atmosphere, compositionThe percentage composition of clean dry air in the Earth's atmosphere is given in the table. The concentrations of CO2, CO, SO2, NO2, and NO are greater in industrial areas. The amount of ozone increases in the ozone layer (see atmospheric layers) and depends on meteorological and geographical variations. The fraction of water vapor in normal air varies from 0.1–2.8% by volume and 0.06–1.7% by mass, with meteorological and geographical variations and a decrease with altitude.
The Earth's primitive atmosphere is thought to have formed as a result of outgassing from the Earth's interior, in proportions similar to those emitted by present-day volcanoes (mainly water vapor and CO2 plus some SO2 and N2 ). The composition has since changed radically, initially as a result of geochemical cycling of the elements and the presence of plant and animal life, and recently by combustion processes and by industrial, technological, and agricultural activities. See also greenhouse effect.