Differences in Arctic and Antarctic PSC Occurrence as observed by lidar in Ny-Ålesund [79ºN, 12ºE] and McMurdo [78ºS, 167ºE]

The extent of springtime Arctic ozone lossdoes not reach Antarctic "ozone hole" dimensionsbecause of the generally higher temperaturesin the northern hemisphere vortex and consequentless polar stratospheric cloud (PSC) particlesurface for heterogeneous chlorine activation.Yet, with increasing greenhouse gases stratospherictemperatures are expected to further decrease.To infer if present Antarctic PSC occurrence canbe applied to predict future Arctic PSC occurrence,lidar observations from McMurdo station (78ºS, 167ºE)and Ny Ålesund (79ºN, 12ºE) have been analysed forthe 9 winters between 1995 (1995/1996) and 2003(2003/2004). Although the statistics may notcompletely cover the overall hemispheric PSCoccurrence, the observations are considered torepresent the main synoptic cloud features asboth stations are mostly situated in the centreor at the inner edge of the vortex. Since thefocus is set on the occurrence frequency of solidand liquid particles, the analysis has beenrestricted to volcanic aerosol free conditions.In McMurdo, by far the largest part of PSCobservations is associated with PSC type Ia.The observed constant background of NATparticles and their potential ability to causedenoxification and irreversible denitrificationis presumably more important to Antarctic ozonechemistry than the scarcely observed PSC type II.Meanwhile in Ny-Ålesund, PSC type II has neverbeen observed, while type Ia and Ib both occurin large fraction. Although they are also foundsolely, the majority of observations revealssolid and liquid particle layers in the sameprofile. For the Ny-Ålesund measurements, thefrequent occurrence of liquid PSC particlesyields major significance in terms of ozonechemistry, as their chlorine activation ratesare more efficient. The relationship betweentemperature, PSC formation, and denitrificationis nonlinear and the McMurdo and Ny-Ålesund PSCobservations imply that for predictedstratospheric cooling it is not possible todirectly apply current Antarctic PSC occurrencedirectly to the Arctic stratosphere. FutureArctic PSC occurrence, and thus ozone loss,will depend on the shape and barotropy of thevortex rather than on the minimum temperatures.