MODIS aerosol optical depth (AOD) product provides a quantitative measure of columnar aerosol abundance over both land and ocean. The satellite-derived AOD has shown to correlate with surfaced-measured PM2.5 (particulate matter particulate matter with aerodynamic diameter < 2.5 μm) concentration. The timely MODIS AOD product from direct broadcasting facilitates near real-time (1-2 hours after satellite overpass) monitoring of PM distribution and movement over the continental US the Infusing Satellite into Environmental Application (IDEA) (http://idea.ssec.wisc.edu). Many questions, however, are raised in regard to correlation between AOD and PM2.5. In this paper, we characterize the correlation between MODIS AOD and surface-measured PM2.5 in different conditions and introduce mean absolute error as a second measure to help assess satellite derived AOD for air quality application.