Anthropogenic forcing on tropospheric ozone and OH since preindustrial times

Abstract

A global three‐dimensional model of tropospheric chemistry is used to investigate the changes in tropospheric O 3 and OH since preindustrial times as a result of fuel combustion and industry, biomass burning, and growth in atmospheric CH 4. Model results indicate a 63% increase of the global tropospheric O 3 burden from preindustrial times to present (80% and 50% in the northern and southern hemispheres, respectively). Anthropogenic emissions of NOx and of CO and hydrocarbons make comparable contributions to the global O 3 increase (60% and 40% respectively), even though the local rate of tropospheric O 3 production is generally NO χ limited. The rise in O 3 production parallels closely the rise in the emissions of CO and hydrocarbon because the O 3 yield per mole of CO or hydrocarbon oxidized has remained constant at 0.7–0.8 mol/mol since preindustrial times. In contrast, the O 3 production efficiency per mole of NO χ emitted has decreased globally by a factor of 2. We find a 9% decrease in the global mean OH concentration (mass‐weighted) since preindustrial times. A linear relationship is found in the model between the global mean OH concentration and the SN/S C3/2 ratio, where SN and S C are the sources of NO χ and of CO and hydrocarbons, respectively. The relative constancy of the global mean OH concentration since preindustrial times reflects the conservation of the SN/S C3/2 ratio despite large increases in both SN and SC. Comparisons of model results with reconstructed nineteenth century observations of O 3 at continental sites indicate a systematic overestimate of about 5 ppbv. Correcting this overestimate would require either a large missing chemical sink for O 3 or a downward revision of the natural NO χ source from lightning (3 Tg N yr−1 in our model). The nineteenth century observations of O 3 over France show no vertical gradient between the boundary layer and the free troposphere, which is inconsistent with our current understanding of tropospheric O 3. The model underestimates preindustrial CO concentrations derived from polar ice cores; these measurements are difficult to reconcile with any reasonable CO emission inventories.