Recent studies have explored the relationship between aerosol optical depth (AOD) measurements by satellite sensors and concentrations of particulate matter with aerodynamic diameters less than 2.5 μm (PM2.5). However, relatively little is known about spatial and temporal patterns in this relationship across the contiguous United States. In this study, we investigated the relationship between US Environmental Protection Agency estimates of PM2.5 concentrations and Moderate Resolution Imaging Spectroradiometer (MODIS) AOD measurements provided by two NASA satellites (Terra and Aqua) across the contiguous United States during 2005. We found that the combined use of both satellite sensors provided more AOD coverage than the use of either satellite sensor alone, that the correlation between AOD measurements and PM2.5 concentrations varied substantially by geographic location, and that this correlation was stronger in the summer and fall than that in the winter and spring.
 WHO (World Health Organization) (2005) Particulate matter air pollution: How it harms health. Factsheet EURO/04/05 Europe.
 EPA (Environmental Protection Agency) (1996) Review of the ambient air quality standard for particulate matter: Policy assessment of scientific and technical information. EPA, Research Triangle Park.
 Miller, K.A., Siscovick, D.S., Sheppard, L., Shepherd, K., Sullivan, J.H., Anderson, G.L., et al. (2007) Long-term exposure to air pollution and incidence of cardiovascular events in women. The New England Journal of Medicine, 356, 447-458. http://dx.doi.org/10.1056/NEJMoa054409
 Pope, C.A., Burnett, R.T., Thun, M.J., Calle, E.E., Krewski, D., Ito, K., et al. (2002) Lung cancer, cardiopulmonary mortality, and long-term exposure to fine particulate air pollution. Journal of the American Medical Association, 287, 1132-1141.
 Schwartz, J., Laden, F. and Zanobetti, A. (2002) The concentration-response relation between PM2.5 and daily deaths. Environmental Health Perspectives, 110, 1025-1029. http://dx.doi.org/10.1289/ehp.021101025
 Liu, Y., Paciorek, C.J. and Koutrakis, P. (2009) Estimating regional spatial and temporal variability of PM2.5 concentrations using satellite data, meteorology, and land use information. Environmental Health Perspectives, 117, 886-892.
 Paciorek, C.J. and Liu, Y. (2009) Limitations of remotely sensed aerosol as a spatial proxy for fine particulate matter. Environmental Health Perspectives, 117, 904-909.
 Kaufman, Y.J., Tanre, D., Remer, L.A., Vermote, E.F., Chu, A. and Holben, B.N. (1997) Operational remote sensing of tropospheric aerosol over land from EOS moderate resolution imaging spectroradiometer. Journal of Geophysical Research D: Atmospheres, 102, 17051-17067.
 Kaufman, Y.J., Tanre, D. and Boucher, O. (2002) A satellite view of aerosols in the climate system. Nature, 419, 215-223. http://dx.doi.org/10.1038/nature01091
 Koelemeijer, R.B.A., Homan, C.D. and Matthijsen, J. (2006) Comparison of spatial and temporal variations of aerosol optical thickness and particulate matter over Europe. Atmospheric Environment, 40, 5304-5315.
 NASA (National Aeronautics and Space Administration) (2009) NASA MODIS Level 1 and Atmospheric Archive and Distribution System (LAADS) at the Goddard Space Flight Center 2009. http://ladsweb.nascom.nasa.gov/
 Engel-Cox, J.A., Holloman, C.H., Coutant, B.W. and Hoff, R.M. (2004) Qualitative and quantitative evaluation of MODIS satellite sensor data for regional and urban scale air quality. Atmospheric Environment, 38, 2495-2509.
 EPA (Environmental Protection Agency) (2010) Technology Transfer Network (TTN) Air Quality System (AQS) 2010.
 Zhang, H., Hoff, R.M. and Engel-Cox, J.A. (2009) The relation between Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth and PM2.5 over the United States: A geographical comparison by US Environmental Protection Agency Regions. Journal of the Air & Waste Management Association, 59, 1358-1369.
 Ross, Z., Jerrett, M., Ito, K., Tempalski, B. and Thurston, G.D. (2007) A land use regression for predicting fine particulate matter concentrations in the New York City region. Atmospheric Environment, 41, 2255-2269.
 Roy, B., Mathur, R., Gilliland, A.B. and Howard, S.C. (2007) A comparison of CMAQ-based aerosol properties with IMPROVE, MODIS, and AERONET data. Journal of Geophysical Research D: Atmospheres, 19, 112.