We use satellite observations in conjunction with a global 3-D chemical transport model (GEOS-Chem) to constrain global dust emissions and to estimate the relative contributions from Asian and African sources to the surface dust in the western U.S.mountain ranges.Model simulated monthly mean dust aerosol optical depths (AODs) from January to June 2006 are compared with corresponding values from the Multi-angle Imaging SpectroRadiometer (MISR) instrument onboard NASA Terra satellite.The ratios are used to adjust model dust emissions for each model grid.The resulting global dust emissions are 267 Tg for March-May 2006, a reduction of global dust emissions by 43% (34% in Asia and 50% in Africa).Results from model simulations with the adjusted dust emissions are in significantly better agreement with surface observations of fine dust from the IMPROVE network in the western U.S.Large discrepancies in the southwest U.S.are due largely to excessive dust emissions from the region.We further apply the OMI aerosol index (AI) to impose daily variations on dust emissions.Our model results show little difference, reflecting the dispersion of the signal of daily variation during the long-range transport of Asian and African dust to the western U.S.Our model results suggest strong influence of African dust emissions on the surface dust in the western U.S.mountain ranges.The African influence is comparable to that of Asian emissions in February-March and May 2006.