Anthropogenic forcing on tropospheric ozone and OH since preindustrial times

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 CH4. 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 NOx 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 NOx 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 S-N/S-C(3/2) ratio, where S-N and S-C are the sources of NOx and of CO and hydrocarbons, respectively. The relative constancy of the global mean OH concentration since preindustrial times reflects the conservation of the S-N/S-C(3/2) ratio despite large increases in both S-N and S-C. 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 NOx 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.