Which major issue is associated with nitrogen oxides?

Nitrogen oxides (NOx), which include nitrogen dioxide (NO2) and nitric oxide (NO), play a significant role in the formation of photochemical smog. This type of air pollution typically occurs in urban areas where there is a high concentration of vehicle emissions and sunlight. When nitrogen oxides react with volatile organic compounds (VOCs) in the presence of sunlight, they lead to the formation of ground-level ozone and particulate matter, which are the main components of photochemical smog.

The relationship between nitrogen oxides and smog is a critical environmental concern, particularly because smog can have harmful effects on human health, such as respiratory issues, and can damage the environment. Other options listed do not directly relate to the primary environmental impact of nitrogen oxides. Increased oxygen levels would not be a consequence of NOx emissions, nor would a reduction of atmospheric pressure or the decomposition of organic matter. Therefore, the connection between nitrogen oxides and the production of photochemical smog is the most relevant issue associated with these pollutants.