NOx

NOx is a collective term for nitrogen oxide species produced during combustion, chiefly nitric oxide (NO) and nitrogen dioxide (NO2). In emissions inventories, NOx usually denotes the sum of NO and NO2; nitrous oxide (N2O) is a separate greenhouse gas and is not always included in NOx figures.

NOx forms when nitrogen in air reacts with oxygen at high flame temperatures. The two main pathways are thermal NOx, through high-temperature air–fuel reactions, and fuel NOx, originating from nitrogen bound in fuels. Natural sources include lightning and certain soil and microbial processes, but human activities dominate urban and industrial NOx emissions.

Environmental and health effects are significant. NOx contribute to ground-level ozone and photochemical smog, and to secondary nitrate aerosols. They irritate the respiratory system, worsen asthma, and are linked to cardiovascular problems. In the atmosphere, NOx play a role in acid deposition and nutrient pollution, and their secondary products can affect ecosystems and visibility.

Control and regulation focus on reducing NOx emissions from vehicles, power plants, and industrial sources. Technologies include three-way catalysts and diesel oxidation catalysts for engines, exhaust gas recirculation to lower combustion temperatures, and selective catalytic reduction using ammonia or urea to convert NOx to nitrogen and water. Low-NOx burners, lean-burn engines, and fuel switching also help reduce NOx formation. Ambient NOx is monitored to enforce regulatory limits and assess air quality.

Measurement and units commonly use NO and NO2 as the primary species, with NOx reported as the total against emissions or ambient concentration standards. Measurement methods include chemiluminescence for NOx, UV photometry for NO2, and optical techniques for ambient NOx monitoring.