Global air quality. Global map showing changes in atmospheric nitrogen dioxide concentrations between 2005 and 2014. Blue denotes lower concentrations and red indicates higher concentrations. Nitrogen dioxide gas is a common emission from vehicles, power plants and industry, and is used as an indicator of general air quality. Over this period air quality has improved in the US, Europe and Japan, due to emission control regulations. Data obtained by the Ozone Monitoring Instrument aboard NASA's Aura satellite.