One of the largest changes over the last century is the massive increase in human population globally. At the same time, the percentage of people living in large cities is increasing rapidly. A large fraction of anthropogenic emissions is released to the atmosphere in these cities, and this trend is expected to continue as more and more countries industrialise while population is still on the rise. As a result, large cities are hotspots of pollution problems and at the same time have a significant impact on air quality on a global scale. One particular problem is the formation of ozone, a secondary pollutant with health and climate implications.

Using measurements from satellite based spectrometers, the spatial distribution and the temporal evolution of nitrogen dioxide NO₂ and formaldehyde HCHO, two important ozone precursors can be determined. These measurements in combination with the results of atmospheric chemistry models such as MOZART can be used to asses the impact of large cities on the atmosphere, to validate model calculations and to estimate future developments. In this PhD, data from the GOME. SCIAMACHY and GOME-2 satellite instruments will be used to derive a consistent data set of tropospheric columns of NO₂ and HCHO, to study their evolution and the importance of large cities on their global budgets. This work will be performed in close collaboration with an international project on the impact of Mega-Cities.