Thanks for your answer MariLiza. And indeed Sander has been very helpful with some of my other questions, so thanks to both of you.
I have been looking at the tropospheric column number density. Having read through some of the links you have shared and looking further into it, I can see that the ppbv number would distort the data. That being said, is there a way to do a similar averaging of NO2 over the total column or tropospheric column as is done with CH4, for instance? Would that achieve a ppb or ppm number? I understand that it would lose some accuracy but at the moment the NO2 number gives a 2D reading in a 3D space. Whereas the CH4 data is sensed under the same conditions, yet it is presented in density. I began working with CH4 data which is already in ppb so I, perhaps naively, expected the conversion to be possible. I would not think of the data as representing the surface NO2 levels as the satellite data doesn’t sense at that altitude, yet the sources of the pollution are absolutely on the ground, as far as my project is concerned.
For anyone else looking at this problem here are some relevant discussions:
All the best.
D