(1) The gaseous envelope that surrounds the earth. It comprises of four major divisions: the troposphere (from sea level to approximately 10 km), the stratosphere (ozone region) which extends from approximately 10 to 50 km, the mesosphere which extends from from approximately 50 to 100 km, and the thermosphere which ranges from approximately 100 to 1000 km or more. There is no sharp boundary between the layers. The pressure drops rapidly as altitude increases (from 1 atm at sea level to 10−13 atm at 1000 km). The chemical entities and reactions that occur in these spheres are the subject of extensive research. (2) The pressure exerted by the air at sea level (14.696 psi), which will support a column of mercury 760 mm high (approximately 30 in.). This is standard barometric pressure, though it varies slightly with local meteorological conditions. It is often used to indicate working pressures of steam. The accepted abbreviation is atm. (3) Any environmental gas or mixture of gases, e.g., an atmosphere of nitrogen or an inert atmosphere.
