Global and regional assimilation of satellite observations of ozone and carbon monoxide have been conducted with the Global Environmental Multiscale model (GEM). The chemical transport is on-line with the meteorological model, and simplified chemistry are considered. CO is to a large degree a chemical tracer in the troposphere. The results of assimilation is a good indicator of the sources and results are compared with the typical emission inventories. Schemes to retrieved chemical sources in data assimilation is also discussed. In ozone assimilation the impact of satellite versus surface data is considered. A discussion on the effect of assimilating Co and O3 on other constituents will be given.