Final answer:
The main source of nitrogen oxides, or NOx, is vehicle emissions, specifically from cars, trucks, and buses, as these contribute significantly more than natural sources like volcanoes.
Step-by-step explanation:
The main pollution source of nitrogen oxides is vehicles. Nitrogen oxides, often abbreviated as NOx, are a group of highly reactive gases. NOx primarily gets into the air from the burning of fuel, with emissions from cars, trucks, and buses as well as power plants and off-road equipment being significant contributors. These emissions occur when atmospheric nitrogen reacts with oxygen at high temperatures, such as those found in a car engine. Apart from vehicles, other sources include industrial processes and the burning of high-sulfur coal or electric utilities. However, compared to vehicles, natural sources like volcanoes contribute much less to the overall NOx levels found in the atmosphere.