Hello Windy,
I've just recently subscribed to your platform after having used the app and web interface for a while now. I'm using it to keep a bit of a daily, personal, record of the NOx/SO2 emissions in the BENELUX, but also particularly Germany's Ruhr area. (I am not a researcher or scientist, mind you. Just hobby based.)
I've found one topic asking about high (EU) SO2 emissions, which was in the period of Italy's active vulcano eruption. Here: SO2 konzentrationen
However, wanted to ask some additional questions and create a new topic instead.
I've often wondered why The Netherlands has such a high NOx/CO(2) emission. Could it be the farmlands, agriculture,
The cars? Perhaps the trucks? Or could it perhaps be another mode of transport, something that isn't focused on as much as it should be: the marine traffic, both at sea as well as inland?
Yes, you've guessed it. In my view it are predominantly the large harbour cities such as: Antwerp, Rotterdam, IJmuiden and the Ruhr area.
Definitely not the small shrimp fishermen from Harlingen...
Specifically Rotterdam, with its many factories and refineries, Antwerp similarly, all of its traffic goes through Dutch territory. IJmuiden (with its large harbours and steel industries). In case of DE, it has the Ruhr area, a large and heavy industrial area, with lots of inland shipping coming and going from there, towards NL via the Maas, Rhine, etc., basically making NL the 'industrial NOx wastebin of the EU', in my opinion, seeing the data from Windly.
Particularly 'beautiful' to see are the The NOx/CO2 data you provide, clearly shows the marine traffic routes due to the amount NOx emitted along the routes from large marine vessels. See following screenshot:

Vessels arrive and depart from IJmuiden/Rotterdam/Antwerp, through the channel of Calais, continuing along the west coast of France and Spain, into the mediterranean sea through the Strait of Gibraltar. From there you can see the emission patterns visiting several other countries along the south coast of the EU and northern african nations. Further down, it'll go through the Suez canal, exiting East-Africa and on to many other SE-Asian nations. What unfortunately isn't clearly visible, are the inland routes, via the Maas/Rhine etc., but heavily used inland shipping routes they are!
Take for example the screenshot of NL/BE/DE from the other day:
As you can see it's a very high concentration, depending on weather conditions. Almost as high as China and India, an area with dense population and industries, which is to be expected to have higher emission rates: 
Suggestion 1: Add marine traffic button to toggle as additional layer. (Data can be pulled from MarineTraffic app)
See marine traffic routes from MarineTraffic app: 
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Then there is the issue of CO(2) emission. We all know humans contribute to the increase to some extent: particularly China/India, but also South-America seems to do its part.
What I cannot understand, is some hotspots on your datamap. The hotspots include Canada/North-America's hotspot. See screenshot here:

This is a massive amount of concentration, compared to other locations. Not even China comes close to this. Considering the geographical location of the concentration, it's not like there are massive cities or industries that could produce such a high concentration either. What I did find interesting is the fact that in/around that highly concentrated area some (passive and active) volcanoes are located. See screenshot here: 
Another example is Sicily, with its vulcano 'Etna'. It is still active, and you can see a clear CO2 'cloud' around the area, getting blown away by winds up high.

Is it safe to assume that something else (other than man made) is causing these extreme amounts of emission rates? To me it would seem so...
Suggestion 2: would it make sense to have the option to toggle an extra layer (just as with pressure toggle button) of worldwide (active + passive) volcanic locations, to more easily pinpoint higher SO2/CO2/NOx concentration/hotspots? So that people can see the difference between cities that emit said concentrations, or natural phenomena, such as volcanic activities, geysers and so on.
Thanks for letting me ask some questions and provide some feedback. If anyone is scientifically capable of shedding some light on this, I would be very grateful. Thanks for your consideration, Windy!