ARTICLEdoi:10.1038/nature10556

Unprecedented Arctic ozone loss in 2011Gloria L. Manney1,2, Michelle L. Santee1, Markus Rex3, Nathaniel J. Livesey1, Michael C. Pitts4, Pepijn Veefkind5,6, Eric R. Nash7,Ingo Wohltmann3, Ralph Lehmann3, Lucien Froidevaux1, Lamont R. Poole8, Mark R. Schoeberl9, David P. Haffner7,Jonathan Davies10, Valery Dorokhov11, Hartwig Gernandt3, Bryan Johnson12, Rigel Kivi13, Esko Kyro13, Niels Larsen14,Pieternel F. Levelt5,6,15, Alexander Makshtas16, C. Thomas McElroy10, Hideaki Nakajima17, Maria Concepcion Parrondo18,David W. Tarasick10, Peter von der Gathen3, Kaley A. Walker19 & Nikita S. Zinoviev16

Chemical ozone destruction occurs over both polar regions in local winter–spring. In the Antarctic, essentially completeremoval of lower-stratospheric ozone currently results in an ozone hole every year, whereas in the Arctic, ozone loss ishighly variable and has until now been much more limited. Here we demonstrate that chemical ozone destruction overthe Arctic in early 2011 was—for the first time in the observational record—comparable to that in the Antarctic ozonehole. Unusually long-lasting cold conditions in the Arctic lower stratosphere led to persistent enhancement inozone-destroying forms of chlorine and to unprecedented ozone loss, which exceeded 80 per cent over 18–20kilometres altitude. Our results show that Arctic ozone holes are possible even with temperatures much milder thanthose in the Antarctic. We cannot at present predict when such severe Arctic ozone depletion may be matched orexceeded.

Since the emergence of the Antarctic ‘ozone hole’ in the 1980s1 andelucidation of the chemical mechanisms2–5 and meteorological con-ditions6 involved in its formation, the likelihood of extreme ozonedepletion over the Arctic has been debated. Similar processes are atwork in the polar lower stratosphere in both hemispheres, but differ-ences in the evolution of the winter polar vortex and associated polartemperatures have in the past led to vastly disparate degrees of spring-time ozone destruction in the Arctic and Antarctic. We show thatchemical ozone loss in spring 2011 far exceeded any previouslyobserved over the Arctic. For the first time, sufficient loss occurredto reasonably be described as an Arctic ozone hole.

Arctic polar processing in 2010–11In the winter polar lower stratosphere, low temperatures inducecondensation of water vapour and nitric acid (HNO3) into polarstratospheric clouds (PSCs). PSCs and other cold aerosols providesurfaces for heterogeneous conversion of chlorine from longer-livedreservoir species, such as chlorine nitrate (ClONO2) and hydrogenchloride (HCl), into reactive (ozone-destroying) forms, with chlorinemonoxide (ClO) predominant in daylight5,7.

In the Antarctic, enhanced ClO is usually present for 4–5 months(through to the end of September)8–11, leading to destruction of mostof the ozone in the polar vortex between ,14 and 20 km altitude7.Although ClO enhancement comparable to that in the Antarcticoccurs at some times and altitudes in most Arctic winters9, it rarelypersists for more than 2–3 months, even in the coldest years10. Thuschemical ozone loss in the Arctic has until now been limited, withlargest previous losses observed in 2005, 2000 and 19967,12–14.

The 2010–11 Arctic winter–spring was characterized by ananomalously strong stratospheric polar vortex and an atypically longcontinuously cold period. In February–March 2011, the barrier to

transport at the Arctic vortex edge was the strongest in either hemi-sphere in the last ,30 years (Fig. 1a, Supplementary Discussion).

The persistence of a strong, cold vortex from December through tothe end of March was unprecedented. In the previous years with mostozone loss, temperatures (T) rose above the threshold associated withchlorine activation (Tact, near 196 K, roughly the threshold for thepotential existence of PSCs) by early March (Fig. 1b, SupplementaryFigs 1, 2). Only in 2011 and 1997 have Arctic temperatures below Tact

persisted through to the end of March, sporadically approaching avortex volume fraction similar in size to that in some Antarctic winters(Fig. 1b). In 1996–97, however, the cold volume remained very limiteduntil mid-January and was smaller than that in 2011 at most timesduring late January through to the end of March (Fig. 1b, Supplemen-tary Figs 1, 2).

Daily minimum temperatures in the 2010–11 Arctic winter werenot unusually low, but the persistently cold region was remarkablydeep (Supplementary Figs 1, 2). Temperatures were below Tact formore than 100 days over an altitude range of ,15–23 km, comparedto a similarly prolonged cold period over only ,20–23 km altitude in1997; below ,19 km altitude, T , Tact continued for ,30 days longerin 2011 than in 1997 (Supplementary Fig. 1b). In 2005, the previousyear with largest Arctic ozone loss7, T , Tact occurred for more than100 days over ,17–23 km altitude, but all before early March.

The winter mean volume of air in which PSCs may form (that is,with T , Tact), Vpsc, is closely correlated with the potential for ozoneloss7,15–17. In 2011, Vpsc (as a fraction of the vortex volume) was thelargest on record (Fig. 1c). Both large Vpsc and cold lingering well intospring are important in producing severe chemical loss7,15,16, and2010–11 was the only Arctic winter during which both conditionshave been met. Much lower fractional Vpsc in 1997 than in 1996, 2000,2005 or 2011 (Fig. 1c) is consistent with less ozone loss that year16,17.

1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA. 2New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA. 3Alfred WegenerInstitute for Polar and Marine Research, D-14473 Potsdam, Germany. 4NASA Langley Research Center, Hampton, Virginia 23681, USA. 5Royal Netherlands Meteorological Institute, 3730 AE De Bilt, TheNetherlands. 6Delft University of Technology, 2600 GA Delft, The Netherlands. 7Science Systems and Applications, Inc., Lanham, Maryland 20706, USA. 8Science Systems and Applications, Inc., Hampton,Virginia 23666, USA. 9Science and Technology Corporation, Lanham, Maryland 20706, USA. 10Environment Canada, Toronto, Ontario, Canada M3H 5T4. 11Central Aerological Observatory, Dolgoprudny141700, Russia. 12NOAA Earth System Research Laboratory, Boulder, Colorado 80305, USA. 13Arctic Research Center, Finnish Meteorological Institute, 99600 Sodankyla, Finland. 14Danish ClimateCenter, Danish Meteorological Institute, DK-2100 Copenhagen, Denmark. 15Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands. 16Arctic and Antarctic Research Institute, StPetersburg 199397, Russia. 17National Institute for Environmental Studies, Tsukuba-city, 305-8506, Japan. 18National Institute for Aerospace Technology, 28850 Torrejon De Ardoz, Spain. 19University ofToronto, Toronto, Ontario, Canada M5S 1A7.

