Observational and theoretical evidence in support of a significant in-situ photochemical source of tropospheric ozone

The latitudinal and seasonal variation of ozone in the troposphere is discussed. Of particular interest is the asymmetrical behavior of this gas with respect to the two hemispheres. These asymmetries, when coupled with a diagnostic photochemical model of the troposphere, lends support to the view that ozone cannot be viewed as an inert tracer of stratospheric origin. We note in our calculations that it is likely that the budgets of carbon monoxide and tropospheric ozone may be quite dependent on each other and discuss these calculations in light of the uncertainty which currently exists about representative global tropospheric background concentrations of the nitrogen oxides. In addition, the seasonal variation of excess 14CO2 (a stratospheric tracer) is examined and compared with the seasonal ozone variation during the same period of observations at the same location and altitudes. The distinct maxima for ozone found during the summer in the lower troposphere are not present for the 14CO2 data. This finding likewise suggests that photochemical processes taking place in the troposphere are an important source term for tropospheric ozone.