RADIATIVE FORCING DUE TO TROPOSPHERIC OZONE AND SULFATE AEROSOLS

The radiative forcing due to tropospheric ozone and sulfate aerosol ch anges is evaluated using a radiative wideband model. The concentration fields of these constituents and their precursors on a 10 degrees x10 degrees grid are computed with a three-dimensional transport/chemistr y model of the troposphere (MOGUNTIA) for the preindustrial (1850) cas e, the contemporary atmosphere (1990) and for the future (2050). We fi nd global and annual average radiative forcings over the industrial pe riod (1850-1990) of 0.38 W m(-2) and -0.36 W m(-2) for tropospheric oz one and sulfate aerosol changes, respectively. These values indicate a n approximate balance, which is also due to the fact that both constit uents are confined to specific regions since their atmospheric residen ce times are relatively short, albeit that ozone changes are more zona lly dispersed. However, both constituents have characteristic seasonal cycles, and their forcing dependencies on several radiative parameter s are quite different. The longwave as well as, the shortwave sensitiv ities are examined so as to estimate the uncertainty ranges of the com puted radiative forcings. Subsequently, we present analytical fits of the (normalized) shortwave forcing by both ozone and sulfate. The effe cts of relative humidity on the optical parameters of the sulfate aero sol and hence on the patterns of radiative forcing are also considered . Our results indicate that the Impact as compared to a fixed relative humidity (at 80%) is quite small on a global scale, but regionally, d ifferences up to 5 W m(-2) are found. By adding the radiative effects of sulfate and ozone over the period 1850-1990, we find that the conti nents of the northern hemisphere are dominated by the negative forcing of sulfate throughout the year, whereas a positive forcing in July an d no effect in January are found over the oceans. Even if we add the e ffects of the well-mixed greenhouse gases over the same period, some s pots of negative forcing remain. New results are presented for the ozo ne and sulfate forcings toward 2050. The radiative forcing of ozone is expected to increase by another 70% over the period 1990-2050 as comp ared to the industrial period, while the negative sulfate forcing is p redicted to increase even stronger by 150% comparing the same periods. Maximum changes of both constituents are shifting southward in the no rthern hemisphere as a result of increasing industrial activities in c ountries with emerging economies.