Ozone Chemistry Changes in the Troposphere and Consequent Radiative Forcing of Climate

Abstract

A global three-dimensional transport/chemistry model of the troposphere has been applied to simulate changes in the chemical composition of the troposphere since pre-industrial times. Calculated increases of tropospheric O3 have been evaluated with a radiative transfer model. Our calculations confirm earlier studies that the combined effects of anthropogenic NOx, CO and CH4 emissions cause a strong enhancement of O3 levels throughout the global troposphere, while this blend of emissions does not seem to have affected global average OH concentrations significantly. In fact, anthropogenic NOx induced photochemistry may have compensated for the strong OH reduction which would have resulted from anthropogenic CO and CH4 emissions alone. Our model simulations suggest that during the past 1.5 Century net photochemical 03 formation in the global troposphere has changed sign, from -87 Tg/yr to 427 Tg/yr, caused mostly by anthropogenic emissions in the mid- latitude northern hemisphere and to a lesser extent by biomass burning emissions in the tropics. We calculate that the global O3 burden in the troposphere has increased by a factor of 1.7, the CO burden by a factor of 2.5 and that of H2O2 by a factor of 2. Preliminary calculations suggest that the global mean radiative forcing of climate by increasing tropospheric O3 is 0.55 W/m2 for the month July, maximizing in the mid-latitude northern hemisphere, and 0.44 W/m2 for January; the annual average, about 0.5 W/m2, is approximately one third of the radiative forcing by increasing CO2.