Air-quality is important for human health, efficient crop production and visibility. Motor vehicles are known to emit large quantities of gaseous pollutants such as nitrogen oxides, which have the potential to significantly degrade air near the Earth’s surface. The rapid growth in the use of automobiles means that this issue is becoming increasingly important for National Governments to focus on. For this they need scientific guidance from institutes such as KNMI as to the growth and extent of the problem. Although there are extensive monitoring networks on the ground whose purpose is to measure the quality of the air, such networks need regular maintenance and calibration which can be rather labour intensive. In the last ten years earth-orbiting satellite instruments have become crucial in monitoring the composition of tropospheric air on a global scale. Moreover, complex computer models have been developed which include the most important physical and chemical processes that occur in the atmosphere. Dr. Nadége Blond, in collaboration with Dr Folkert Boersma (now at Harvard), Dr Henk Eskes (KS-CK) and Dr. Ronald van der A (KS-AK), has recently used existing ground-based measurements of NO2 (a ubiquitous nitrogen oxide) to test the quality of atmospheric data retrieved from the European Space Agencies environmental satellite known as SCIAMACHY. Moreover, a simultaneous comparison of model output from a recently upgraded version of the CHIMERE regional air-quality model was also performed. The results give confidence that such datasets are accurate enough to be of value for assessing the extent to which anthropogenic activity changes the regional composition of the air. The picture below shows the spatial distribution of such pollution given as an annual mean as derived from both SCIAMACHY and CHIMERE. From this it can be seen that there is a high degree of correlation between the distribution of nitrogen dioxide (NO2) calculated by the model and that measured by the satellite.