NO₂ is a reddish-brown gas emitted by fossil fuel combustion, especially diesel engines. It is a primary pollutant that also acts as a precursor of tropospheric ozone and secondary particles (PM2.5).
In Spanish cities
Madrid, Barcelona, and other large cities frequently exceed the EU annual limit (40 μg/m³) at traffic stations. Traffic restrictions (Madrid ZBE, Barcelona ZBE) aim to reduce these levels. NO₂ concentrations peak during rush hours and are lowest at dawn. It is one of the most easily measured pollutants from satellites (Sentinel-5P/TROPOMI).