*Author to whom correspondence should be addressed.

Atmospheric Environment Vol. 32, No. 21, pp. 3665—3668, 1998( 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain1352—2310/98 $19.00#0.00PII: S1352–2310(98)00087–9

ON THE ANTARCTIC ORIGIN OF LOW OZONE EVENTS ATTHE SOUTH AMERICAN CONTINENT DURING THE

SPRINGS OF 1993 AND 1994

A. PEREZ* and F. JAQUEDepartamento de Fısica de Materiales C-IV, Universidad Autonoma de Madrid, 28007 Madrid, Spain

(First received 26 August 1997 and in final form 22 February 1998. Published August 1998)

Abstract—This paper reports the detection of several low total ozone events at mid-latitudes in the SouthAmerican continent during the springs of 1993 and 1994. During these ozone depletions the total ozonecolumn reached values as low as 150 Dobson Units at high latitudes (56S). It was found that thetemperature of the lower stratosphere for the 450 K isentropic surface decreased with each low ozone eventobserved, suggesting that masses of ozone poor air were transported from the cold polar vortex to the SouthAmerican continent. Isentropic back trajectory analyses corroborate the Antarctic origin of the observedlow ozone events. ( 1998 Elsevier Science Ltd. All rights reserved

Key word index: Ozone, stratosphere, trajectory, transport.

INTRODUCTION

Since the detection by Farman et al. (1985) of a largedecrease of the ozone column over the Antarcticaduring the austral spring, considerable efforts havebeen made to study the so-called ozone hole. Theunique conditions required for the formation of theozone hole make it unlikely that this phenomenonwould occur away of the polar vortex (Solomon,1990). However, several low ozone events at mid-latit-udes have been reported and attributed to the trans-port of ozone poor air from Antarctic regions tomid-latitude regions (Kirchhoff et al., 1996, 1997a—c;Atkinson et al., 1989).

These low ozone events are usually sudden in oc-currence and very pronounced in terms of total ozonedecrease. Recently Tocho et al. (1996) have reportedozone depletion events over the South American con-tinent during the springs of 1993 and 1994 at latitudesas low as 37S. These ozone events were tentativelyattributed to the transport of ozone poor air fromAntarctic latitudes to the South American continenton the basis of an analysis of Potential Vorticity maps.

In this work, in addition to total ozone column dataobtained from ground stations and satellites, com-plementary analyses of the lower stratosphere temper-ature and air mass back trajectories during springtime of 1993 and 1994 in different locations in SouthAmerica are presented.

EXPERIMENTAL RESULTS AND DISCUSSION

Ground station data of the total ozone column presentedin this work were obtained by means of instruments based onthe differential optical absorption of the ultraviolet (UV)B solar radiation by the atmosphere. Our ozone-meter, de-signed for a low cost and easy operation, consists of twoEGG detectors incorporating blue silicon photodiodes(spectral range 250—900 nm) and integrated interference fil-ters centred at 313 and 300 nm with 10 nm bandwidth. Twoquartz-based telescope assemblies and an equatorial mountallow sun tracking for direct solar radiation measurements.The instruments were calibrated with a Brewer spectro-photometer in Punta Arenas (Chile) and with a Dobsonspectrophotometer N°13 in Lisbon (Portugal), showing vari-ations less than 3%. In 1993 a network using these instru-ments was established at several sites in Argentina and Chile,(Tocho et al., 1996). Ground based total ozone data shown inthis work were taken at around noon solar time on clear skydays.

Satellite total ozone column data were obtained by theTotal Ozone Mapping Spectrometer (TOMS) instrument onboard the Meteor 3 spacecraft for the springs of 1993 and1994. The TOMS data were provided by the National Aero-nautics and Space Administration (NASA) Goddard SpaceFlight Center.

Temperature and isentropic wind fields for the 450 Kisentropic surface were provided by the Scientific ComputingDivision (SCD) of the NCAR (National Center for Atmo-spheric Research) at Boulder, Colorado. Data were providedevery 24 h, at 00Z and on a 2.5°]2.5° latitude/longitudegrid.

Back trajectories were calculated by the use of a FOR-TRAN code, developed for this purpose. The 450 K analysedmeridional and zonal winds were integrated in time witha linear spatial interpolation. Time steps were fixed at30 min. Temporal interpolation was carried out by a fourthorder Lagrange polynomial. The selected numerical integra-tion method was the Euler method (Elsgoltz, 1983, Ch. 6).

