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. durin g the winter of 1992-1993, returned to levels slightly above normal du ring 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 al so high. Low ozone values in 1992-1993 were believed to be related to heterogeneous chemistry on the Pinatubo volcanic aerosol. This interpr etation is strengthened by these observations since the particle surfa ce area available for heterogeneous processes in the stratosphere dimi nished substantially at midlatitudes during 1993 and was not replenish ed by transport from the equatorial reservoir during the winter as had occurred during the previous winter. However, the observation of cont inued unusually high ozone above 24 km in winter suggests that this ph enomenon, thought to also have been at least partially due to heteroge neous chemistry, is mainly related to dynamics. Unusually high total o zone 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 h igh ozone above 24 km in midlatitudes at this time.