Radiative forcing due to tropospheric ozone and sulfate aerosols

Abstract

The radiative forcing due to tropospheric ozone and sulfate aerosol changes is evaluated using a radiative wideband model. The concentration fields of these constituents and their precursors on a 10°×10° grid are computed with a three‐dimensional transport/chemistry model of the troposphere (MOGUNTIA) for the preindustrial (1850) case, the contemporary atmosphere (1990) and for the future (2050). We find global and annual average radiative forcings over the industrial period (1850–1990) of 0.38 W m−2 and −0.36 W m−2 for tropospheric ozone and sulfate aerosol changes, respectively. These values indicate an approximate balance, which is also due to the fact that both constituents are confined to specific regions since their atmospheric residence times are relatively short, albeit that ozone changes are more zonally dispersed. However, both constituents have characteristic seasonal cycles, and their forcing dependencies on several radiative parameters are quite different. The longwave as well as the shortwave sensitivities are examined so as to estimate the uncertainty ranges of the computed radiative forcings. Subsequently, we present analytical fits of the (normalized) shortwave forcing by both ozone and sulfate. The effects of relative humidity on the optical parameters of the sulfate aerosol 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, differences 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 continents of the northern hemisphere are dominated by the negative forcing of sulfate throughout the year, whereas a positive forcing in July and no effect in January are found over the oceans. Even if we add the effects of the well−mixed greenhouse gases over the same period, some spots of negative forcing remain. New results are presented for the ozone and sulfate forcings toward 2050. The radiative forcing of ozone is expected to increase by another 70% over the period 1990–2050 as compared to the industrial period, while the negative sulfate forcing is predicted to increase even stronger by 150% comparing the same periods. Maximum changes of both constituents are shifting southward in the northern hemisphere as a result of increasing industrial activities in countries with emerging economies.