One of the advantages of satellite observations for aerosols in contrast with in-situ measurements is homogeneous data over wide areas. They can detect dust storms, biomass burnings, and urban and trans-boundary air pollutions. Aerosol data from satellites have been accumulated for a few decade, which means that they are being obtained on a time scale of climate change. Principal parameters of atmospheric aerosols obtained from passive satellite sensors are the aerosol optical thickness (AOT) and Ångström exponent (AE). AOT is an index of the aerosol column loading, and AE, which is an aerosol size index, is used for discriminating roughly between coarse natural aerosols and fine anthropogenic ones.

Retrieved data on atmospheric aerosols from satellites have been used for evaluating global and regional aerosol numerical models which treat all principal tropospheric aerosols, i.e., soil dust, sea salt, black carbon (BC), organic matter (OM), sulfate, and nitrate. This evaluation is significant because aerosol optical parameters (e.g., AOT and AE) are directly related to estimation of the aerosol direct radiative effect as well as they can compare on a global scale. In the inter-comparison project of global aerosol models, AeroCom, simulated results of aerosol parameters are compared not only among models but also with satellite data (e.g., Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging Spectroradiometer (MISR)). Comparisons of the cloud droplet and ice crystal effective radii simulate by models with those retrieved from satellite observations are also important to evaluate the aerosol first indirect effect. The Phase I experiment of the AeroCom contributed to the 4th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4). The AeroCom proceeds with the Phase II experiments (http://aerocom.met.no/cgi-bin/aerocom/surfobs_annualrs.pl) according to the common protocols, in which about 10 models participate, toward the IPCC 5th Assessment Report (AR5).

Observations from active satellite sensors are essential to evaluate vertical profiles of aerosols and clouds simulated by models. Long-range transport of aerosols much depends on their vertical profiles. Relative altitude between aerosol and cloud layers affects not only the aerosol indirect effect but also magnitude and sign of the aerosol direct radiative forcing. Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and CloudSat are commonly used for evaluating numerical models. Earth Clouds, Aerosols and Radiation Explorer (EarthCARE) mission will also provide valuable spatial three-dimensional data for aerosols and clouds.

The traditional method of using satellite data in aerosol modeling studies is comparison as described above. To use satellite data more directly, data assimilation methods are being developed. Both Ensemble Kalman Filter (EnKF) and 4-dimensional variational (4D-Var) methods are incorporated into a global aerosol transport and climate model, SPRINTARS. This assimilation system uses aerosol optical properties from satellites, and therefore the modified aerosol direct radiative forcing as well as the modified AOT are estimated. There are still large uncertainties in estimating the aerosol effect on the climate system as reported in the IPCC AR4. It is expected that the uncertainties reduce by estimation with aerosol models adopting the assimilation methods in conjunction with satellite data.