8.1.1 Constituents of the atmosphere

It is convenient to subdivide the atmosphere into a ``clean dry'' component, water vapor, and aerosols (water droplets, as well as ice crystals, salt grains & dust particles, which serve as condensation seeds for water).

8.1.1.0.1 Abundances

 

Table 8.1: Main constituents of the dry air in the troposphere

Name	Molec. mass	Normal abund.	Name	Molec. mass	Normal abund.
	amu	(% in volume)		amu	(% in volume)
N2	28.013	78.084	He	4.003	0.0005
O2	32.000	20.946	Kr	83.8	0.0001
Ar	39.948	0.934	CH4	16.043	0.0002
CO2	44.010	0.033v	H2	2.016	0.00005
Ne	20.183	0.0018	N2O	44.013	0.00005

 

Table 8.1 gives the composition of the ``clean dry'' air in the ``standard US'' model throughout the troposphere (i.e at altitudes $\leq 20$ km). Altogether, N₂, O₂, and Ar represent 99.96 % in volume. Except for CO₂, whose abundance at ground level may vary between day and night by up to a factor of 2, this composition is remarkably homogeneous and constant. It is completed by a number of trace components, most of which are unstable (SO₂, O₃, NO, CO, ...) and whose abundance (in volume) never exceeds 10^-5.

The abundance of water is highly variable, but hardly exceed 1% in mass, even locally. Most of the water in the air is in the form of vapor. Even inside the clouds, precipitation and turbulence insure that the mass of water droplets per cm^-3 seldom equals that of water vapor.

Despite their low abundances, water, which has a large absorption cross section in the IR and a large specific heat of vaporization, ( $L_w\sim 600$ cal/g), ozone and carbon dioxyde, which have large IR absorption cross sections, are the major actors of the thermal balance of the air.

8.1.1.0.2 Absorption spectrum

Ozone is responsible for most of the absorption of the solar radiation in the UV (especially between 1800 and 2900 Å). In the visible, the air is fairly transparent except for scattering by aerosols (mostly water droplets and/or ice crystals when there are clouds). In the infrared, H₂O, CO₂ and, around 10$\mu$m, O₃ are very efficient absorbers of the solar and ground radiation.

By clear weather, the atmospheric absorption at mm wavelengths is dominated by rotational and fine structure lines of molecules in their ground electronic and vibrational state. The strongest transitions are electric dipole transitions from polar molecules, like water and ozone. Intrinsically weaker (typically by a factor of 10^2-3), but of considerable practical importance, are the magnetic dipole transitions from O₂. When there are clouds, the thin water droplets scatter and absorb the short (mm) radio waves.