Continuous global monitoring of ground-level particulate matter less than 2.5 μ in diameter (PM 2.5) is important for two reasons. One, it contributes to our understanding of aerosol changes as human activity and land use evolve globally. Two, it guides environmental policy as related to air quality; PM 2.5 can adversely affect health and contribute to morbidity and mortality. I’m using a global chemical transport model (GEOS-Chem) to estimate ground-level PM 2.5 from satellite-observed ‘column’ aerosol loading.