Variations in the free chlorine content of the stratosphere (1991-1997): Anthropogenic, volcanic, and methane influences

Remote sensing of chlorine monoxide (ClO) by the Microwave Limb Sounder exp eriment aboard the Upper Atmosphere Research Satellite (UARS) has provided global measurements of variations in stratospheric free chlorine for 1991-1 997. Linear trends were obtained from a multiple regression analysis of thi s data set at low latitudes and midlatitudes. ClO increases in the upper st ratosphere (2 hPa) are significantly larger than expected from trends in ch lorine source gases alone. Much of the upper stratospheric ClO variability can be explained by changes in CH4, as measured by the UARS Halogen Occulta tion Experiment. Decreasing ClO in the lower stratosphere is consistent wit h a relaxation from a chemically perturbed state attributed to the 1991 Mt. Pinatubo eruption.