The atmosphere is generally assumed to be in an hydrostatic equilibrium: i.e. particles and molecules do not escape in large quantity due to the Earth gravity, and do not collapse at the surface as the gas volume is governed by the temperature and pressure conditions (i.e. the ideal gas law). As a consequence, the atmospheric pressure obeys the barometric equation and exponentially decreases with respect to altitude, about a factor of 10 every 17 km. Generally speaking, the average surface pressure at sea level is 1013 hPa but drops to about 0.02 hPa at 80 km. This means that 99.9995% of the atmosphere density is located below this altitude, close to the surface. The temperature dependency on altitude leads to divide the atmosphere in multiple vertical layers with either positive or negative temperature gradient (see Figure below):
- The troposphere (Greek: well mixed region), or lowest part of the atmosphere, typically extends from 0 to 9 km at the poles, 17 km at the equator. The top of the troposphere is named “tropopause height”. Within this layer, temperature steadily decreases at a rate of 6.5C per km. This negative gradient leads to convective and turbulent mixing and actually provides our weather. Internally, one usually distinguish the planetary boundary layer (PBL), closest layer to the surface, and the free troposphere. The PBL height is determined by the vertical transport or buoyancy due to thermal convection: the air rises as the Sun heats the surface, expanding then its volume due to the lower pressure at higher altitude. It is quite variable, from less than 500 m in winter to 2000 m in summer, and presents a strong diurnal cycle. Air pollution is mostly produced at the surface and usually remains confined because of the temperature inversion. Transport to the free troposphere is limited although possible. Changes in the troposphere directly affect life and our environment. Therefore, observations of tropospheric composition changes is of high importance (Burrows et al., 2011).
- The stratosphere (Greek: stratified region) from above the tropopause height up to about 50 km. This atmospheric region is characterized by a positive temperature gradient: i.e. temperature rises with increasing altitude. This is primarily due to the reaction of shortwave UV radiation with O3 – Ozone and O2. Reaction of UV with O2 leads to O3 formation and heat as a byproduct. O3 absorbs UV radiation (because of its low stability) and emits thermal infrared and then heats the surrounding layers. Ozone concentration steadily rises from the tropopause to about 40 km, explaining then in part the constant rise of temperature. Such a temperature profile creates very stable atmospheric conditions. Furthermore, stratospheric H2O – Water Vapour concentration is very low. Consequently, the stratosphere is almost completely free of clouds or other forms of weather.
- And higher layers: mesosphere (Greek: middle) from 50 to 80 km, thermosphere (Greek: heated region) from 80 km to 700 km, and exosphere from 700 to 10,000 km.