CHRISTINE Code for High ResolutIon Satellite mapping of optical ThIckness and إNgstrom Exponent. Part II: First application to the urban area of Athens, Greece and comparison to results from previous contrast-reduction codes

There is an increasing demand for exploiting satellite data in urban air quality assessment. High spatial resolution satellite data can be used to retrieve the aerosol optical thickness (AOT), as an air quality indicator, over urban areas. One of the methods to achieve this applies the contrast-reduction principle to a set of two satellite images, one of which has minimum aerosol content and is used as a reference. Previous satellite image processing codes that followed this approach were subject to surface changes which may have occurred in the time interval between the processed images acquisition. In order to eliminate this potential source of AOT miscalculation the CHRISTINE Code for High Resolution Satellite Mapping of Optical Thickness and إngstrom Exponent was developed. This new code takes into consideration contrast reduction in more than one spectral band, and applies the إngstromʹs law to isolate atmospheric and surface components. The code underwent its first testing using Landsat satellite data acquired before 2001 (when air pollution was at its peak) over the study area of Athens (Greece). Results showed that CHRISTINE can effectively separate contrast modifications attributed to atmospheric changes from those due to surface changes. Comparison against the previous SMA Satellite Mapping of Aerosols code showed an average improvement of 21% in terms of area over which AOT could be retrieved with high confidence. CHRISTINE also approximates the aerosol size distribution over the studied area. These preliminary findings show that the new code can be used to counteract for spatial deficiencies in urban monitoring networks. In the case of Athens the application to archived satellite data also allowed hindcasts for the period prior to ground based aerosol measurements.