Why does India's air look different from space?

There is something very distinct about the air over India and the surrounding countries in South Asia.

It is the presence of formaldehyde - a colourless gas that is naturally released by vegetation but also from a number of polluting activities.

The elevated concentrations have been observed by Europe's new Sentinel-5P satellite, which was launched last October to track air quality worldwide.

It is information that will inform policies to clean up the atmosphere.

Compared to the major constituents like nitrogen and oxygen, the formaldehyde signal is actually very small; in every billion air molecules just a few will be HCHO. But it can be a signifier of more general pollution problems, says Isabelle De Smedt from the Royal Belgian Institute for Space Aeronomy (BIRA-IASB).

"The formaldehyde column is composed of different sorts of volatile organic compounds, and the source can be from vegetation - so, from natural origin - but also from fires and pollution," she told BBC News.

"It depends on the region but 50-80% of the signal is from some biogenic origin. But above that you have pollution and fire. And the fire can be from coal burning or wildfires, but in India, yes, you have a lot of agricultural fires."

India also uses considerable quantities of wood in the home for cooking and heating.

When volatile organic compounds are brought together with nitrogen dioxide (NO₂, from fossil fuel burning) and sunlight, reactions will produce ground-level ozone.

This is a severe respiratory irritant that can lead to significant health problems.

Notice how the Himalaya Mountains essentially corral the air on the plains, preventing it from moving north.

The relative low in formaldehyde concentration in north-west India is centred over the desert lands of Rajasthan, where, obviously, there is much less vegetation and fewer people.

Sentinel-5P was procured and launched by the European Space Agency for the European Union's Copernicus Earth-monitoring programme.

The satellite's Tropomi instrument can detect the presence in the atmosphere of a suite of trace gases in addition to formaldehyde, including nitrogen dioxide, ozone, sulphur dioxide (SO₂), methane, carbon monoxide (CO) and aerosols (small droplets and particles).

All affect the air we breathe and therefore our health, and a number of them also play a role in climate change.

The Tropomi instrument itself represents a remarkable step-change on the capability of its predecessor spectrometer system known as Omi, which still flies today on an American space agency satellite.

"We already had really good data, but we needed many more days of observations, sometimes years of observations, to get this kind of quality," said Dr De Smedt.

"The new (India) map contains four months of data. Tropomi can do in one month what Omi did in six.

"We now see much faster the details, the small emissions, the cities - the kind of signals we didn't see so well before. We needed 10 years of data to see the emissions around Tehran, for example. In this map you can see them from only four months of Tropomi data."

After a test and commissioning phase, S5P will go fully operational at the end of the month for some of its data products, such as nitrogen dioxide and carbon monoxide.

Others, like formaldehyde, will have to wait until the Autumn.

Overall, the Tropomi investigations on S5P are led from the Netherlands Met Office (KNMI). The BIRA-IASB heads up the HCHO and SO₂ analyses.

The bus, or chassis, of the satellite was assembled by Airbus in the UK, making S5P Britain's biggest single industrial contribution to the Sentinel series of satellites that have been procured for Copernicus.

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