Aerosol optical thickness and PM 2 . 5 1 Intercomparison between Satellite - Derived Aerosol Optical Thickness and PM

We explore the relationship between column aerosol optical thickness (AOT) derived from the Moderate Resolution Imaging SpectroRadiometer (MODIS) on the Terra/Aqua satellites and hourly fine particulate mass (PM2.5) measured at the surface at seven locations in Jefferson county, Alabama for 2002. Although the MODIS AOT is a column value and the PM2.5 mass is representative of near-surface conditions, results indicate that there is a good correlation between the satellite-derived AOT and PM2.5 (linear correlation coefficient, R=0.7) indicating that most of the aerosols are in the lower boundary layer. The maximum PM2.5 and AOT values occur during the summer months due to enhanced photolysis, while the PM2.5 has a distinct diurnal signature with maxima in the early morning (6:00~8:00AM) due to increased traffic flow and restricted mixing depths during these hours. Using simple empirical linear relationships derived between the MODIS AOT and 24hr mean PM2.5 we show that the MODIS AOT can be used quantitatively to estimate air quality categories (e.g., good, moderate, unhealthy for special groups, unhealthy and hazardous) as defined by the U.S. Environmental Protection Agency (EPA) with an accuracy of more than 90% in cloud-free conditions for large spatial scales. Vertical profiles of aerosol concentration from current ground-based or future space-borne lidars such as the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and aerosol hygroscopiscity measurements will further enhance our capability to effectively use satellite data for air quality studies. A concerted effort is also needed to create a data base of high temporal resolution PM2.5 data sets to further explore these relationships. Aerosol optical thickness and PM2.5 3