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Factors playing secondary parts in governing interannual variabilityin ozone destruction, including vortex strength, structure and posi-tion relative to the cold region, also favour large loss in 2011 (Sup-plementary Figs 2, 3, Supplementary Discussion). However, despitethe fraction of the vortex with T , Tact and mid-March temperaturessporadically approaching those seen in the Antarctic (Fig. 1b,Supplementary Fig. 1a), even in 2011 temperatures were much higher,and the cold regions much smaller, than those in most Antarcticwinters.

Satellite trace-gas and PSC measurements highlight the stark con-trast between polar processing in 2010–11 and that in typical Arcticwinters, and the parallels with Antarctic conditions (Figs 2, 3). In 2011,PSCs or aerosols were abundant until mid-March (Fig. 3a; consistentwith a deep region with T , Tact, Fig. 3b), much later than usual in theArctic18–20, with vortex-average amounts at some altitudes similar tothose in the Antarctic and dramatically larger than the near-zero valuesat that time in most Arctic winters. Furthermore, PSCs in 2011spanned an altitude range comparable to that in the Antarctic, anuncommon occurrence in the Arctic18–20. Particles in long-lastingPSCs can grow large enough to sediment, resulting in denitrification,permanent removal of HNO3 from the stratosphere7,12. By late March2011 no PSCs remained (Fig. 3a), yet HNO3 mixing ratios were muchlower than observed in any previous Arctic winter (Fig. 2a). The con-tinuing depression in HNO3 after PSCs had evaporated indicates

denitrification. Albeit less severe than in typical Antarctic winters(Fig. 2b, c, 3c), the extent and degree of denitrification in 2011 wereunmatched in the Arctic, approaching the range of Antarctic condi-tions for the first time.

Decreasing HCl and increasing ClO signify chlorine activation(Fig. 2d–i). Some ClO enhancement has occurred in all recentArctic winters, but has never been as prolonged and extensive as thatin 2011. In late February, high ClO pervaded the sunlit portion of thevortex. The 2011 values vastly exceed the range previously observed inthe Arctic from late February through to the end of March. They alsobriefly lie outside the Antarctic seasonal envelope, primarily becausethe higher solar zenith angles of the Antarctic measurements usedhere lead to ,30% lower ClO under fully activated conditions. In lateFebruary, HCl values (unaffected by solar zenith angle issues) fallalong the lower boundary of the Antarctic envelope, confirming thepicture seen in ClO. The vertical extent of chlorine activation was alsocomparable to that in the Antarctic (Fig. 3d, e).

In previous cold Arctic winters, chlorine was deactivated (convertedfrom ozone-destroying forms into less reactive reservoir species) bymid-March11; even in 1997, ClO started to decline by late February(Fig. 2g). In 2011, by contrast, ClO began decreasing rapidly only abouta week earlier than is typical in the Antarctic. ClO data in late February1997 indicate that not only were maximum values lower than those inearly March 2011, but also the vertical range of enhancement wasshallower, with weaker activation at low altitudes than in 2011(Fig. 3e), consistent with the higher altitudes and decreasing extent(Figs 1b, 3b, Supplementary Fig. 2) of T , Tact.

When chlorine is deactivated, whether it is converted first into HClor ClONO2 depends sensitively upon HNO3 and ozone abundances. Inthe Arctic, chlorine is normally deactivated through initial reformationof ClONO2. In the severely denitrified and ozone-depleted Antarcticvortex, production of ClONO2 is suppressed and that of HCl highlyfavoured11,12,21. In March 2011, the recovery of HCl followed a muchmore Antarctic-like pathway than has been observed in any otherArctic winter.

The largest Arctic chemical ozone loss was previously observed in2005, followed closely by 2000 and 19967,12–14. Although low tempera-tures persisted until the end of March 1997, the ozone loss in that yearwas far less. No previous year rivals 2011, when the evolution of Arcticozone more closely followed that typical of the Antarctic (Fig. 2j). Ozoneprofiles in late March 2011 resemble typical Antarctic late-winter pro-files much more strongly than they do the average Arctic one (Fig. 3f).Because mixing in April 2011 (for example, lamination events largerthan that shown in Fig. 3f) entrained ozone-rich air into the vortex, theslight decrease in vortex-averaged ozone at a potential temperature of485 K from 26 March to 20 April (from ,1.8 to ,1.6 p.p.m.v., Fig. 2j)indicates continuing chemical loss during this interval.

Estimates of chemical ozone lossChemical loss is difficult to quantify in the Arctic, where transportfrom above replenishes ozone in the lower stratospheric vortex,obscuring the signature of chlorine-catalysed destruction12,22,23. Theevolution of the long-lived trace gas nitrous oxide (N2O) reflects steadydownward transport throughout the 2010–11 winter–spring, indi-cating that subsidence partially masked chemical loss. Horizontaltransport can also confound the signature of chemical loss, bringingair into the vortex that has either higher24 or lower14 concentrations ofozone, depending on the altitude and latitude from which it originates.

Representative results from two types of chemical loss calcula-tions24–28 based on balloon-borne and satellite observations are shownin Fig. 4. The differences (up to ,0.4 p.p.m.v. at the end of March2011) in estimates derived from the various methods and data setsimply some uncertainty in the chemical loss determination. Year-to-year differences in the amount of ozone loss are very similar whenobtained from any method/data set combination, however, indicatinga high degree of precision in the relative amount of calculated loss

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Figure 1 | Meteorology of the Arctic lower stratosphere. a, Vortex strength(as indicated by maximum potential vorticity49 (PV) gradients) at 460 Kpotential temperature (,18 km altitude, ,65 hPa level). b, Fraction of vortexvolume at potential temperatures between 390 and 550 K with a temperatureless than the chlorine activation threshold (Tact). Light (dark) grey shadingshows range of Arctic (Antarctic) values for 1979–2010. Antarctic dates areshifted by six months (top axis in a) to show the equivalent season. c, Wintermean Vpsc during the past 32 years, expressed as a fraction of vortex volume.Red, orange, green, purple and blue lines/bars show the 2010–11, 2004–05,1999–2000, 1996–97 and 1995–96 Arctic winters, respectively.

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between different years. Chemical destruction was severe between,16 and 22 km altitude, with the largest loss exceeding 2.5 p.p.m.v.by 26 March 2011 (Fig. 4a). By 31 March 2011, chemical loss wasnearly double that in 2005 from ,18 to above 22 km, and similar tothat in 2005 at lower altitudes (Fig. 4b, c). From ,18 to 20 km, morethan 80% of the ozone present in January had been chemicallydestroyed by late March. Chemical removal in 1996 and 2000 startedat a rate similar to that in 2011 (Fig. 4c), but ceased by late March;maximum losses in 2000 approached those in 2011, but extended overa much smaller vertical range (Fig. 4b). Loss in 1996, 2000 and 2005considerably exceeded that in 1997, with greater destruction at loweraltitudes in those years contributing more to total column loss7,12,13.Chemical loss in 2011 was two to three times larger than that in 1997,and about twice that in 1996 and 2005 above ,16 km; from ,15 to23 km it was comparable to that in the Antarctic ozone hole in 198529.

