NOx emissions derived from space

Satellite measurements provide information of atmospheric column densities of several trace gases, e.g. NO2.
This allows to infer the respective emissions by various approaches. 
Here we focus on two empirical approaches that do not involve chemical models: 
- from the decay patterns downwind from regions of large emissions, like megacities or industrial areas, the NOx lifetime and the respective emissions can be derived simultaneously.
- from the divergence, i.e. the spatial derivative of the horizontal flux, point sources can clearly be identified due to the strong local gradients, 
and their emissions can be quantified.

We discuss the potential and limitations of these methods and present recent improvements. 
In particular, in addition to annual and monthly means, we investigate how far emissions can be derived for individual orbits.
Also potential applications to other species like SO2, CO, or CH4 are discussed.  
The resulting NOx emissions from point sources, megacities, and ship tracks are presented, which were compiled within ESA's World Emission project.