NOx emissions refer to the collective term for nitrogen oxides NO and NO2, which are significant components of harmful air pollution. These gases are formed during combustion processes such as engines, power plants, and boilers, as well as industrial processes with very high temperatures. NOx pollution is emitted by automobiles, trucks, and various non-road vehicles, as well as industrial sources such as power plants, industrial boilers, cement kilns, and turbines. NOx often appears as a brownish gas and is a strong oxidizing agent that plays a major role in the atmospheric reactions with volatile organic compounds (VOC) that produce ozone (smog) on hot summer days.
NOx emissions contribute to respiratory diseases, the formation of ground-level ozone, and acid rain. They are particularly a local/regional problem, but NOx can also be transported over longer distances with air currents. In the upper atmosphere, NOx is involved in various processes that are important for the greenhouse gas budget, with both cooling and warming effects. NOx is a significant contributor to air pollution, and various regulations have been implemented to control NOx emissions from stationary and mobile sources.
