Not health advice. This map shows satellite measurements for exploring and learning. It is not an air-quality forecast or a measure of the air you breathe. For decisions about your health or activities, use your local official air-quality service and follow medical advice.
The gas
Nitrogen dioxide (NO₂) is a reddish-brown gas made mostly when fuel burns hot: road traffic, power stations, industry, ships and aircraft. Lightning, soils and wildfires add some too. NO₂ breaks down within hours in daylight, so it stays close to where it is made. That is why the map lights up over cities, highways, ports and power plants and fades over oceans and forests.
NO₂ is a common air pollutant: it helps form ozone and fine particles, and many countries set limits on it. Because it is short-lived and comes mostly from burning fuel, it is also a good sign of human activity, which is why lockdowns, holidays and new emission rules show up in it.
The measurement
The Ozone Monitoring Instrument (OMI) on NASA’s Aura satellite has measured NO₂ since October 2004. It looks down at sunlight scattered back from Earth and works out how much NO₂ the light passed through. Aura crosses each place once a day in the early afternoon (around 1:45 pm local time).
The number is a tropospheric column: how many NO₂ molecules there are above each square centimetre of ground, from the surface up to roughly 10–15 km. The legend gives it in units of 10¹⁵ molecules per cm². It is not the same as the concentration at street level, which can be much higher or lower than the column suggests.
Reading the map
- Gaps are normal. OMI cannot see through cloud, snow and ice confuse it, and since 2007 part of its view has been blocked (the “row anomaly”), leaving stripes. Gaps show as the plain dark map, or are filled faded from the 1–2 days before (switch this off in Settings). Longer stretches with no data at all are hatched on the timeline.
- One day is noisy. Wind, rain, sunshine and the day of the week change NO₂ a lot. To compare years, compare the same season and look at several days.
- The grid is coarse. Each cell is about 25 km across, so a city is a handful of cells and a single road or chimney cannot be seen.
- City numbers are the median of the cells within about 30 km of the place, read from the same colours as the map. They are rounded to the colour bands of the scale.