Ambient fine particulate matter (PM2.5) is a leading environmental risk factor for premature mortality. We use aerosol optical depth (AOD) retrieved from two satellite instruments, MISR and SeaWiFS, to produce a unified 15-year global time series (1998–2012) of ground-level PM2.5 concentration at a resolution of 1° x 1°. The GEOS-Chem chemical transport model (CTM) is used to relate each individual AOD retrieval to ground-level PM2.5. Four broad areas showing significant, spatially coherent, annual trends are examined in detail: the Eastern U.S. (−0.39 ± 0.10 μg m–3 yr–1), the Arabian Peninsula (0.81 ± 0.21 μg m–3 yr–1), South Asia (0.93 ± 0.22 μg m–3 yr–1) and East Asia (0.79 ± 0.27 μg m–3 yr–1). Over the period of dense in situ observation (1999–2012), the linear tendency for the Eastern U.S. (−0.37 ± 0.13 μg m–3 yr–1) agrees well with that from in situ measurements (−0.38 ± 0.06 μg m–3 yr–1). A GEOS-Chem simulation reveals that secondary inorganic aerosols largely explain the observed PM2.5 trend over the Eastern U.S., South Asia, and East Asia, while mineral dust largely explains the observed trend over the Arabian Peninsula.