Anthropogenic forcing on tropospheric ozone and OH since preindustrial times

A global three-dimensional model of tropospheric hemistry is used to investigate the changes in tropospheric 03 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 03 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 03 increase (60% and 40% respectively), even though the local rate of tropospheric 03 production is generally NOx limited. The rise in 03 production parallels closely the rise in the emissions of CO and hydrocarbon because the 03 yield per mole of CO or hydrocarbon oxidized has remained constant at 0.7-0.8 mol/mol since preindustrial times. In contrast, the 03 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 SN/Sc 3/2 ratio, where S•v and Sc 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 f the SN/Sc 3/2 ratio despite large increases inboth S•v and Sc. Comparisons of model results with reconstructed nineteenth century observations of 03 at continental sites indicate a systematic overestimate of about 5 ppbv. Correcting this overestimate would require either a large missing chemical sink for 03 or a downward revision of the natural NOx source from lightning (3 Tg N yr -1 in our model). The nineteenth century observations of 03 over France show no vertical gradient between the boundary layer and the free troposphere, which is inconsistent with our current understanding of tropospheric 03. The model underestimates preindustrial CO concentrations derived from polar ice cores; these measurements are difficult to reconcile with any reasonable CO emission inventories.