Single ozone-sonde station measurements in early April 2011 suggestcontinuing ozone loss (Fig. 4c).

Although the meteorology during March–April was similar in 1997and 2011, ozone loss was much more pronounced in 2011. Photo-chemical box model simulations (Supplementary Fig. 4, Supplemen-tary Discussion) elucidate how early winter conditions set the stage forrecord springtime ozone destruction in 2011. Chlorine activationbrought on by enduring cold from December through to the end ofFebruary led to ,0.7–0.8 p.p.m.v. lower ozone at the beginning ofMarch 2011 (Figs 2j, 4c). The early onset of continuous cold alsofacilitated formation of PSC particles large enough to sediment, result-ing in ,4 p.p.b.v. less HNO3 by March in 2011 than in 1997 (Fig. 2a).The degree of denitrification has a profound impact on the severity ofspringtime Arctic ozone loss30. By delaying chlorine deactivation,lower HNO3 by 1 March was responsible for ,0.6 p.p.m.v. more ozone

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Figure 2 | Chemical composition in the lower stratosphere. a–l, Maps (right)and vortex-averaged time series (left) at 485 K potential temperature (,20 km,,50 hPa) for four different gases: HNO3 (a, b, c), HCl (d, e, f), ClO (g, h, i) andO3 (ozone; j, k, l); mixing ratios from Aura MLS are shown. Averaging for thetime series is done within the white contour shown on the maps. Blue (purple)triangles on time series, 1995–96 (1996–97) values from UARS MLS. Linecolours/shading as in Fig. 1, but shading is for Aura MLS measurements from

2005–10. Antarctic dates are shifted by six months (top axis on time series) toshow the equivalent season. Vertical lines show dates of maps in 2011 (2010) inthe Arctic (Antarctic). Black overlays on HNO3 maps, Tact (,196 K at thislevel); HNO3 may be sequestered in PSCs at lower temperatures. Dotted black/white contour on ClO maps, 92u SZA, poleward of which measurements weretaken in darkness. Yellow/black triangles on ozone maps, locations of theprofiles in Fig. 3.

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loss after that date in 2011 than in 1997 (Supplementary Fig. 4,Supplementary Discussion). The effects of denitrification and early-winter loss together account for the disparity in ozone depletion inthese two winters (,1.5 p.p.m.v. more loss at 460 K in 2011 than in1997, Fig. 4c, Supplementary Fig. 4). Loss as severe as that in 2011 thus

requires T , Tact, with consequent chlorine activation and ozonedestruction, early in winter (as in 1996, 2000 and 2005, but not in1997), a cold period and region before March sufficient to allow wide-spread denitrification, and the persistence of a cold polar vortex intoApril (as in 1997, but not in 1996, 2000 or 2005).

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Figure 3 | Vertical composition information. a, Red, PSCs/aerosol amountsaveraged in the vortex over a week centred around 25 February 2011; dark blue,the average for the same week in 2007–10; grey, the average over the equivalentperiod (centred on 28 August) for the Antarctic in 2006–10; lavender, the Arcticaverage for a week centred around 26 March 2011. (In late winter–spring,maximum PSC altitudes are generally higher in the Arctic because early winterPSC activity redistributes HNO3 and water vapour to lower altitudes in theAntarctic18). b–f, Daily average profiles of MERRA temperatures (b) and MLSHNO3 (c), HCl (d), ClO (e) and ozone (f). Red lines, data from a 4u3 15ulatitude 3 longitude box around 79uN, 12uE; in c, f, taken on 26 March; in

b, d, e, on 6 March 2011. Lavender, 7-day average for 2005–10 (1980–2010 forb) centred on the same location and days. Grey, profiles in a similar box in theAntarctic (79u S, 12uE) on 26 September for c, f, and on 8 September 2010 forb, d, e. Dotted black line in b, approximate Tact (195 K), see text. Purple line inb, 7-day average around 6 March 1997, centred on same location. Purple line ine, a midday ClO profile from UARS MLS on 26 February 1997 averaged in an8u3 30u box centred at the same Arctic location. A high-resolution ozone-sonde profile at Ny Alesund on 26 March 2011 (black in f) agrees well withMLS; lamination, a signature of mixing with ozone-rich extra-vortex air, isapparent as a local maximum near 60 hPa.

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Figure 4 | Chemical ozone loss estimates. a, Chemical loss as a function oftime and potential temperature from passive subtraction of MLS and ATLASpassively-transported ozone (initialized with December MLS data).b, Chemical loss from ozone sondes in unmixed vortex air as a function of‘spring equivalent potential temperature’48 (black contours in a). Shading,Antarctic range defined by 1985 (the first year with profile measurementsinside the ozone hole29) and 2003 (a recent year with a severe ozone hole). The2003 Antarctic curve is shifted by six months minus 10 days because ozonesondes that year predominantly sampled the outermost vortex, where ozone

loss begins earliest. c, Ozone at a spring equivalent potential temperature of465K (white contour in a), near the level of maximum chemical loss. Shading,the region below the minimum reached in the 1985 Antarctic ozone hole. InApril 2011 most soundings sampled the disturbed vortex edge; only two weremade in air uninfluenced by mixing (red dots). Error bars, 1s uncertaintiesbased on the scatter of individual ozone-sonde measurements. Line colours asin Fig. 1; 1998–99 (a winter with no ozone loss) is shown in cyan. NH, NorthernHemisphere; SH, Southern Hemisphere.

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Column ozoneTotal column ozone is a predominant factor determining exposure ofEarth’s surface to ultraviolet radiation7,12. In the context of previousArctic winters, 2011 was truly remarkable: the fraction of the Arcticvortex in March with total ozone less than 275 Dobson units (DU) istypically near zero, but reached nearly 45% in 2011 (Fig. 5a). Becauseof the dynamically-driven correlation between total ozone and lower-stratospheric temperature23,31–34 (Supplementary Discussion), theabiding cold in 1997 and 2011 would have led to lower March totalozone than in other Arctic winters even without chemical loss;dynamical conditions in March–April 1997 particularly favouredlow total ozone33 (Supplementary Discussion). In March 2011, however,the area of low total ozone covered more than twice as much of thevortex as in 1997, and the daily vortex ‘ozone deficit’ (SupplementaryFig. 5a) was 30–50 DU larger, consistent with the greater chemical loss(Fig. 4). Maximum 2011 vortex fractions of low ozone approachedthose in early Antarctic ozone holes (Fig. 5a). The close correspond-ence between the vortex and both low total ozone and the large Arctictotal ozone deficit (Fig. 5b, d) implies that low total ozone in March2011 resulted primarily from chemical loss31,32 (SupplementaryDiscussion). The ozone deficit in the Antarctic (Fig. 5e) shows amaximum over 0–90uW, and a minimum over 90–200uE, reflectinga vortex position in 2010 different to that in the reference state (whichis less robust than that for the Arctic). Differences in morphology deepin the vortex are, however, minimal. The 2011 Arctic ozone deficit wasat least comparable to that in the 2010 Antarctic vortex core at anequivalent time.

