Global nitrogen dioxide levels, July 2014. Major sources of tropospheric nitrogen dioxide (NO2) include industrial emissions, automobile traffic, forest and brush fires, microbiological soil emissions, lightning, and aircraft. More than half of the total NO2 emissions are estimated to be anthropogenic, mainly from the burning of fossil fuels for energy production, transportation, and industrial activities. NO2 has a relatively short lifetime (about a day) and is therefore concentrated near its sources. This image shows the global NO2 levels on 25 July 2014. The NO2 levels are colour-coded (key at bottom right). The data is from the Ozone Measuring Instrument (OMI) on NASA's Aura spacecraft.