospheric sounding (MIPAS‐FT) was operated on board a German research aircraft (Transall C‐160) to record infrared emission spectra of the atmosphere inside and outside the Arctic vortex. Measurements were made during four campaigns between December 4, 1992, and March 29, 1993, in the European Arctic as well as over central and southern Europe (82°N–37.5°N). We present the retrieved zenith column amounts of the stratospheric trace gases ClONO2, HNO3, and O3 of this period. Inside the polar vortex, the column amounts of ClONO2 and HNO3 were considerably enhanced already in early December, up to 3.1×1015 cm−2 and 2.7×1016 cm−2, respectively. Around the end of January, low ClONO2 (1×1015 cm−2) and high HNO3 column amounts (up to 3.7×1016 cm−2) were observed inside the vortex, whereas a highly variable “collar” of ClONO2 had developed at the vortex edge. During March, after temperatures had been above the threshold for polar stratospheric clouds (PSCs) for several weeks, we measured lower HNO3 (below 2.5×1016 cm−2) and very high ClONO2 column amounts (up to 6×1015 cm−2) inside the vortex. Thus a major part of the reactive chlorine had been converted into ClONO2, and the potential for rapid ozone depletion was reduced markedly in the region observed. On March 10, when the polar vortex extended southward to the Mediterranean, ClONO2 column amounts as high as 4.6×1015 cm−2 were observed at 40°N. At the end of March, considerable amounts of ClONO2 (up to 3.4×1015 cm−2) were measured also far outside the vortex.