An echo of the AntarcticIn the absence of chemical ozone loss, downward transport duringwinter results in a springtime maximum in total ozone; because thistransport is stronger in the Arctic, background ozone levels there are,100 DU higher than those in the Antarctic7,23. Therefore Arcticspring total ozone could, even after chemical destruction comparableto that in an Antarctic ozone hole (commonly defined by values lessthan 220 DU; refs 7, 12), exhibit only a weak maximum in total ozonerather than a well-defined minimum. Examination of the long-termozone-sonde record in the Arctic shows that abundances near 250 DUor less are well below typical autumn values, thus appearing as a ‘hole’in total ozone. Dynamical processes can result in transient regions ofvery low total ozone (Supplementary Discussion, Supplementary Figs5, 6) and/or local minima in lower-stratospheric ozone profiles (forexample, via ozone-poor extra-vortex air transported into the polarvortex14,24). For an interhemispheric comparison of chemical loss, it isthus important to verify that observed Arctic ozone decreases wereprimarily related to chemical, rather than dynamical, processes.

Figure 4 shows that the precipitous decline in Arctic ozone inFebruary–March 2011 resulted from chemical loss of similar mag-nitude to that in the Antarctic in the mid-1980s. Observed ozonebetween ,15 and 20 km altitude decreased to values matching theminima in early Antarctic ozone holes and those reached at the cor-responding time in some recent Antarctic winters (Figs 2j–l; 3f). Inlate March–early April, most ozone-sonde profiles in the vortex hadmixing ratios less than 1 p.p.m.v., with values ,0.7 p.p.m.v. over anapproximately 2-km altitude region, and some dipping to 0.5 p.p.m.v.(Supplementary Fig. 7). Minimum total ozone in spring 2011 wascontinuously below 250 DU for ,27 days (Supplementary Fig. 5b),with a maximal area below that level of ,2 3 106 km2 (roughly fivetimes the area of Germany or California). Values dropped to ,220–230 DU for about a week in late March 2011.

In these respects, chemical ozone destruction in the 2011 Arcticpolar vortex attained, for the first time, a level clearly identifiable as anArctic ozone hole. On the other hand, although the magnitude ofchemical depletion was comparable to that in the Antarctic, totalozone values remained higher and, because the areal extent of theArctic vortex was much smaller (,60% the size of a typicalAntarctic vortex), the low-ozone region was more confined.

The Arctic winter stratosphere exhibits striking interannual vari-ability. The past decade has included the four most dynamically active(hence among the warmest) Arctic winters in the past 32 years (ref.35) and now the two coldest winters with largest ozone loss7,12–14,extending the previously noted trend of the coldest winters becomingcolder13,16. Had implementation of the Montreal Protocol not curbedthe increase in stratospheric halogen loading, formation of an Arcticozone hole would have already become common even in moderatelycold winters36. Even with the lower anthropogenic halogen levelsactually reached, the potential for Antarctic-like ozone loss in theArctic in the event of a persistently cold winter–spring such as thatin 2010–11 has been recognized for decades5,22. Despite temperaturesthat were generally far higher than those in Antarctic winter, Arcticchemical ozone destruction in 2011 rivalled that in some Antarcticozone holes. The development of an Arctic ozone hole under condi-tions only slightly more extreme than those in some previous Arcticwinters raises the possibility of yet more severe depletion as lower-stratospheric temperatures decrease. More acute Arctic ozonedestruction could exacerbate biological risks from increased ultra-violet radiation exposure, especially if the vortex shifted over denselypopulated mid-latitudes, as it did in April 2011.

Our present understanding of what drives variability in the Arcticwinter stratosphere is incomplete. Stratospheric temperatures andvortex evolution depend on the atmosphere’s radiative propertiesand propagation of wave activity37,38, which are being modified byincreasing greenhouse gas concentrations. Day-to-day troposphericdisturbances can lead to stratospheric warming or cooling, depending

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Figure 5 | Total column ozone. a, Time series of the fraction of 460 K vortexarea with total ozone below 275 Dobson units (DU) in February–April in theArctic (bottom axis), and in August–October in the Antarctic (top axis). Linecolours/shading as in Fig. 1. 2005–2011 values are from OMI; earlier values arefrom TOMS (Total Ozone Mapping Spectrometer) instruments50. Maps showOMI total ozone (b, c) and ozone deficit (d, e) in the Arctic (Antarctic) on26 March 2011 (26 September 2010). Overlays as in Fig. 2 but at 460 K.

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on their geographical location and the stratospheric vortex structure,which controls their upward propagation39,40. Current climate modelsdo not fully capture either the observed short-timescale patterns ofArctic variability or the full extent of the observed longer-term coolingtrend in cold stratospheric winters; nor do they agree on future cir-culation changes that affect trends in transport41,42. Our ability topredict when conditions similar to, or more extreme than, those in2011 may be realized is thus very limited. Improving our predictivecapabilities for Arctic ozone loss, especially while anthropogenichalogen levels remain high, is one of the greatest challenges in polarozone research. Comprehensive stratospheric data sets, such as thoseused here, are critical to meeting that challenge.

METHODS SUMMARYMERRA (Modern Era Retrospective-analysis for Research and Applications43)fields are used for temperature and vortex analysis and for vortex averaging ofcomposition measurements. The CALIOP (Cloud-Aerosol Lidar withOrthogonal Polarization) on the CALIPSO (Cloud-Aerosol Lidar and InfraredPathfinder Satellite Observations) satellite44 provides PSC/aerosol information.

Trace gas profiles are from the Microwave Limb Sounder (MLS)45 on NASA’sAura satellite. Only daytime ClO measurements are used. Northern (southern)high latitudes are sampled near midday (in late afternoon), thus the average solarzenith angle (SZA) of MLS Antarctic measurements is ,7u higher than that in theArctic. Reactive chlorine partitioning shifts away from ClO at higher SZAs7,12,leading to ,30% lower ClO measured in the Antarctic than in the Arctic underfully activated conditions. An instrument anomaly disrupted MLS measurementsfrom 27 March to 20 April 2011. UARS (Upper Atmosphere Research Satellite)MLS measurements, used for 1995–1996 and 1996–1997 analyses, are sparsebecause of the UARS yaw cycle and other measurement gaps26.

Total column ozone is measured by the Dutch-Finnish Ozone MonitoringInstrument (OMI)46 on Aura. Total ozone ‘deficit’ is the difference between dailyvalues and a reference that is minimally affected by chemical loss.

