NO2gas

NO2 gas, or nitrogen dioxide, is a chemical compound with the formula NO2. It is a reddish-brown, pungent, toxic gas that is a prominent component of air pollution. At room temperature, NO2 exists mainly as a monomer, but it readily forms the dimer N2O4, especially when cooled. The two species are in equilibrium: 2 NO2 ⇌ N2O4. The balance shifts toward NO2 at higher temperatures, giving the characteristic brown color; N2O4 is colorless.

NO2 is produced by the high-temperature combustion of fossil fuels, such as in motor vehicles and power plants, and in some industrial processes. In the atmosphere, nitric oxide (NO) is oxidized to NO2, forming a pool of nitrogen oxides (NOx) that influence air chemistry.

In the atmosphere, NO2 participates in photochemical reactions that generate ozone and contribute to smog, especially in urban areas with sunlight and volatile organic compounds. It can also react with water to form nitric and nitrous acids (HNO3 and HNO2), contributing to acid deposition and nitrate aerosol formation.

Uses of NO2 include its role as an oxidizing agent in certain chemical syntheses and as an intermediate in the production of nitric acid via the NOx catalytic cycle. It may be employed in nitration reactions under controlled conditions, though such uses are specialized.

Safety and environmental aspects: NO2 is toxic and irritating to the eyes, throat, and lungs. Acute exposure can cause coughing, shortness of breath, and bronchospasm; chronic exposure is associated with respiratory problems. It is a major urban air pollutant and a contributor to acid rain and particulate matter formation. Concentrations in workplaces and the environment are subject to regulatory limits and monitoring.