Nitrogen dioxide is a brown, acrid gas produced primarily by high- temperature combustion in road vehicles. In French cities it has two defining traits: it is highly localised (concentrations drop by an order of magnitude within 200 m of a busy street) and its trend is - for once - improving.
Electrification is working
Airparif reports a 48 % reduction in annual mean NO2 levels in Ile-de-France between 2005 and 2022, driven largely by the phasing out of older Euro 3-4 diesel vehicles. The WHO 2021 annual guideline (10 microgrammes/m3) is now within reach for most neighbourhoods - but not on the busiest arterials.
The legal story
France was found in breach of the EU NO2 limit in a 2019 CJEU ruling (Case C-636/18), and continues to pay a 10 million euro semester fine until it reaches compliance. The implementation tool - zones a faibles emissions (ZFE) - remains politically contested.
What ClearSpot does with NO2
Our air module treats NO2 separately from PM2.5 because it behaves differently. A cell can be "clear" for PM2.5 at the annual scale and "not clear" for NO2 on a single weekday morning. We therefore display both verdicts in the dashboard and only flip the home-page chip when either pollutant breaches its threshold.
- NO2 threshold: 25 microgrammes/m3 (rolling 1 h mean) or 10 microgrammes/m3 annual mean, whichever fires first.
- Refresh cadence: 15 minutes from OpenAQ, hourly from Atmo France.