Ozone (O3) is a key component of photochemical smog. Ozone is formed by a complex reaction between nitrogen dioxide and hydrocarbons in the presence of sunlight. It is considered to be a criteria pollutant in the troposphere—the lowermost layer of the atmosphere—but not in the upper atmosphere, where it occurs naturally and serves to block harmful ultraviolet rays from the Sun. Because nitrogen dioxide and hydrocarbons are emitted in significant quantities by motor vehicles, photochemical smog is common in cities where sunshine is ample and highway traffic is heavy. Certain geographic features, such as mountains that impede air movement, and weather conditions, such as temperature inversions in the troposphere, contribute to the trapping of air pollutants and the formation of photochemical smog.