RECOVERY OF STRATOSPHERIC OZONE OVER THE UNITED-STATES IN THE WINTER OF 1993-1994

Total ozone levels, which were 10-15% below normal over the U.S. during the winter of 1992-1993, returned to levels slightly above normal during the winter of 1993-1994. Investigation of ozone vertical profiles indicates that in the region where severe depletion occurred in 1992-1993 (25% reductions at 12-22 km), ozone had returned to normal, while above this region, ozone was abnormally high. Thus total ozone was also high. Low ozone values in 1992-1993 were believed to be related to heterogeneous chemistry on the Pinatubo volcanic aerosol. This interpretation is strengthened by these observations since the particle surface area available for heterogeneous processes in the stratosphere diminished substantially at midlatitudes during 1993 and was not replenished by transport from the equatorial reservoir during the winter as had occurred during the previous winter. However, the observation of continued unusually high ozone above 24 km in winter suggests that this phenomenon, thought to also have been at least partially due to heterogeneous chemistry, is mainly related to dynamics. Unusually high total ozone levels in high northerly latitudes during the winter of 1993-1994 and especially in early February, associated with warm stratospheric temperatures during December and January, are probably the source of high ozone above 24 km in midlatitudes at this time.