nitrogenoxide

Nitrogen oxides, or NOx, refer to a group of nitrogen-oxygen compounds that are primarily produced and studied due to their roles in air pollution and atmospheric chemistry. The most significant NOx species are nitric oxide (NO) and nitrogen dioxide (NO2). In the atmosphere NO2 readily forms equilibrium with dinitrogen tetroxide (N2O4), depending on temperature. Nitrous oxide (N2O) is sometimes included in broader discussions of NOx, but it is less reactive and is often treated separately.

Formation and sources: NOx are produced mainly by high-temperature combustion, such as in motor vehicle engines, power plants, and industrial furnaces, where nitrogen in the air reacts with oxygen. Natural sources include lightning and certain soil and microbial processes. The balance of NO and NO2 released depends on combustion conditions and subsequent atmospheric chemistry.

Atmospheric chemistry and environmental impact: NO reacts with ozone to form NO2, and together NOx drive photochemical smog and tropospheric ozone formation. NO2 can hydrolyze in water to produce nitric acid, contributing to acid deposition and nitrate aerosol formation. NOx influence the atmospheric concentration of reactive nitrogen and can impact climate indirectly through aerosol and ozone interactions.

Health and regulation: Exposure to NOx irritates the eyes and respiratory tract and can worsen asthma and other lung conditions. Regulatory frameworks aim to limit ambient NOx concentrations, with controls such as catalytic converters on vehicles and selective catalytic or non-catalytic reduction technologies at industrial facilities to reduce emissions.

Uses and detection: NO serves as a pharmaceutical inhaled vasodilator in some medical treatments. NOx are important industrial precursors, notably for nitric acid production via the Ostwald process. Monitoring methods include chemiluminescence, spectroscopy, and electrochemical sensors.