The impact of greenhouse gases on past changes in tropospheric ozone

The impact of changes in the abundance of greenhouse gases (GHGs) on the evolution of tropospheric ozone (O-3) between 1960 and 2005 is examined using a version of the Goddard Earth Observing System chemistry-climate model (GEOS CCM) with a combined troposphere-stratosphere chemical mechanism. Simulations are performed to isolate the relative role of increases in methane (CH4) and stratospheric ozone depleting substances (ODSs) on tropospheric O-3. The 1960 to 2005 increases in GHGs (CO2, N2O, CH4, and ODSs) cause increases of around 1-8% in zonal-mean tropospheric O3 in the tropics and northern extratropics, but decreases of 2-4% in most of the southern extratropics. These O-3 changes are due primarily to increases in CH4 and ODSs, which cause changes of comparable magnitude but opposite sign. The CH4-related increases in O-3 are similar in each hemisphere (similar to 6%), but the ODS-related decreases in the southern extratropics are much larger than in northern extratropics (10% compared to 2%). This results in an interhemispheric difference in the sign of past O-3 change. Increases in the other GHGs (CO2 and N2O) and SSTs have only a small impact on the total burden over this period, but do cause zonal variations in the sign of changes in tropical O-3 that are coupled to changes in vertical velocities and water vapor. Citation: Lang, C., D. W. Waugh, M. A. Olsen, A. R. Douglass, Q. Liang, J. E. Nielsen, L. D. Oman, S. Pawson, and R. S. Stolarski (2012), The impact of greenhouse gases on past changes in tropospheric ozone, J. Geophys. Res., 117, D23304, doi:10.1029/2012JD018293.