Air contaminated with ozone, nitrogen oxides, and hydrocarbons, with or without natural fog being present. In the presence of sunlight, hydrocarbons and NO_x are involved in a complex series of chemical reactions that eventually creates ozone and other oxidants as secondary pollutants. However, ozone is also destroyed by NO_x. Photochemical air pollution levels are generally proportional to concentrations of nitrogen oxides and hydrocarbons; they also increase with strong solar intensity and high ambient temperatures, which increase biogenic volatile organic emissions to the atmosphere from vegetation. The pollutant levels are inversely proportional to wind speed and inversion height. See also smog.