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.
Nitrogen dioxide (NO₂)
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 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.
Particles (aerosol optical depth)
Aerosol optical depth (AOD) says how much sunlight the particles in the air block on the way down: smoke, desert dust, haze from pollution, sea salt and volcanic ash. Around 0.1 is clear air; above 1 the sky is thick with smoke or dust. It has no unit. The layer merges two methods (“Dark Target” and “Deep Blue”) from the MODIS instrument on NASA’s Terra satellite, which passes once a day in the late morning (around 10:30 am local time), daily since February 2000.
AOD covers the whole column of air, so it is not the same as PM2.5 at ground level. It cannot be measured through cloud or over snow and ice.
Smoke & dust (UV aerosol index)
The UV aerosol index from OMI picks out particles that absorb ultraviolet light, mainly smoke and desert dust. Unlike most measurements it also works above clouds, so it follows smoke plumes far out over the oceans. Values above about 1 mean smoke or dust; the higher and thicker the plume, the higher the index. It has no unit, and is a sign of particles rather than an amount.
Carbon monoxide (CO)
Carbon monoxide comes from fires and from burning fuel. It lasts one to two months in the air, long enough to cross oceans, so fire seasons show up as huge plumes. This layer is from the AIRS instrument on NASA’s Aqua satellite (daytime pass, daily since September 2002), on a 1° grid. It is the amount at about 5 km up (the 500 hPa pressure level), in parts per billion by volume (ppbv), not the air at street level.
Sulfur dioxide (SO₂)
Sulfur dioxide comes from burning coal and oil, from metal smelters and from volcanoes. This layer is from the OMPS Nadir Mapper on the Suomi NPP satellite (early-afternoon pass, daily since January 2012). It gives the column of SO₂ in Dobson units (1 DU = 2.69×10¹⁶ molecules per cm²), worked out as if the gas sat in the lowest kilometre or so of air. That suits power plants and smelters; for volcanic plumes higher up the numbers are only rough.
Only clear plumes are drawn. Where the map shows nothing, no SO₂ stood out from the noise that day (or there was no reading), so the gaps are not filled from earlier days for this layer.
Ozone
This is total ozone from OMI, in Dobson units: all the ozone above each place, about 90% of it in the high ozone layer (roughly 15–35 km up) that shields the ground from ultraviolet light. A typical value is around 300 DU. Every southern spring (September–October) the Antarctic ozone hole drops below 220 DU. It is not the ground-level ozone in summer smog, which is a small part of the total. OMI needs sunlight, so the polar winter is left blank.
Reading the map
- Gaps are normal. Most of these instruments 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 these measurements a lot. To compare years, compare the same season and look at several days.
- The grid is coarse. Each cell is about 10–100 km across depending on the layer, so a city is a handful of cells and a single road or chimney cannot be seen.
- Different satellites, different times. Terra passes in the late morning; Aura, Aqua and Suomi NPP in the early afternoon. MODIS has flown since 2000, AIRS since 2002, OMI since 2004 and OMPS since 2012, so each layer’s timeline starts on its own first day.
- 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.
- Layers are not interchangeable. Each has its own units and scale, so compare a place with itself on another day, not one layer with another.