Nitrogen dioxide (NO₂) is a gas that is considered a pollutant in large quantities. The primary sources of nitrogen dioxide on Earth are automobile exhaust, smoke from industry, and the burning of fossil fuels. It is also produced when nitric oxide (NO), another nitrogen oxide that forms during combustion, reacts with oxygen. 

NASA’s Earth-observing instruments collect data on concentrations of trace gases in the atmosphere, including nitrogen dioxide and other nitrogen oxides. These datasets are used to track historical and near real-time global and regional emissions, enabling scientists to study the effects of nitrogen oxides on human health, air quality, and weather patterns.