According to the findings, US, Europe and Japan have improved air quality owing to emission control regulations.
    
"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. The science team analysed year-to-year trends in nitrogen dioxide levels around the world.

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