NO2

NO2, or nitrogen dioxide, is a reactive nitrogen oxide with the chemical formula NO2. It is a reddish-brown gas at room temperature and has a sharp, acrid odor. NO2 is a radical species, containing an unpaired electron, which makes it highly reactive toward many organic and inorganic compounds.

NO2 forms primarily in the atmosphere by oxidation of nitric oxide (NO) produced during combustion. Major sources include motor vehicle exhaust, power plants, industrial processes, and other high-temperature combustion. Indoor sources such as gas stoves and heaters can also release NO2. In the atmosphere, NO2 can dissolve in water droplets and form nitrous and nitric acids, while in the air it participates in photochemical reactions that generate ozone and secondary aerosols.

In atmospheric chemistry, NO2 is an intermediate in the NOx cycle. It reacts with hydroxyl radicals to form nitric acid, contributes to the formation of nitrate aerosols, and, upon sunlight exposure, participates in the photochemical production of ozone. These processes link NO2 to smog formation and acidification of precipitation.

Health and environmental effects are a primary concern. NO2 and other NOx compounds irritate the airways, exacerbate asthma and other respiratory conditions, and can contribute to reduced lung function with prolonged exposure. High concentrations can cause more serious health effects. NO2 also affects vegetation and contributes to environmental issues such as acid rain and nutrient imbalances in ecosystems.

Uses and safety: NO2 is used as a chemical reagent and oxidant in laboratory and industrial processes and is an intermediate in the production of nitric acid via the Ostwald process. Handling requires proper ventilation and controls to limit inhalation exposure; exposure limits are established by regulatory agencies to protect public health.