Nitrogen oxides (also known as oxides if nitrogen, and abbreviated as NOx) is a collective term used to refer to two species of oxides of nitrogen: nitric oxide (NO) and nitrogen dioxide (NO2). Nitric oxide is a colorless, flammable gas with a slight odour. Although somewhat toxic, its odour is insufficient to provide warning. Nitrogen dioxide is a reddish brown, nonflammable, gas with a detectable smell. In significant concentrations it is highly toxic, causing serious lung damage with a delayed effect. Nitrogen dioxide is a strong oxidizing agent that reacts in the air to form corrosive nitric acid, as well as toxic organic nitrates. It also plays a major role in the atmospheric reactions that produce ground-level ozone or smog.
Globally, quantities of nitrogen oxides produced naturally by bacterial and volcanic action, and lightning, outweigh man-made emissions. Man-made emissions are mainly due to fossil fuel combustion from both stationary sources, such as power generation (24%), and mobile sources, such as transport (49%). Other atmospheric contributions come from non-combustion processes, for example nitric acid manufacture, welding processes and the use of explosives.
In the atmosphere, nitrogen oxides mix with water vapour producing nitric acid. This acidic pollution can be transported by win over many hundreds of miles, and deposited as acid rain.