TROPOSPHERIC OZONE AND CLIMATE-CHANGE

Considerable attention has been paid in recent years to photo chemical smog pollution close to the earth's surface and to stratospheric ozon e depletion. There is reason to suspect that the next round of scienti fic concern will be devoted to the perturbations in the ''free troposp here.'' Tropospheric ozone has been building up in many regions of the northern hemisphere. Ozone changes in the upper troposphere will exer t a considerable impact on global warming. This could affect moisture levels, cloud amount and distribution, precipitation, and atmospheric dynamics on different scales. This paper analyzes: (1) the physical an d chemical processes contributing to changes in tropospheric ozone con centration; (2) the observational evidence of previous ozone change; a nd (3) results drawn from computer modelling of past and future radiat ive forcing caused by rising ozone concentrations in the upper troposp here. The solar and longwave radiative model developed by Wang et al. (1 991) was used for calculating the change in radiative forcing to th e troposphere-surface system that can be ascribed to changing concentr ations in ozone and other greenhouse gases. Nitric oxide emission from aircraft are a prime suspect for the observed increases in upper trop ospheric ozone. The inference can be drawn that a radiative forcing of 0.2 to 0.35 Wm-2 Will result from a doubling of aircraft emissions ov er the next two decades. This will amount to 10 to 25 percent of the r adiative forcing attributable to CO2 alone for the same period. The ef fect of doubling aircraft emissions will increase as stratospheric ozo ne concentrations recover from the recent buildup of harmful chloroflu orocarbons. A large fraction of the radiative forcing that occurred du ring the 1970 to 1990 period can be attributed to increases in troposp heric ozone as opposed to increases in other greenhouse gases.