Implications of upper stratospheric trace constituent changes observed by HALOE for O3 and ClO from 1992 to 1995

Abstract

Measurements from the Halogen Occultation Experiment (HALOE) of the upper stratosphere show increases in HCl and H2O and decreases in CH4 and O3 during the period 1992–1995. These changes all coincide with the decline of solar cycle 22. Using a simple photochemical model, we find that 4 major components contribute to the O3 decrease: 1) an increase in total chlorine as indicated by increasing HCl 2) an additional increase in reactive chlorine due to repartitioning of Cly by the decreasing CH4 3) a decrease in odd oxygen production due to decreased solar flux and 4) an increase in odd hydrogen loss due to increasing H2O. At 2 mbar, the Cly repartitioning is the largest cause of O3 changes. Because the Cly repartitioning coincides with decreasing solar flux, some recent observational estimates of the long‐term response of upper stratospheric O3 to solar UV irradiance variability may be too large. Compared with the HALOE O3 data, the model O3 exhibits a larger negative trend. This appears to be because the model O3 is more sensitive to the increased H2O than is the observed O3. The 20–30% decline in CH4 also implies a large increase in ClO which exceeds that expected from CFCs. Observations of ClO from the Microwave Limb Sounder (MLS) support this inference.