Measurements from MLS and the Match network of balloon-borne ozonesoundings (ozone sondes)47 are used to estimate chemical ozone loss in two ways.The difference between calculated ‘passive’ (influenced only by transport) ozoneand observed ozone is computed, with passive ozone obtained using MLS nitrousoxide14, a ‘reverse trajectory’ model25,26, and the ATLAS (Alfred Wegener InstituteLagrangian Chemistry/Transport System) model27. Vortex ozone is also examinedon the surfaces on which it subsides12,14,28,48, with descent rates from modelledradiative heating/cooling rates averaged over the polar vortex48.

Photochemical box model runs were performed using the chemical modelfrom ATLAS27 to test the sensitivity of ozone loss to initial ozone amounts anddenitrification.

Full Methods and any associated references are available in the online version ofthe paper at www.nature.com/nature.

Received 3 May; accepted 7 September 2011.

Published online 2 October 2011.

1. Farman, J. C., Gardiner, B. G. & Shanklin, J. D. Large losses of total ozone inAntarctica reveal seasonal ClOx/NOx interaction. Nature 315, 207–210 (1985).

2. Solomon, S., Garcia, R. R., Rowland, F. S. & Wuebbles, D. J. On the depletion ofAntarctic ozone. Nature 321, 755–758 (1986).

3. Molina, L. T. & Molina, M. J. Production of Cl2O2 from the self-reaction of the ClOradical. J. Phys. Chem. 91, 433–436 (1987).

4. Anderson, J. G., Brune, W. H.& Proffitt,M.H.Ozone destruction by chlorine radicalswithin the Antarctic vortex: the spatial and temporal evolution of ClO-O3

anticorrelation based on in situ ER-2 data. J. Geophys. Res. 94, 11465–11479(1989).

5. Solomon, S. Stratospheric ozone depletion: a review of concepts and history. Rev.Geophys. 37, 275–316 (1999).

6. Schoeberl, M. R. & Hartmann, D. L. The dynamics of the stratospheric polar vortexand its relation to springtime ozone depletions. Science 251, 46–52 (1991).

7. World Meteorological Organization. Scientific Assessment of Ozone Depletion:2010 (Report 52, Global Ozone Research and Monitoring Project, 2011).

8. Solomon, P. M. et al. High concentrations of chlorine monoxide at low altitudes inthe Antarctic spring stratosphere: secular variation. Nature 328, 411–413 (1987).

9. Waters, J. W. et al. Stratospheric ClO and ozone from the Microwave Limb Sounderon the Upper Atmosphere Research Satellite. Nature 362, 597–602 (1993).

10. Santee, M. L., Manney, G. L., Waters, J. W. & Livesey, N. J. Variations and climatologyof ClO in the polar lower stratosphere from UARS Microwave Limb Soundermeasurements. J. Geophys. Res. 108, 4454, http://dx.doi.org/10.1029/2002JD003335 (2003).

11. Santee, M. L. et al. A study of stratospheric chlorine partitioning based on newsatellite measurements and modeling. J. Geophys. Res. 113, D12307, http://dx.doi.org/10.1029/2007JD009057 (2008).

12. World Meteorological Organization. Scientific Assessment of Ozone Depletion:2006 (Report 50, Global Ozone Research and Monitoring Project, 2007).

13. Rex, M. et al. Arctic winter 2005: implications for stratospheric ozone loss andclimate change. Geophys. Res. Lett. 33, L23808, http://dx.doi.org/10.1029/2006GL026731 (2006).

14. Manney, G. L. et al. EOS MLS observations of ozone loss in the 2004–2005 Arcticwinter. Geophys. Res. Lett. 33, L04802, http://dx.doi.org/10.1029/2005GL024494 (2006).

15. Harris,N.R.P., Lehmann,R.,Rex,M.&vonderGathen,P.Acloser lookatArcticozoneloss and polar stratospheric clouds. Atmos. Chem. Phys. 10, 8499–8510 (2010).

16. Rex, M. et al. Arctic ozone loss and climate change. Geophys. Res. Lett. 31, L04116,http://dx.doi.org/10.1029/2003GL018844 (2004).

17. Tilmes, S., Muller, R., Engel, A., Rex, M. & Russell, J. M. III. Chemical ozone loss in theArctic and Antarctic stratosphere between 1992 and 2005. Geophys. Res. Lett. 33,L20812, http://dx.doi.org/10.1029/2006GL026925 (2006).

18. Poole, L. R. & Pitts, M. C. Polar stratospheric cloud climatology based onStratospheric Aerosol Measurement II observations from 1978 to 1989.J. Geophys. Res. 99, 13083–13089 (1994).

19. Fromm,M. D. et al. An analysis of Polar Ozone and Aerosol Measurement (POAM) IIArctic stratospheric cloud observations, 1993–1996. J. Geophys. Res. 104,24341–24357 (1999).

20. Pitts, M. C., Poole, L. R. & Thomason, L. W. CALIPSO polar stratospheric cloudobservations: second-generation detection algorithm and compositiondiscrimination. Atmos. Chem. Phys. 9, 7577–7589 (2009).

21. Douglass, A. R. et al. Interhemispheric differences in springtime production of HCland ClONO2 in the polar vortices. J. Geophys. Res. 100, 13967–13978 (1995).

22. Manney, G. L. et al. Chemical depletion of ozone in the Arctic lower stratosphereduring winter 1992–93. Nature 370, 429–434 (1994).

23. Tegtmeier, S., Rex, M., Wohltmann, I. & Kruger, K.Relative importance ofdynamicaland chemical contributions to Arctic wintertime ozone. Geophys. Res. Lett. 35,L17801, http://dx.doi.org/10.1029/2008GL034250 (2008).

24. Rex, M. et al. In situ measurements of stratospheric ozone depletion rates in theArctic winter of 1991/1992: a Lagrangian approach. J. Geophys. Res. 103,5843–5853 (1998).

25. Manney, G. L. et al. Lagrangian transport calculations using UARS data. Part II:ozone. J. Atmos. Sci. 52, 3069–3081 (1995).

26. Manney, G. L. et al. Variability of ozone loss during Arctic winter (1991–2000)estimated from UARS Microwave Limb Sounder measurements. J. Geophys. Res.108, 4149, http://dx.doi.org/10.1029/2002JD002634 (2003).

27. Wohltmann, I., Lehmann, R. & Rex, M. The Lagrangian chemistry and transportmodel ATLAS: simulationandvalidationof stratospheric chemistryandozone lossin the winter 1999/2000. Geosci. Model Dev. 3, 585–601 (2010).

28. von der Gathen, P. et al. Observational evidence for chemical ozone depletion overthe Arctic in winter 1991–92. Nature 375, 131–134 (1995).

29. Gernandt, H. The vertical ozone distribution above the GDR-research base,Antarctica in 1985. Geophys. Res. Lett. 14, 84–86 (1987).

30. Rex, M. et al. Prolonged stratospheric ozone loss in the 1995–96 Arctic winter.Nature 389, 835–838 (1997).

31. Manney, G. L., Froidevaux, L., Santee, M. L., Zurek, R. W. & Waters, J. W. MLSobservations of Arctic ozone loss in 1996–97. Geophys. Res. Lett. 24, 2697–2700(1997).

