Type of air pollution characterized by high levels of ozone and low visibility, typically found in cities located in a valley (e.g., Los Angeles, Denver, Mexico City). Sunlight, oxides of nitrogen, and hydrocarbons (the latter two of which arise from automobile exhaust) are all required in order for smog formation to occur. The most severe episodes occur when a strong temperature inversion caps the location and traps the pollutants. The degradation in visibility is associated with the light scattering due to particulate matter. See London (sulfurous) smog.