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?<p><a href="http:&#x2F;&#x2F;aqicn.org&#x2F;map&#x2F;europe&#x2F;" rel="nofollow">http:&#x2F;&#x2F;aqicn.org&#x2F;map&#x2F;europe&#x2F;</a> 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.<p>You can easily see this by just looking at the air quality reports in Vienna: <a href="http:&#x2F;&#x2F;aqicn.org&#x2F;map&#x2F;europe&#x2F;#@g&#x2F;48.183&#x2F;16.4118&#x2F;12z" rel="nofollow">http:&#x2F;&#x2F;aqicn.org&#x2F;map&#x2F;europe&#x2F;#@g&#x2F;48.183&#x2F;16.4118&#x2F;12z</a>