Nitrogen oxides (NOx) are a mixture of nitric oxide (NO) and nitrogen dioxide (NO2). They are produced from a variety of sources, including:

 Nitrogen oxides (NOx) are a mixture of nitric oxide (NO) and nitrogen dioxide (NO2). They are produced from a variety of sources, including: 

Transportation: Road traffic is the main outdoor source of NOx. 

Fuel combustion: Power plants, industrial activities, and residential energy use all emit NOx when they burn fossil fuels.

Indoor appliances: Poorly maintained or unflued gas, wood, oil, kerosene, or coal-burning appliances can be significant sources of NOx indoors. 

Lightning: Lightning is a natural source of NOx, and scientists estimate that lightning produces 8.6 million tonnes of NOx each year. 

Soil: Microbial processes in soils also produce small amounts of NOx. 

NOx are a key component of photochemical smog, which gives smog its yellowish-brown color. They are highly reactive gases that can be harmful to health and the environment.