Analysis of upper stratospheric Umkehr ozone profile data for trends and the effects of stratospheric aerosols

The effect of stratospheric aerosols on Umkehr estimates of long-term ozone depletion associated with chlorofluoromethanes (CFMs) is considered in a statistical time series trend analysis. Time series models are estimated using monthly averages of Umkehr measurements made over the last 15 to 20 years. The time series regression models incorporate seasonal, trend and noise factors and an additional factor to account for the effects of atmospheric aerosols on the Umkehr measurements. The analysis indicates a statistically significant relation with atmospheric aerosol transmission in the Umkehr layers and implies that the relation is an important factor in any time series trend analysis of Umkehr data. Taking this relation into account, statistically significant negative trends were found in the Upper Umkehr layers. It is pointed out that upper stratospheric ozone could be sensitive to long-term solar variability as well as other possible influences in addition to CFM-induced effects, and therefore the cause or causes of the estimated trend cannot be unambiguously estimated using current Umkehr data.