The primary sources of nitrogen oxides in the atmosphere include vehicle emissions, industrial processes, and natural events such as lightning.

Nitrogen oxides, commonly abbreviated as NOx, are a group of gases composed mainly of nitrogen and oxygen. The two most prevalent forms of nitrogen oxides are nitric oxide ($\text{NO}$) and nitrogen dioxide ($\text{NO}_2$). These gases are produced when nitrogen gas ($\text{N}_2$) reacts with oxygen ($\text{O}_2$), a reaction that can occur due to both anthropogenic (human-caused) activities and natural processes.

One of the principal sources of nitrogen oxides is the combustion of fossil fuels, especially in transportation. When gasoline or diesel is burned in an engine, the high temperatures generated cause nitrogen and oxygen in the air to react, resulting in the formation of nitrogen oxides. Consequently, regions with heavy traffic often experience elevated levels of nitrogen oxides in the atmosphere.

Industrial activities also significantly contribute to nitrogen oxide emissions. Many factories and power plants burn fossil fuels to generate energy, which can lead to substantial nitrogen oxide emissions. Additionally, certain industrial processes, such as the production of nitric acid and the manufacturing of explosives, generate nitrogen oxides as by-products.

Natural events can further contribute to the formation of nitrogen oxides. For instance, during thunderstorms, the intense heat and pressure produced by lightning can cause nitrogen and oxygen in the air to react, resulting in nitrogen oxides. Other natural sources include volcanic eruptions and wildfires, both of which can emit nitrogen oxides into the atmosphere.

Furthermore, nitrogen oxides can form in the atmosphere through the reaction of nitrogen and oxygen in the presence of sunlight, a phenomenon known as photochemical smog. This process is a significant contributor to air pollution in many urban areas.