A global three-dimensional model of tropospheric chemistry is used to inves
tigate the changes in tropospheric O-3 and OH since preindustrial times as
a result of fuel combustion and industry, biomass burning, and growth in at
mospheric CH4. Model results indicate a 63% increase of the global troposph
eric O-3 burden from preindustrial times to present (80% and 50% in the nor
thern and southern hemispheres, respectively). Anthropogenic emissions of N
Ox and of CO and hydrocarbons make comparable contributions to the global O
-3 increase (60% and 40% respectively), even though the local rate of tropo
spheric 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 producti
on 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-weighte
d) since preindustrial times. A linear relationship is found in the model b
etween 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 preindust
rial times reflects the conservation of the S-N/S-C(3/2) ratio despite larg
e increases in both S-N and S-C. Comparisons of model results with reconstr
ucted nineteenth century observations of O-3 at continental sites indicate
a systematic overestimate of about 5 ppbv. Correcting this overestimate wou
ld require either a large missing chemical sink for O-3 or a downward revis
ion 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 gra
dient between the boundary layer and the free troposphere, which is inconsi
stent with our current understanding of tropospheric O-3 The model underest
imates preindustrial CO concentrations derived from polar ice cores; these
measurements are difficult to reconcile with any reasonable CO emission inv
entories.