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The extent of springtime Arctic ozone loss does not reach Antarctic ``ozonehole' dimensions because of the generally higher temperatures in thenorthern hemisphere vortex and consequent less polar stratospheric cloud(PSC) particle surface for heterogeneous chlorine activation. Yet, withincreasing greenhouse gases stratospheric temperatures are expected tofurther decrease. To infer if present Antarctic PSC occurrence can beapplied to predict future Arctic PSC occurrence, lidar observations fromMcMurdo station (78° S, 167° E) and Ny\uc5lesund (79° N,12° E) have been analysed for the 9 winters between 1995 (1995/1996) and2003 (2003/2004). Although the statistics may not completely cover theoverall hemispheric PSC occurrence, the observations are considered torepresent the main synoptic cloud features as both stations are mostlysituated in the centre or at the inner edge of the vortex. Since the focusis set on the occurrence frequency of solid and liquid particles, theanalysis has been restricted to volcanic aerosol free conditions. InMcMurdo, by far the largest part of PSC observations is associated with NATPSCs. The observed persistent background of NAT particles and theirpotential ability to cause denoxification and irreversible denitrificationis presumably more important to Antarctic ozone chemistry than the scarcelyobserved ice PSCs. Meanwhile in Ny-\uc5lesund, ice PSCs have never beenobserved, while solid NAT and liquid STS clouds both occur in largefraction. Although they are also found solely, the majority of observationsreveals solid and liquid particle layers in the same profile. For theNy-\uc5lesund measurements, the frequent occurrence of liquid PSC particlesyields major significance in terms of ozone chemistry, as their chlorineactivation rates are more efficient.<P  style="line-height: 20px;">The relationship between temperature, PSC formation, and denitrification isnonlinear and the McMurdo and Ny-\uc5lesund PSC observations imply that forpredicted stratospheric cooling it is not possible to directly apply currentAntarctic PSC occurrence to the Arctic stratosphere. Future Arctic PSCoccurrence, and thus ozone loss, is likely to depend on the shape andbarotropy of the vortex rather than on minimum temperature alone

Differences in Arctic and Antarctic PSC occurrence as observed by lidar in Ny-Ålesund (79° N, 12° E) and McMurdo (78° S, 167° E)