Pollutants Nitrogen Dioxide (NO2)

Description:
NO₂ is part of a family of highly reactive gases called nitrogen oxides (NO_x), which also include nitric oxide (NO) and nitrous oxide (N₂O). NO₂ is a smog-forming chemical that can smell slightly sweet and is reddish brown in color. NO_x can react with SO₂ and other chemicals in the air to form acid rain.

Source:
NO₂ forms when fuel is burned at high temperatures. For example, when a nitrogen (N₂)-oxygen (O₂) mixture is heated at temperatures over 1100°C, the nitrogen and oxygen will combine to form nitrogen oxide (NO). If the cooling process is slow, the gases will decompose back to their original state. If the cooling process is fast, such as in the internal combustion engines of motor vehicles, the nitrogen oxides will not decompose but instead stay in the NO state. These gases are released into the atmosphere and combine with ozone (O₃) to produce NO₂. Most NO₂ in the atmosphere is formed in this way, although some is released directly from the source.

In the presence of sunlight, NO₂ also reacts with other pollutants, such as VOCs, to help form ground-level ozone. Once this ozone is formed, it is again scavenged by NO to form NO₂. In the absence of other competing reactions, a "photostationary state" is formed where concentrations of NO, NO₂, and O₃ are all interrelated. Ozone levels therefore tend to be higher in rural areas where automobile driving is less frequent and consequent NO levels are low.

NO₂ is primarily the result of gases from motor vehicle exhaust and stationary fuel combustion sources like electric utilities and industrial boilers. It can also be produced from gas stoves and heaters. NO₂ absorbs light and can lead to the brownish haze over metropolitan areas.

In addition, nitrogen oxides contribute to the formation of acid rain. NO_x, when in the presence of water vapor in the atmosphere, partially converts into nitric acid, HNO₃. Nitric acid and sulfuric acid combine to create acid rain.