Pressure, Density, and Mixing Ratios

Convection ceases at the tropopause level, and the temperature in the stratosphere and mesosphere is determined strictly by radiation balance. At altitudes above 20 km the absorption of solar ultraviolet radiation becomes increasingly important. The temperature peak at the stratopause has its origin in the absorption of near-ultraviolet radiation by stratospheric ozone. In fact, the existence of the ozone layer is in itself a consequence of the ultraviolet (UV) irradiation of the atmosphere. The enormous temperature increase in the thermosphere is due to the absorption of extremely shortwaved and thus energetic radiation coupled with the tenuity of the atmosphere, which prevents an effective removal of heat by thermal radiation. Instead, the heat must be carried downward by conduction toward denser layers of the atmosphere, where H20 and C02 are sufficiently abundant to permit the excess energy to be radiated into space. 

At the pressures and temperatures prevailing in the atmosphere, air and its gaseous constituents have the properties of an ideal gas to a very good approximation. Even water vapor can be treated in this way. Absolute temperature T, total pressure p, and air density p thus are related by the ideal gas law 

Since pressure and density are additive properties of ideal gases, one finds that Eq. (1-1) applies also to each gaseous constituent separately  

The quantities vj V and p, are called concentrations. Although they may be used to specify the abundance of a constituent in a parcel of air, it is preferable to use mixing ratios for this purpose because they are independent of pressure and temperature. For a constituent that mixes well in the atmosphere, the mixing ratio will be constant. 

NA = 6.02 x 1023 molecules/mol is Avogadro s number and fcB = RJ Na is Boltzmann s constant. The customary unit for n, is molecules/cm3, and we shall adhere to it although it does not conform to SI units. 

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