| The aerosol indirect effect remains one of the most important uncertainties in the projection of future climate. Here we first provided an observational evidence of the change in radiative forcing due to the aerosol indirect effect. Based on the observations of aerosol and cloud properties, we used a cloud parcel model to estimate the cloud optical depth at a polluted site and a clean site. We also determined the cloud optical depth required to fit the surface measured downward SW fluxes. Results from both methods show that the cloud optical depth is larger at the polluted site given a value of cloud liquid water path. From this good agreement we concluded that the aerosol indirect effect has a significant influence on the radiative fluxes.; Then, we used 3-D meteorological fields together with a radiative transfer model to calculate the global first aerosol indirect forcing. We also examined the spatially-resolved uncertainty in estimates of this forcing by perturbing the values of each parameter in the calculation. The global mean forcing calculated in the reference case is -1.30 Wm-2, and the global mean relative uncertainty is 130%. The aerosol burden calculated by chemical transport models and the cloud fraction were found to be the most important sources of uncertainty.; We also studied the aerosol indirect effect by nitrate and nitric acid gas. The indirect effect of nitrate on TOA radiative flux was found to be comparable to that by anthropogenic sulfate in some places. A substitution method that accounts for the gas phase nitric acid effect on CCN activation was developed and added to the cloud nucleation parameterization.; Anthropogenic aerosols may also change the formation of ice crystals in cirrus clouds, which has effects on both the SW and LW radiative balance of the earth. The indirect forcing due to anthropogenic aerosols was calculated by a radiative transfer model using aerosols from different emissions. The results show that anthropogenic aerosols may have two effects on cirrus clouds: increase in the ice number concentration (Ni) by increasing the ice nuclei concentration, or decrease Ni by lowering the maximum supersaturation. |
