Geophysical Research Letters, 28, 1187-1190, 2001.

Is Upper Stratospheric Chlorine Decreasing?

Darryn W. Waugh,1 David B. Considine,2,3 and Eric L. Fleming,3,4

1Department of Earth and Planetary Science, Johns Hopkins University, Baltimore, Maryland.
2University of Maryland, College Park, Maryland
3NASA Goddard Space Flight Center, Greenbelt, Maryland.
4Emergent Information Technologies Inc., Arlington, Virginia.

Abstract

The monthly-mean the total chlorine abundance (ClT) at 55 km inferred from HALOE HCl observations increases from 1992 to 1997 and then subsequently decreases. The pre-1997 increase is consistent with surface measurements of ClT time-lagged by around 6 years. However, a decrease after 1997 is inconsistent with such a time lag, which would predict a peak in late 1999. Accounting for stratospheric mixing processes produces an expected stratosphere ClT which is in agreement with the HALOE ClT time series considering the uncertainty in the HALOE data. However, the peak in ClT is still predicted to occur in later 1999 rather than 1997. We find that the expected ClT at 55 km is insensitive to reasonable low frequency changes in transport, chlorine partitioning, anomalous buildup of organic chlorine at 55 km, and tropospheric rainout of inorganic chlorine. At present, we are unable to explain how upper stratospheric ClT could decrease as early as 1997.

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