Photochemical pollution, also called photo-oxidant pollution, is caused by the transformation of primary pollutants, chemical precursors, into oxidant forms when exposed to sunlight.
After exposure to ultraviolet rays, photochemical reactions transform nitrogen oxides (NOx) and volatile organic compounds (VOC) into tropospheric ozone (O3) and other oxidant compounds (aldehydes, hydrogen peroxide, etc.)
Photochemical pollution is therefore responsible for ozone peaks and their harmful effects on human populations as well as plants. Moreover, oxidant forms also cause the acidification of soil and water.
Cartography of the tropospheric ozone, a source of photochemical pollution, in August from 1979 to 2000. The blue areas indicate a decrease in levels, the yellow areas indicate an increase in levels. © Jack Fishman / Nasa Langley Research Center