Aerosol retrieving over urban area using SPOT/HRG imagery

Retrieving aerosol optical depth (AOD) over urban areas from satellite imagery is a challenging issue because of the large variance in surface reflectivity, especially for high spatial resolution imagery. However, based on the concept of the blurring effect of aerosols in the visible imagery, the reflectance contrast between the bright and dark objects can provide the information of aerosol loading. The principle is that, after the correction of the solar incidence and satellite view angle, the remaining variation of the apparent reflectance over an unchanged surface is affected by the pollutants. The overall AOD can be derived from the dispersion of the apparent reflectance over urban areas with a referenced image (under a clear atmosphere). The results show that the referenced AOD should be as small as possible to retrieve more accurate AOD. The size of the target window (i.e. AOD spatial resolution) is also another factor that affects the accuracy of AOD retrieval. Validating by several different sizes, it reveals that the window size at 51×51 (5.1 km×5.1 km) can provide the most accurate results. Examples of the AOD derivation over Taipei City (Taiwan) using this method show that most of the cases exhibit the retrieving feasibility (error <30%). To overcome the retrieved uncertainties, more parameters of the metadata such as azimuth angle and satellite orientation will be examined further.