No abstract

[1] We have examined measurements of chlorine monoxide, ClO, in the lower stratosphere by the Microwave Limb Sounder (MLS) on the Aura satellite in the austral spring of 2005. These measurements have been compared to those of a ground-based spectrometer at Scott Base, Antarctica. The data analysis is performed in both cases by subtracting nighttime measurements from daytime ones. The transition from full darkness to full daylight at the high latitude of Scott Base limits the time during which both day and night measurements are made. After further selection for good observing conditions and the position of the polar vortex, 16 valid profile comparisons are made. The day-to-day variability of ClO is observed to be large, $30% of its peak value. The daily column densities of the two instruments are correlated with a significance of 3s with most of the mean difference arising from 2 days. The statistical agreement between MLS and the Scott Base instrument is good. Scott Base values are on average marginally but not significantly larger, by 0.10 ± 0.07 ppb, or 11 ± 8% (1s), in peak mixing ratio, than the MLS values.

Our ground‐based millimeter‐wave measurements of chlorine monoxide (ClO) in the ozone layer above Hawaii since 1992, together with a few measurements from the same site using an earlier version of the same instrument during the 1980's, provide a unique data set for the determination of the long‐term trend of active chlorine in the middle stratosphere. The trend shows a rapid rise of 58% from 1982 to a broad maximum in 1994–1997 and a substantial decline of 1.5%/year from the maximum through late 2004. About 1/3 of the decline may be due to changes in methane. Both the rise and the decline are consistent with that expected from the rise and decline of total chlorine in the troposphere, which reached a maximum in 1992–1993, then declined sharply after implementation of international agreements to phase out chlorofluorocarbon (CFC) production starting with the Montreal Protocol.