EFFECT OF OZONE DEPLETION ON ATMOSPHERIC CH4 AND CO CONCENTRATIONS

GLOBAL surface-based measurements of atmospheric methane and carbon mo noxide concentrations revealed a marked and unexpected decrease in the ir growth-rates in 1991 and 1992, particularly in the Northern Hemisph ere(1,2). Changes in emissions are unlikely to be the sole reason for the sudden reduction in the concentrations of these source gases(2,3). The unprecedentedly large depletion of Stratospheric ozone observed i n 1991 and 1992 (ref. 4) may have contributed to the sharp decrease in the growth rates of both CH4 and CO by exposing the troposphere to mo re ultraviolet radiation. This would have resulted in increased concen trations of the hydroxyl radical, OH., which is the major atmospheric sink for both CH4 and CO. Here we present two-dimensional model simula tions which allow us to assess the significance of the link between st ratospheric ozone depletion and the observed trends of CH4 and CO. We find that the low values in stratospheric ozone concentration can acco unt for almost half of the 1992 decrease in the CH4 and CO growth rate s.