Natural sources of sulphur dioxide include release from volcanoes, biological decay and forest fires.
Actual amounts released from natural sources in the world are difficult to quantify.
Natural sources of nitrogen oxides include volcanoes, oceans, biological decay and lightning strikes. Ozone is a secondary photochemical pollutant formed near ground level as a result of chemical reactions taking place in sunlight. Natural sources of particulate matter are less important than man-made sources. These include volcanoes and dust storms. However, such sources do account for intense high particulate pollution episodes, occurring over relatively short times scales.