Natural and human-related sources of ozone-forming trace gases in southernAfrica

Nitric oxide (NO) reacts with hydrocarbons and carbon monoxide (CO) in the presence of sunlight to form ozone in the lower atmosphere. Tropospheric oz one can be harmful to plants and animals, and is usually regarded as a symp tom of industrial or vehicular pollution. The cloud of tropospheric ozone w hich forms over southern Africa every spring probably has its main origin i n natural emissions of the ozone-forming trace gases, including CO from veg etation fires, emissions of NO from soils, and hydrocarbons from plants. Th e observed levels of ozone are not dangerous to humans, but form a high bac kground level to which pollutants of human origin are added.