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Benchmarking MERIS derived high resolution aerosol optical thickness over Athens on the basis of MODIS products

Adrianos Retalis(1) and Nicolaos Sifakis(1)

(1) National Observatory of Athens, I. Metaxa & Vas. Pavlou, Pendeli, 15236, Athens, Greece

Abstract

Satellite sensors with low spatial resolution, such as TOMS and AVHRR, have been effective in tracking aerosols on a global scale. Sensors with higher resolution, such as MODIS and MERIS, provide us with information relevant to aerosol concentrations at larger geographic scales useful for regional and, to a certain extent, local applications. Nonetheless MODIS products do not fully exploit the resolution of the sensor, and the application of MERIS is still limited to scientific studies. The current study aimed at combining MERIS and MODIS data to map the horizontal distribution of aerosols over an urban area in an operational context. More specifically we processed MERIS images to retrieve relative aerosol optical thickness (AOT) values over the city of Athens, then we used MODIS products to benchmark these relative values. First, cloud-free MERIS Level 2 full resolution images were selected in the period from May to September 2003. Then ground based air pollution data referring to hourly concentrations of PM10, SO2, NO2, NO, O3 and CO were gathered from the national pollution-monitoring network covering the area of Athens, and were analysed for defining prevailing atmospheric conditions for the dates when the satellite images were acquired. This assisted in the categorisation of available images to highly, moderately or not polluted. The latter category was necessary in order to be used as “reference” for relative AOT retrieval using the DTA (Differential Textural Analysis) algorithm and code. The resulted AOT maps had a MERIS equivalent spatial resolution of 300×300 m2, and were benchmarked on the basis of information provided by the MODIS product “MOD04_L2” at a resolution of 10×10 km2. Final AOT maps compared successfully to PM10 ground based measurements when available meteorological information from back-trajectories (HYSPLIT model) was considered. This led to identification of cases of dust transportation from the Sahara that could impact satellite derived AOT values but not PM10.

 

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry