Aerosol climatologies based on consistent and high quality sources of information over extended areas are not available. Yet, such information is needed for estimating their radiative effects, for correctly computing surface radiative fluxes by methods of remote sensing, as inputs to climate models, as well as for the evaluation of satellite based estimates of this parameter. To respond to such needs, several attempts have been made to obtain aerosol climatologies, based on inventories of sources and by applying transport models to distribute the emitted particulates in space. Recently, consistent observations from numerous locations are becoming available from the Aerosol Robotic Network (AERONET) network. This network is most dense over the United States. The observations are not evenly distributed, and not of sufficient density. However, they are extremely valuable since they represent “true” values at the sites where they are observed. In this paper, an effort has been made to develop aerosol climatology over the United States, at a monthly time scale, by combining ground observations and transport model estimates, using objective analysis methods.