32. Manney, G. L., Santee, M. L., Froidevaux, L., Waters, J. W. & Zurek, R. W. Polar vortexconditions during the 1995–96 Arctic winter: meteorology and MLS ozone.Geophys. Res. Lett. 23, 3203–3206 (1996).

33. Petzoldt, K. The role of dynamics in total ozone deviations from their long-termmean over the Northern Hemisphere. Ann. Geophys. 17, 231–241 (1999).

34. Hood, L. L., Soukharev,B.E., Fromm,M.&McCormack, J. P.Originof extremeozoneminima at middle to high northern latitudes. J. Geophys. Res. 106, 20925–20940(2001).

35. Manney, G. L. et al. Aura Microwave Limb Sounder observations of dynamics andtransport during the record-breaking 2009 Arctic stratospheric major warming.Geophys. Res. Lett. 36, L12815, http://dx.doi.org/10.1029/2009GL038586(2009).

36. Newman, P. A. et al. What would have happened to the ozone layer ifchlorofluorocarbons (CFCs) had not been regulated? Atmos. Chem. Phys. 9,2113–2128 (2009).

37. Newman, P. A., Nash, E. R. & Rosenfield, J. E. What controls the temperatures of theArctic stratosphereduring the spring? J.Geophys.Res. 106,19999–20010 (2001).

38. Polvani, L. M. & Saravanan, R. The three-dimensional structure of breaking Rossbywaves in the polar wintertime stratosphere. J. Atmos. Sci. 57, 3663–3685 (2000).

39. Orsolini, Y. J., Karpechko, A. Y. & Nikulin, G. Variability of the Northern Hemispherepolar stratospheric cloud potential: the role of North Pacific disturbances. Q. J. R.Meteorol. Soc. 135, 1020–1029 (2009).

40. Woollings, T., Charlton-Perez, A., Ineson, S., Marshall, G. & Masato, G. Associationsbetween stratospheric variability and tropospheric blocking. J. Geophys. Res. 115,D06108, http://dx.doi.org/10.1029/2009JD012742 (2010).

41. Butchart, N. et al. Multimodel climate and variability of the stratosphere.J.Geophys. Res. 116,D05102, http://dx.doi.org/10.1029/2010JD014995 (2011).

42. Charlton-Perez, A. et al. The potential to narrow uncertainty in projections ofstratospheric ozone over the 21st century. Atmos. Chem. Phys. 10, 9473–9486(2010).

43. Reinecker, M. M. et al. MERRA — NASA’s modern-era retrospective analysis forresearch and applications. J. Clim. 24, 3624–3648 http://dx.doi.org/10.1175/JCLI-D-11-00015.1 (2011).

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44. Hunt, W.H.et al. CALIPSO lidardescription andperformance assessment. J. Atmos.Ocean. Technol. 26, 1214–1228 (2009).

45. Waters, J. W. et al. The Earth Observing System Microwave Limb Sounder(EOS MLS) on the Aura satellite. IEEE Trans. Geosci. Rem. Sens. 44, 1075–1092(2006).

46. Levelt, P. F. et al. The Ozone Monitoring Instrument. IEEE Trans. Geosci. Rem. Sens.44, 1093–1101 (2006).

47. Rex, M. et al. Chemical ozone loss in the Arctic winter 1994/95 as determined bythe Match technique. J. Atmos. Chem. 32, 35–59 (1999).

48. Rex, M. et al. Chemical depletion of Arctic ozone in winter 1999/2000. J. Geophys.Res. 107, 8276, http://dx.doi.org/10.1029/2001JD000533 (2002).

49. Hoskins, B. J., McIntyre, M. E. & Robertson, A. W. On the use and significance ofisentropic potential-vorticity maps. Q. J. R. Meteorol. Soc. 111, 877–946 (1985).

50. McPeters, R. D. et al. Earth Probe Total Ozone Mapping Spectrometer (TOMS) DataProducts User’s Guide (NASA Technical Publication 1998-206895, 1998).

Supplementary Information is linked to the online version of the paper atwww.nature.com/nature.

Acknowledgements We thank the MLS (especially A. Lambert, D. Miller, W. Read,M. Schwartz, P. Stek, J. Waters), OMI (especially P. K. Bhartia, G. Jaross, G. Labow),CALIPSO and Match science teams, as well as A. Douglass, J. Joiner and the Auraproject, for their support. We also thank W. Daffer and R. Fuller for programmingassistanceat JPL; themanyobserverswhoseworkwent intoobtaining theozone-sondemeasurements; the ozone scientists who participated in the discussion of the 2011Arctic ozone loss and appropriate definition of an Arctic ozone hole (including, but notlimited to, N. Harris, G. Bodeker, G. Braathen, M. Kurylo, R. Salawitch); and especiallyP. Newman and K. Minschwaner for discussions and comments. Meteorologicalanalyses were provided by NASA’s Global Modeling and Assimilation Office (GMAO)and by the European Centre for Medium-Range Weather Forecasts. We thankS. Pawson of GMAO for advice on usage of the MERRA reanalysis. Ozone-sondemeasurements at Alert, Eureka, Resolute Bay, Churchill and Goose Bay were funded byEnvironment Canada. Additional ozone sondes were flown at Eureka as part of the

Canadian Arctic Atmospheric Chemistry Experiment (ACE) Validation Campaign andwere funded by the Canadian Space Agency. Academy of Finland provided partialfunding for performing and processing ozone-sonde measurements in Jokioinen andSodankyla. Ozone soundings and work at AWI were partially funded by the EC DGResearch through the RECONCILE project. Work at the Jet Propulsion Laboratory,California Institute of Technology, and at Science Systems and Applications Inc., wasdone under contract with NASA.

Author Contributions G.L.M. and M.L.S. led analysis of MLS data; M.R. led analysis ofozone-sonde data; G.L.M. led the meteorological data analysis. M.R., G.L.M., N.J.L. andI.W. did chemical ozone loss calculations. R.L. and M.R. performed and analysedchemical box model calculations. M.C.P. and L.R.P. provided CALIPSO/CALIOP dataanalyses; E.R.N. and P.V. provided TOMS and OMI data analyses. L.F., M.L.S., G.L.M. andN.J.L. provided expertise on MLS data usage; D.P.H., P.V. and P.F.L. provided expertiseon OMI data usage. J.D., V.D., H.G., B.J., R.K., E.K., N.L., A.M., C.T.M., H.N., M.C.P., D.W.T.,P.v.d.G., K.A.W. and N.S.Z. were responsible for performing and processingozone-sonde measurements. All authors contributed comments on the manuscript.G.L.M., M.L.S. and M.R. jointly compiled and synthesized the results. G.L.M. and M.L.S.wrote the paper.

