Evaluation of MODIS aerosol optical thickness over Europe using sun photometer observations

Satellite retrieved aerosol optical thickness (AOT) may be useful to improve the insight in PM distributions in Europe in combination with models and ground-based measurements. To use AOT in mapping or assimilation experiments, it requires well-validated satellite data. We have compared the AOT retrieved by MODIS (collection 4) to sun photometer data from the AERONET network in Europe and found a good temporal correlation between MODIS and AERONET. However, we also found a large positive bias of about 50% in the MODIS AOT data, which is in accordance with earlier findings. We highlight the strong seasonal signature in the overestimation of AOT by MODIS with a maximum during summer. After correction for the bias, the accuracy of MODIS AOT retrievals agrees with reported uncertainties and the residuals show a normal distribution. We have introduced a simple method for the evaluation of the possible extent of cloud contamination and hypothesise that on average, up to one-third of the MODIS retrievals may be cloud contaminated. For some stations in central Europe, this percentage was found to be larger than >50%. The consequences of a bias between satellite and in situ data for their use in the mapping of aerosol levels are discussed.