Validation of PM MAPPER aerosol optical thickness retrievals at 1×1 km2 of spatial resolution

The polar-orbiting MODerate resolution Imaging Spectrora-diometer (MODIS) on-board Terra and Aqua satellites is a key instrument for the daily monitoring of global aerosol properties over a large spectral range. Its aerosol retrieval algorithm is set to a size of 10×10 km² of spatial resolution, and hence may not be adequate for detailed analysis at local scale. PM MAPPER is a software system capable of handling the multispectral data acquired by the MODIS sensors. It produces maps of Aerosol Optical Thickness (AOT) at increased spatial resolution up to 1×1 km², which are then available online in a GIS environment. This article shows the validation results of these products, obtained by comparison with AOT measurements of several ground-based radiometers of the A Erosol RObotic NETwork (AERONET) over Europe, for a period of 3 years (2007-2009). They show a good correlation between satellite products and ground measurements. Different sizes of the spatio-temporal window to associate satellite and ground observations have been tested, and trends have been searched by tuning the comparisons for different years, seasons and land-cover classes. An optimal spatio-temporal window for this kind of validation is also suggested. This could be used for other purposes as well, e.g. to perform improvements of AOT retrieval algorithm with machine learning techniques.