Author Information CALIOP data are publicly available at http://eosweb.larc.nasa.gov/PRODOCS/calipso/table_calipso.html,MLS data athttp://disc.sci.gsfc.nasa.gov/Aura/data-holdings/MLS, OMI data at http://disc.sci.gsfc.nasa.gov/Aura/data-holdings/OMI/omto3_v003.shtml, and GEOS-5 MERRA analyses through http://disc.sci.gsfc.nasa.gov/mdisc/data-holdings/merra/. The balloon-borne Antarcticozone-sonde data recorded in 1985 and the following years are publicly available athttp://dx.doi.org/10.1594/PANGAEA.547983. Reprints and permissions informationis available at www.nature.com/reprints. The authors declare no competing financialinterests. Readers are welcome to comment on the online version of this article atwww.nature.com/nature. Correspondence and requests for materials should beaddressed to G.L.M. (Gloria.L.Manney@jpl.nasa.gov) or M.L.S.(Michelle.L.Santee@jpl.nasa.gov).

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METHODSData sets. Modern Era Retrospective-analysis for Research and Applications(MERRA)43 fields, from the Goddard Earth Observing System Version 5.2.0(GEOS-5) data assimilation system, are used for the temperature and vortexanalysis. The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) onthe Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations(CALIPSO) satellite44 provides PSC/aerosol information. CALIOP measure-ments began in April 2006. Trace gas profile measurements are from theMicrowave Limb Sounder (MLS)45 on NASA’s Aura satellite, and the predecessorMLS instrument26 on the Upper Atmosphere Research Satellite (UARS). Totalcolumn ozone data are from the Dutch-Finnish Ozone Monitoring Instrument(OMI)46 on board Aura. The historical total ozone record comprises data fromNimbus-7 and Earth Probe Total Ozone Mapping Spectrometer (TOMS)50. AuraMLS and OMI measurements are available from August 2004 through to thepresent. UARS MLS measurements were obtained from September 1992 throughto early 2000, with increasingly sparse sampling in the later years26. TOMS dataare available beginning in 1979, but no TOMS instrument was taking measure-ments during the 1995–96 Arctic winter.

Measurements from the Match network of balloon-borne ozone soundings(ozone sondes)47 are used in some of the chemical ozone loss estimates.Temperature and vortex analysis. Potential vorticity49 (PV) is used to definethe vortex, with a contour of ‘scaled’ PV of 1.4 3 10–4 s21 (in vorticity units)demarking the vortex edge51,52. Vortex strength is diagnosed as the maximumdaily gradient in PV as a function of equivalent latitude (the latitude that wouldenclose the same area between it and the pole as a given PV contour)51–53. ScaledPV multiplied by 104 is used in the calculation, resulting in units for its gradient of1024 (s degrees equivalent latitude)21.

The temperature threshold for chlorine activation, Tact, is estimated using theformula for nitric acid trihydrate formation54, which depends on pressure, HNO3

and H2O. Climatological HNO3 and H2O profiles are used, derived from UARSdata. The area with T , Tact is calculated on seven isentropic surfaces in the lowerstratosphere: 390, 410, 430, 460, 490, 520 and 550 K; Tact on these levels is 197.5,197.2, 196.8, 196.5, 195.9, 195.3 and 194.5 K, respectively. To get the volume withT , Tact from 380 through 565 K, the areas at each of the seven levels are mul-tiplied by the estimated altitude associated with that layer and summed. Thealtitude range associated with each layer is obtained from a standard potentialtemperature profile as a function of altitude derived from high latitude temper-ature soundings taken during the 1988–89 through to 2001–02 winters (the sameprofile was used for Vpsc calculations in refs 13, 16 and 48). These thicknesses are1.29088, 1.19995, 1.36770, 1.46281, 1.30554, 1.18199 and 1.07382 km for theseven levels listed above. Vortex volume is calculated from vortex area in thesame manner. Winter mean Vpsc is calculated over 16 December through to 15April. Previous studies have shown that Vpsc scaled by the vortex area is a goodproxy for chlorine activation and ozone loss potential17. Additional temperatureand vortex diagnostics are described in Supplementary Information.Polar stratospheric cloud and aerosol information. Particulate backscatteraveraged over the polar vortex derived from CALIOP data is used to providePSC/aerosol information. Total attenuated backscatter at 532 nm, b(z), is one ofthe basic CALIOP Level 1B data products. b(z) is the sum of the particulatebackscatter (due to liquid aerosol and PSCs), bp(z), and molecular backscatter,bm(z). bm(z) is calculated using GEOS-5 molecular density profiles (included inthe CALIOP Level 1B data files) and a theoretical value for the molecular scatter-ing cross-section55. Profiles of bp(z) are then produced by subtracting bm(z) fromb(z). Vortex-averaged profiles of bp(z) are produced by averaging all CALIOPbp(z) profiles located inside the vortex edge (defined using information availablein GEOS-5 Derived Meteorological Product (DMP) files for the nearly-coincidentAura MLS data52) over the selected time interval.MLS trace gas profile measurements and analysis. Trace gas profile measure-ments of HNO3, HCl, ClO, ozone and N2O (a long-lived tracer used to assessdescent) are from Aura MLS45 version 3 retrievals; data quality screening is asrecommended in the MLS data quality document56. MLS data are retrieved onpressure surfaces; potential temperature as a function of pressure from MLSDMPs52 calculated from GEOS-5 analyses is used to interpolate to isentropicsurfaces. Vortex averages of MLS data are calculated using the 1.4 3 1024 s21

scaled PV contour to define the vortex edge, using PV values from the MLSDMPs52. Active chlorine is in the form of ClO mainly during the daytime, andthus measured ClO amounts vary with the solar zenith angle (SZA) at which themeasurements are taken. Only daytime ClO measurements are used here.Northern high latitudes are sampled near midday local time, southern highlatitudes are sampled in late afternoon, thus the SZA of Aura MLS Antarcticmeasurements is ,7u higher on average than that in the Arctic. Reactive chlorinepartitioning shifts away from ClO at higher SZAs7,12, leading to ,30% lower ClOmeasured by Aura MLS in the Antarctic than in the Arctic under fully activated

conditions. MLS measurements are unavailable from 27 March through to20 April 2011 because of an instrument anomaly. Upper Atmosphere ResearchSatellite (UARS) MLS measurements, used for analysis of 1995–96 and 1996–97,are sparse because of the UARS yaw cycle and other measurement gaps26. Thetime of day of UARS measurements varied through the yaw cycle, in the middle ofwhich no daytime ClO measurements were obtained10; thus ClO values shown in1995–96 and 1996–97 near those dates (including the mid-February 1996 mea-surements shown in Fig. 2g) are not representative of the degree of chlorineactivation.Chemical loss calculations. Chemical ozone loss is quantified by two methods,both widely used for such calculations7,12,24–28,47,48. In the ‘passive subtraction’method25–27, a transport model is used to calculate the evolution of ozone inthe absence of chemical changes (‘passive’ ozone). The difference between passiveozone and observed ozone provides an estimate of chemical loss.

