Why is the climate forcing of sulfate aerosols so uncertain?

Sulfate aerosol particles have strong scattering effect on the solar radiat ion transfer which results in increasing the planet albedo and, hence, tend to cool the earth-atmosphere system. Also, aerosols can act as the cloud c ondensation nuclei (CCN) which tend to increase the albedo of clouds and co ol the global warming. The ARPEGE-Climat version 3 AGCM with FMR radiation scheme is used to estimate the direct and indirect radiative forcing of sul fate aerosols. For minimizing the uncertainties in assessing this kind of c ooling effect, all kinds of factors are analyzed which have been mixed in t he assessment process and may lead to the different results of the radiativ e forcing of aerosols, It is noticed that one of the uncertainties to asses s the climate forcing of aerosols by GCM results from the different definit ion of radiative forcing that was used. In order to clarify this vague idea , the off-line case for considering no feedbacks and on-line case for inclu ding all the feedbacks have been used for assessment. The direct forcing of sulfate aerosols in off-line case is -0.57 W/m(2) and -0.38 W/m(2) for the clear sky and all sky respectively. The value of on-line case appears to b e a little larger than that in off-line case chiefly due to the feedback of clouds. The indirect forcing of sulfate aerosols in off-line case is -1.4 W/m(2) and -1.0 W/m(2) in on-line case. The radiative forcing of sulfate ae rosols has obvious regional characteristics, There is a larger negative rad iative forcing over North America, Europe and East Asia. If the direct and indirect forcing are added together, it is enough to offset the positive ra diative forcing induced by the greenhouse gases in these regions.