Nitrogen oxides are regulated criteria pollutants formed during which process?

Nitrogen oxides, commonly referred to as NOx, are primarily formed during the burning process, especially in high-temperature environments such as internal combustion engines and fossil fuel combustion in power plants. This occurs because, at elevated temperatures, nitrogen in the air reacts with oxygen to produce various nitrogen oxides.

The regulatory status of nitrogen oxides as criteria pollutants stems from their significant impact on air quality and public health; they contribute to the formation of ground-level ozone and smog, exacerbating respiratory problems and other health issues. Understanding that nitrogen oxides are a byproduct of combustion processes highlights the importance of regulating emissions from vehicles and industrial sources to protect air quality.

In contrast to the other processes listed: evaporation and condensation do not involve chemical reactions that form nitrogen oxides, and photosynthesis is a biological process that uses carbon dioxide and releases oxygen, with no relation to the formation of nitrogen oxides.