Here, passive ozone is obtained in three different ways. First, MLS observationsof N2O, a long-lived species unaffected by chemical processes, are used to cal-culate vertical motion, and that estimate of descent is then used to calculate howinitial MLS ozone profiles would have evolved in the absence of chemical loss14.Second, a ‘reverse trajectory’ transport model25,26 is used to transport an initialstate based on MLS-observed ozone with no chemistry. Finally, the ATLAS(Alfred Wegener Institute Lagrangian Chemistry/Transport System) chemistryand transport model is run in passive mode28, initialized with MLS ozone.

Vortex ozone is also examined in relation to the surfaces on which it is sub-siding12,14,28,48. The descent rates used here are obtained by averaging radiativeheating/cooling rates from the radiation calculation used in the ATLAS modelover the polar vortex48. These rates are then used to examine vortex-averagedMLS and ozone-sonde data on surfaces of ‘spring equivalent potential temper-ature’48, defined as the potential temperature at which air originating at a givenlevel arrived at the end of March. Since the air descended on these surfaces, ozonewould have been constant on each such surface in the absence of chemical loss.

The ozone-sonde data used here are all from electrochemical concentration cell(ECC) sondes, made by different manufacturers. Ozone-sonde data quality wasassessed in an intercomparison experiment57 and is discussed in ref. 47. Forchemical loss calculations using ozone-sonde data, the profiles are first examinedusing a procedure for detecting lamination in the profiles; such lamination (anexample is shown in Fig. 3f) is associated with mixing in of extra-vortex air, whichmay obscure the signature of chemical loss. Profiles that have been significantlyaltered by mixing processes, as indicated by lamination, are excluded from thevortex averages used in the chemical loss calculations. 2010–11 Arctic ozone-sonde data are provided as Supplementary Information.

Results from the ATLAS model passive subtraction calculations, and from thecalculations on spring equivalent potential temperature surfaces using the Matchnetwork ozone-sonde data, are shown in Fig. 4; all panels show vortex averages.These results have been compared with the results from the other methodsdescribed above. While absolute ozone values obtained from different methods/data sets vary significantly (up to ,0.4 p.p.m.v. at the end of March 2011), theyear-to-year variations in chemical loss calculated using all three methods agreeclosely, indicating a high degree of precision in the relative amount of calculatedloss between different years.

The Alfred Wegener Institute chemical box model, also used as the chemicalmodule in ATLAS, simulates 175 reactions between 48 chemical species in thestratosphere27,58 This model was used to perform conceptual runs (Supplemen-tary Fig. 4), started on 1 March with identical initial mixing ratios of all speciesexcept HNO3 and O3. For these two species values corresponding to 1997(3 p.p.m.v. O3, 10 p.p.b.v. HNO3) and 2011 (2.2 p.p.m.v. O3, 6 p.p.b.v. HNO3)(compare Figs 2a and 4c) were combined to yield four sets of initial conditions.Initial ClOx was 2 p.p.b.v., corresponding to the vortex-averaged ClOx derived byATLAS from MLS ClO measurements on 1 March 2011. An air parcel at 70uN,460 K potential temperature, with a temperature of 193 K throughout March, wasused. Heterogeneous reactions took place on liquid aerosols, rather than solid(nitric acid trihydrate, NAT) PSCs, since the widespread existence of the latter isinconsistent with MLS observations of gas-phase HNO3 values (Fig. 2a) largerthan those the microphysical module predicts if NAT is present. A sensitivityrun showed that sporadically occurring solid PSCs did not change the resultssignificantly.Column ozone and ozone deficit calculation. OMI total ozone data wereprocessed with version 8.5 of the TOMS algorithm and have been extensivelyvalidated59. TOMS data were processed with version 8 of the algorithm. The OMIand TOMS total ozone data used in this study were averaged on a fixed global1u3 1u latitude 3 longitude grid. Averages were computed by area-weightingobservations based on the overlap of their instantaneous field-of-view with eachgrid cell. Only data that satisfy quality criteria based on measurement path lengthand algorithm diagnostic criteria were included in the averaged samples.

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Individual total ozone retrievals included in the samples are expected to have aroot-mean-squared error of 1–2%.

Total ozone ‘deficit’ is calculated as the difference between daily values and areference that is minimally affected by chemical ozone loss. The reference for theArctic is the daily mean over all Arctic winters from 1978–79 through to 2009–10,from OMI starting in 2004–05 and from TOMS for earlier years50. The Antarcticreference state is the daily mean of TOMS measurements for 1979 through to1981. Because the Antarctic reference state is based on only three years’ data foreach day, variations in vortex position are not effectively averaged out; thisreference is thus less robust than that for the Arctic, so patterns in daily mapsmay partially reflect differences in vortex position between the reference and thefocus day.

51. Manney, G. L., Zurek, R. W., Gelman, M. E., Miller, A. J.& Nagatani, R. The anomalousArctic lower stratospheric polar vortex of 1992–1993. Geophys. Res. Lett. 21,2405–2408 (1994).

52. Manney, G. L. et al. Solar occultation satellite data and derived meteorologicalproducts: Sampling issues and comparisons with Aura MLS. J. Geophys. Res. 112,D24S50, http://dx.doi.org/10.1029/2007JD008709 (2007).

53. Butchart, N. & Remsberg, E. E. The area of the stratospheric polar vortex as adiagnostic for tracer transport on an isentropic surface. J. Atmos. Sci. 43,1319–1339 (1986).

54. Hanson, D. & Mauersberger, K. Laboratory studies of the nitric acid trihydrate:implications for the south polar stratosphere. Geophys. Res. Lett. 15, 855–858(1988).

55. Hostetler, C. A. et al.CALIOP algorithm theoretical basis document. Calibration andLevel 1 data products (Technical Report, NASA Langley Research Center, 2006);available at Æ http://www-calipso.larc.nasa.gov/resources/pdfs/PC-SCI-201v1.0.pdfæ.

56. Livesey, N. J. et al. Version 3.3 Level 2 data quality and description document.(Technical Report JPL D-33509, Jet Propulsion Laboratory, 2010); available atÆhttp://mls.jpl.nasa.gov/data/v3-3_data_quality_document.pdfæ.

57. Smit, H.G.et al. Assessment of the performance of ECC-ozonesondesunder quasi-flight conditions in the environmental simulation chamber: insights from theJulich Ozone Sonde Intercomparison Experiment (JOSIE). J. Geophys. Res. 112,D19306, http://dx.doi.org/10.1029/2006JD007308 (2007).

58. Kramer, M. et al. Intercomparison of stratospheric chemistry models under polarvortex conditions. J. Atmos. Chem. 45, 51–77 (2003).

59. McPeters, R. et al. Validation of the Aura Ozone Monitoring Instrument totalcolumn ozone product. J. Geophys. Res. 113, D15S14, http://dx.doi.org/10.1029/2007JD008802 (2008).

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