Sky
Ammonia has been detected in the upper troposphere (shown here in orange), up to 15 kilometres above sea level, for the first time Nasa/JSC Gateway to Astronaut Photography of Earth

The gas ammonia is usually only found in the atmosphere in tiny trace amounts, but scientists have now detected it at significant levels in the upper troposphere for the first time.

The gas was at levels of 33 molecules per trillion air molecules over north India and south-east China, which have a huge consumption of nitrogen-based fertilisers. For comparison, in the rest of the world ammonia levels were below the lowest detectable limit of about 3-5 molecules per trillion air molecules.

This is the first time ammonia has been detected more than 10km in the atmosphere. It was only found at significant levels in the upper troposphere during the summer monsoon season, with the highest levels confined to the air over south-east China and north India.

"Observations show that ammonia is not washed out completely when air ascends in monsoon circulation," study author Michael Höpfner of the Institute for Meteorology and Climate Research at the Karlsruhe Institute of Technology in Germany, said in a statement.

"Hence, it enters the upper troposphere from the boundary layer close to the ground, where the gas occurs at relatively high concentrations."

Every cloud has an ammonium-salt lining

The finding is surprising in seeing just how far the pollutant ammonia released by human activity can spread in the atmosphere.

"The detection of enhanced amounts of ammonia in the western part of the Asian monsoon anticyclone [for] several years suggests that its lifetime is long enough to survive transport to areas far from the source region," the study authors write in the paper.

However, the finding may have beneficial but unintended consequences. Ammonia reacts with acids in the atmosphere to form ammonium salts, such as ammonium sulphate and ammonium nitrate. These salts act as aerosol particles.

The tiny aerosol particles act as a seed for moisture droplets to form, which can lead to cloud formation. Increased cloud cover may act to help cool the climate.

The research is published in the journal Atmospheric Chemistry and Physics.