How does nitrogen dioxide contribute to smog and acid rain?

Nitrogen dioxide ($N{O}_2$) is a significant air pollutant that contributes to both photochemical smog and acid rain through its chemical reactivity in the atmosphere.12

Photochemical Smog

Nitrogen dioxide plays a central role in the formation of photochemical smog, a reddish-brown haze common in urban areas. The process typically begins with the emission of nitrogen oxides from sources like vehicle exhaust. In the presence of sunlight, these compounds undergo a series of complex reactions with hydrocarbons in the lower atmosphere. This process leads to the production of ozone, peroxyacetyl nitrate, and other irritant compounds that characterize smog.12

Acid Rain

Nitrogen dioxide contributes to acid rain by reacting with water, oxygen, and other chemicals in the atmosphere to form nitric acid. When this acid mixes with cloud moisture, it lowers the pH of precipitation, which then falls to the ground as acid rain. This process is a major environmental concern, as the resulting acidic precipitation can have harmful effects on both natural ecosystems and human-made environments.13