Smog over Denver, Colorado, appears as a hazy sky above the city.

UCAR/Caryle Calvin

Although smog originally denoted a blend of smoke and fog, it now refers more generally to dense, visible air pollution, as seen here north of Denver. In cities such as Denver, automobiles have been a primary culprit for pollution and smog. Nitric oxide from automobile exhaust combines with oxygen in the air to form the brown gas nitrogen dioxide. When hydrocarbons and nitrous oxide from auto emissions become exposed to sunlight, a photochemical reaction creates ground-level ozone pollution.