Trends in tropospheric aerosol loads and corresponding impact on direct radiative forcing between 1950 and 1990: A model study

Global aerosol optical thicknesses and radiative properties need to be known for the study of decadal temperature change. Aerosol distributions have been developed from global transport models for a mixture of sulfate and carbonaceous aerosols from fossil fuel burning, including also contributions from other major aerosol types such as soil dust and sea salt. Between the years 1950 and 1990 the aerosol distributions change due to changes in emissions of SO2 and carbon particles from fossil fuel burning. The optical thickness of fossil fuel derived aerosols increased by nearly a factor of 3 during this period, with particularly strong increase in eastern Asia. In countries where environmental laws came into effect since the early 1980s (e.g., United States and western Europe), emissions and consequently aerosol optical thicknesses did not increase considerably after 1980, resulting in a sl lift in the global distribution pattern. In addition to the optical thickness, aerosol single scattering albedos may have changed during this period due to different trends in absorbing black carbon and reflecting sulfate aerosols. However, due to uncertainties in the emission trends, which are especially large in the case of carbonaceous aerosols, such change cannot be determined with any confidence. Radiative forcing of this aerosol distribution is calculated for several scenarios. Uncertainties in the contribution of the strongly absorbing black carbon aerosol leads to a range in top-of-atmosphere forcings of approximate to -0.5 to +0.1 Wm(-2).