Pollutants, especially smog and smog-forming chemicals, including VOCs, are trapped close to the ground. As people continue driving, and sources other than motor vehicles continue to release smog-forming pollutants into the air, the smog level keeps getting worse. Ozone NOx - Nitrogen Oxides. They are produced from burning fuels, including gasoline and coal. Nitrogen oxides are smogformers, which react with volatile organic compounds to form smog.
Nitrogen oxides are also major components of acid rain.
Ozone is not emitted directly into the air but is formed by the reaction of VOCs and NOx in the presence of heat and sunlight. Ground-level ozone forms readily in the atmosphere, usually during hot summer weather. VOCs are emitted from a variety of sources, including motor vehicles, chemical plants, refineries, factories, consumer and commercial products, and other industrial sources. Nitrogen oxides are emitted from motor vehicles, power plants, and other sources of combustion. Changing weather patterns contribute to yearly differences in ozone concentrations from city to city.