Although ground-level PM<sub>2.5</sub> monitoring sites provide accurate measurements, their spatial coverage within a given region is limited and thus often insufficient for exposure and epidemiological studies. Satellite data expand spatial coverage, enhancing our ability to estimate location- and/or subject-specific exposures to PM<sub>2.5</sub>. Recently, a new Multi- Angle Implementation of Atmospheric Correction (MAIAC) algorithm was developed for MODIS which provides AOD at 1 km resolution. Using MAIAC data, the relationship between MAIAC AOD and PM<sub>2.5</sub> as measured by the 27 EPA ground monitoring stations was investigated. These results were also compared to conventional MODIS 10 km AOD retrieval (MOD04) for the same days and locations. The coefficients of determination for MOD04 and for MAIAC are R2=0.45 and 0.50, respectively, suggesting that AOD is a reasonably good proxy for PM<sub>2.5</sub> ground concentrations. Finally, we studied the relationship between PM<sub>2.5</sub> and AOD at the intra-urban scale (≤10km) in Boston. The fine resolution results indicated spatial variability in particle concentration at a sub-10 kilometer scale.