Surface level ozone was measured continuously for the period January-April 1992 at two sites near Alert, Northwest Territories, Canada. Also, vertical profiles of ozone and meteorological parameters were measured at Alert with tethersondes for a selected period in April. Three ozone depletion events with ozone <0.5 ppb were recorded during April; tethersonde data were available for one of these events. Except for these ozone depletions the surface ozone concentration was lowest in January, in the range 25-30 ppb, and then progressively increased to a high in April, in the range of 40-45 ppb. Analysis of the present and some past ozone profile data from upper air sondes showed that ozone depletion at polar sunrise was associated with a neutral potential temperature profile within the boundary layer (i.e., low stability) capped by an elevated inversion at several hundred meters above the surface.