3665

Fig. 1. Total ozone and 450 K isentropic temperature for (a) Rio Grande, 1993, (b) Tandil, 1993, (c) RioGrande, 1994. (]): TOMS total ozone data; (s): ground stations total ozone data; (d): temperature.

For a time span of a week or so, the motion of air parcels inthe atmosphere can be assumed to be adiabatic and inviscid,that is, potential temperature is conserved following themotion (Holton, 1992). Then, if the potential temperature isused as the vertical coordinate, motion of air can be de-scribed in a bidimensional reference frame. In the lowerstratosphere radiative heating and cooling have associatedtime constants much larger than the dynamic time constant,and have been neglected in our calculations (Brasseur andSolomon, 1995). Finally, the trajectories were plotted usingGrADS (Grid Analysis and Display System) tool from theCenter of Ocean—Land—Atmosphere Studies.

Figure 1a—c show the total ozone column retrievals fromground station (s) and satellite (]) at the Argentina loca-tions of Rio Grande (54S, 68W) (a) and Tandil (37S, 59W) (b)during the austral spring of 1993 and at Rio Grande (c),during 1994 austral spring. In the same figure the 450 K

isentropic surface temperatures are also given (d). As can beseen the two set of ozone data, ground station and satellite,show an excellent agreement at the three locations.

Figure 1a shows the total ozone column and 450 K isen-tropic temperature evolution in 1993 for the period fromJulian day 245 (2 Sep. 93) to 280 (7 Oct. 93) at Rio Grande(Argentina, 54S, 68W). Three remarkable low ozone eventson days 255 (12 Sep. 93), 270 (27 Sep. 93) and 277 (4 Oct. 93)were clearly observed during this period of time. It must benoted that the depletion on days number 255 (12 Sep. 93) and270 (27 Sep. 93) were nearly 50% of the pre-depletion value,reaching total ozone values as low as about 200 DU.

In connection with the 450K isentropic temperature, notethe good correlation found between the low temperaturevalues and the total ozone depletions in agreement withprevious studies (Kirchhoff et al., 1997b). For the abovementioned low ozone events on the 255 and 270 Julian days,

3666 A. PEREZ and F. JAQUE

Fig. 2. Isentropic back trajectories on the 450 K isentropicsurface arriving at: (a) Rio Grande on the Julian day 270 of1993, (b) Tandil on the Julian day 272 of 1993 and (c) Rio

Grande on the Julian day 290 of 1994.

the temperature decreased about 20 K. The total ozonecolumn and the 450 K isentropic temperature evolution inPunta Arenas (Chile) a location near Rio Grande, showedozone depletions and temperature decreases similar to thatobserved in Rio Grande.

Figure 1b presents total ozone and 450 K isentropic tem-perature evolution for Tandil during the same period of timeas in Figure 1a. As can be observed, ozone minima tookplace on days 257, 272 and 279, only two days later than atRio Grande, showing a less pronounced total ozone descentof about a 15% of the pre-depletion value. This featuresuggested a possible common origin for the ozone events atthese two locations. From Figure 1b it can be seen, that eachozone depletion is accompanied by a decrease of the isen-tropic temperature, although not as large as that recorded atRio Grande.

Figure 1c shows the total ozone and 450 K isentropictemperature evolution for Rio Grande, during the springof 1994 and also from Julian day 245 (2 Sep. 94) to 300(27 Oct. 94). Low ozone events were clearly observed on days258 (15 Sep. 94), 275 (2 Oct. 94), and 290 (17 Oct. 94). Notethat the total ozone descent on day 290 reached a minimumca. 150 DU, which represented a 60% of the pre-depletionvalue. This means a decrease lower than that observed in1993 in the same location. These ozone minima during 1994in Rio Grande were also accompanied by correspondingisentropic temperature decreases (20 K), as was previouslyobserved at Tandil and Rio Grande in 1993.

As most of the photochemical ozone damage depletion liesat about the 450 K isentropic layer (Solomon, 1990), thefeatures described above suggest that the high correlationbetween total ozone depletions and the 450 K isentropictemperatures decreases arises from the horizontal advectionof cold polar air toward mid latitudes. However, the Eulerianvariation (at a fixed location in space) of the isentropictemperature are not completely due to the horizontal advec-tion of cold or warm air.

