What is a common source of nitrogen oxides in urban areas?

Vehicle exhaust is a common source of nitrogen oxides (NOx) in urban areas due to the high volume of motor vehicle traffic that characterizes cities. When fuel is burned in vehicles, particularly in internal combustion engines, nitrogen from the atmosphere reacts under high temperatures to form nitrogen oxides. Urban environments often experience heavy traffic congestion, leading to increased emissions from cars, trucks, and buses. This makes vehicle exhaust a significant contributor to air pollution, especially in densely populated areas.

Industrial emissions, while also a source of nitrogen oxides, are generally less prevalent in urban areas compared to the impact of vehicles. Residential heating can produce emissions as well, particularly in areas that rely on oil or gas for heating, but this tends to be less significant in comparison to the continuous, high-volume emissions from cars. Commercial power generation can contribute to nitrogen oxides, but power plants are often located away from urban centers and are not as direct a source of pollution affecting city air quality as vehicle emissions. Therefore, vehicle exhaust stands out as the most common source of nitrogen oxides in urban contexts.