Nitrogen oxides (NOx stands for an indeterminate mixture of NO and NO2) are mainly formed from N2 and O2 during the combustion of automotive fuels at high temperatures. Catalytic converters are used to reduce emissions. Nitric oxide, however, creates a reddish-brown smog in urban air that causes heart and lung problems and may cause cancer. Nitrogen oxides also cause acid rain because they combine with water to produce nitric acid (HNO3) and other acids. Natural sources of nitrogen oxides include nitrogen oxides produced during the metabolism of certain soil bacteria.

Ground and satellite measurements of tropospheric nitrogen oxides (NO2) in India.

Nitrogen oxides (NOx), one of six standard air pollutants, have adverse effects on individuals and ecosystems. Rapid industrial development over the past three decades has led to increased levels of ambient air pollutants, including nitrogen oxides, leading to worsening air quality. Over the past decade, India has made great strides in measuring nitrogen oxides using ground-based and spaceborne sensors.

Nitrogen oxides, NO and NO2, collectively known as NOx, are formed in three ways during combustion. Of the NOx emitted from the combustion chamber, approximately 85-90% is NO and 5% or 10% is NO2. The three types of NOx formed are through thermal, fuel and cue mechanisms.

NOx emissions from coal combustion may come from three sources. Thermal NOx is primarily formed by the reaction of nitrogen and oxygen in the combustion air. Fuel NOx is a component formed primarily from the conversion of nitrogen in fuel to nitrogen oxides. NOx is formed when the hydrocarbon-based fragments in the flame zone react with nitrogen to form nitrogen atoms, which in turn produce NO.