To get further insight on the air masses origin, isentropicback trajectories were calculated around the days whereozone minima were detected. Figures 2a and b show the450 K trajectory of the air mass parcels that arrived at RioGrande (A) on September 27 of 1993 and Tandil (C) onSeptember 29 of 1993, respectively. From Fig. 2a it can beseen that the air mass lying above Rio Grande is located nearlatitude 65S two days earlier. The air mass arriving at Tandilon September 29 of 1993 was near latitude 65S four daysearlier.

In Fig. 2c the trajectory of the air parcel arriving at RioGrande (A) on October 17 of 1994 is shown. The air masstrajectory indicates that this air parcel crossed near 75S twodays before, a latitude well inside the Antarctic circle.

These examples of air mass back trajectories presentedhere covering latitudes from 54S to 41S, strongly supporta polar vortex origin for the low ozone events detectedduring the springs of 1993 and 1994 at mid latitudes of theSouth American continent.

CONCLUSIONS

Stratospheric ozone observations were made atthe Southern edge of the South American continent.Several very low ozone events were described. Thesehave been shown to result from ozone hole eventsproduced at polar latitudes and transported aroundthe south pole to the observation sites by the zonalstratospheric winds. This has been demonstrated withthe calculation of backward trajectories. As alsoshown by other research, the low ozone events arecorrelated with low temperature events.

Acknowledgements—The authors wish to thank Chi-FanShih of the Scientific Computing Division of the NationalCenter for Atmospheric Research for providing his supportwith GrADS and NCAR Reanalyses data handling. The

On the Antarctic origin of low ozone events 3667

authors are also grateful to Professor J. Gonzalo and Dr. S.Mackin for helpful discussions and critical reading of themanuscript.

REFERENCES

Atkinson, R. J., Matthews, W. A., Newman, P. A. and Plumb,R. A. (1989) Evidence of the mid-latitude impact of Ant-arctic ozone depletion, Nature 340, 290—294.

Brasseur, G. and Solomon, S. (1995) Aeronomy of the MiddleAtmosphere, Reidel, Dordrecht, Holland.

Cordoba, C., Perez, A., Aguirre de Carcer, I., Jaque, F.,Henriques, D. and Carvalho, F. (1997) Comparison oftotal ozone measurements from a Differential Optical Ab-sorption Filter Instrument and a Dobson Spectro-photometer. International Journal of Remote Sensing 18.

Elsgoltz, L. (1983) Ecuaciones Diferenciales y Calculo»ariacional. Mir, Moscu.

Farman, J. G., Gardner, B. G. and Shanklin, J. D. (1985)Large losses of total ozone in Antarctica reveal seasonalClO

x/NO

xinteraction. Nature 315, 207—210.

Holton, J. R. (1992) An Introduction to Dynamic Meteoro-logy. Dmowska and Holton, Academic Press, London.

Kirchhoff, V. W. J. H., Schuch, N. J., Pinheiro, D. K., Harris,J. M. (1996) Evidence for an ozone hole perturbation at30° south. Atmospheric Environment 30, 1481—1488.

Kirchhoff, V. W. J. H., Casiccia, C. A. R. and Zamorano, F.(1997a) The ozone hole over Punta Arenas, Chile. Journalof Geophysical Research 102, 8945—8953.

Kirchhoff, V. W. J. H., Sahai, Y., Casiccia, C. A. R.,Zamorano, F. and Valderrama, V. (1997b) Observationsof the 1995 ozone hole over Punta Arenas, Chile. Journalof Geophysical Research 102, 16,109—16,120.

Kirchhoff, V. W. J. H., Zamorano, F. and Casiccia, C. (1997c)UV-B enhancements at Punta Arenas, Chile. Journal ofPhotochemistry and Photobiology 38, 174—177.

Solomon, S. (1990) Progress towards a quantitative under-standing of Antarctic ozone depletion. Nature 347,347—354.

Tocho, J., Da Silva, L., Crino, F., Perez, A. and Jaque, F.(1996) Ozone depletion over the South American conti-nent during the springs of 1993, 1994 and 1995. XIIIQuadrennial Ozone Symposium, L’Aquila, Italy.

3668 A. PEREZ and F. JAQUE