[aselzer]

How is there significantly less particulate matter and therefore less are quality in Cologne and Dortmund, which are located in a very densely populated area with a lot of traffic, than in Vienna?

http://aqicn.org/map/europe/ tends to only shows areas where PM2.5 is even measured as polluted, so I guess this might be inaccurate due to a lack of sensors in some areas, falsely believing they are less polluted.

You can easily see this by just looking at the air quality reports in Vienna: http://aqicn.org/map/europe/#@g/48.183/16.4118/12z