NASA lists most polluted cities in the world; India, China lead

India, China and the Middle East, with their fast-growing economies and expanding industry, have seen growing air pollution, according to NASA scientists who tracked the trends over the last decade in various regions and 195 cities around the globe.

"These changes in air quality patterns aren't random," said Bryan Duncan, an atmospheric scientist at NASA's Goddard Space Flight Centre in US, who led the research. Using new, high-resolution global satellite maps of air quality indicators, NASA scientists tracked air pollution trends over the last decade in various regions and 195 cities around the globe.

Duncan and his team examined observations made from 2005 to 2014 by the Dutch-Finnish Ozone Monitoring Instrument aboard NASA's Aura satellite.

One of the atmospheric gases the instrument detects is nitrogen dioxide, a yellow-brown gas that is a common emission from cars, power plants and industrial activity. Nitrogen dioxide (NO2) can quickly transform into ground-level ozone, a major respiratory pollutant in urban smog.

Nitrogen dioxide hotspots, used as an indicator of general air quality, occur over most major cities in developed and developing nations.

According to the findings, US, Europe and Japan have improved air quality owing to emission control regulations.

The science team analysed year-to-year trends in nitrogen dioxide levels around the world.

To look for possible explanations for the trends, the researchers compared the satellite record to information about emission controls regulations, national gross domestic product and urban growth.

"With the new high-resolution data, we are now able to zoom down to study pollution changes within cities, including from some individual sources, like large power plants," said Duncan.

Previous work using satellites at lower resolution missed variations over short distances.

This new space-based view offers consistent information on pollution for cities or countries that may have limited ground-based air monitoring stations.

The resulting trend maps tell a unique story for each region. The US and Europe are among the largest emitters of nitrogen dioxide.

Both regions also showed the most dramatic reductions between 2005 and 2014. Nitrogen dioxide has decreased from 20 to 50 per cent in the US, and by as much as 50 per cent in Western Europe. China, the world's growing manufacturing hub, saw an increase of 20 to 50 per cent in nitrogen dioxide, much of it occurring over the North China Plain.

The study was published in the Journal of Geophysical Research.