ABSTRACT
The atmosphere is the aeriform fluid that enshrouds the Earth. It is a dusty gas. Much of its mass is contained in its lowermost layer.
The composition of the atmosphere mostly reflects the presence of life on the planet’s surface. The chief gases (by volume) are nitrogen (78.08 per cent) and oxygen (20.95 per cent), neither of which is abundant on any other terrestrial-like planet (Mercury, Venus, and Mars). Water vapour (1 per cent, but variable) is a smallish, but highly significant, component. Next comes argon (0.93 per cent), an inert gas. The remaining gases are minor in quantity, though some play leading roles in biogeochemical cycles and in the regulation of atmospheric temperature. They include carbon dioxide (360 ppm and rising), neon (18 ppm), helium (5.2 ppm), methane (1.5 ppm), krypton (1.0 ppm), hydrogen (0.5 ppm), ozone (0.4 ppm), nitrous oxide (0.33 ppm), xenon (0.087 ppm), carbon monoxide (0.12 ppm), ammonia (0.01 ppm), nitrogen dioxide (0.001 ppm), and sulphur dioxide (0.0002 ppm). The figures for the minor constituents are for the lower atmosphere and vary somewhat with altitude and sometimes latitude. Some water vapour is split by ultraviolet light into hydrogen atoms and hydroxyl (OH) radicals; the latter, mainly because it is a catalyst in ozone destruction, has a material influence on atmospheric chemistry.
