relative concentrations (see Section 2.6) of atomic oxygen (O) and nitrogen (N)
are greatest lower down, while the lighter hydrogen (H) and helium (He) are
found to predominate higher up.
Figure 3.2 shows the various layers of the atmosphere. The part where grav-
itational settling occurs is usually termed the heterosphere, because of the varying
composition. The better-mixed part of the atmosphere below is called the homo-
sphere. Turbopause is the term given to the boundary that separates these two
parts. The heterosphere is so high (hundreds of kilometres) that pressure is
extremely low, as emphasized by the logarithmic scale in the figure.
In a mixture of gases like the troposphere, Dalton’s law of partial pressure
(Box 3.1) is obeyed. This means that individual gases in the atmosphere will
decline in pressure at the same rate as the total pressure. This can all be
conveniently represented by the barometric equation:
ppz=- 0 exp()zH eqn. 3.134 Chapter Three
Box 3.1 Partial pressureThe total pressure of a mixture of gases is
equal to the sum of the pressures of the
individual components. The pressure–volume
relationship of an ideal gas (i.e. a gas
composed of atoms with negligible volume
and which undergo perfectly elastic collisions
with one another) is defined as:
eqn. 1
where pis the partial pressure, Vthe
volume, nthe number of moles of gas,
Rthe gas constant and Tthe absolute
temperature. Real gases behave like ideal
gases at low pressure and we denote a
mixture of gases (1, 2, 3) through the use
of subscripts:Hence:
eqn. 2
or
eqn. 3
where pTis the total pressure of the mixture.
The implication that partial pressure piis a
function of nimeans that the barometric law
(Section 3.2):pVT =++(n n n RT12 3)(pp pV nnnRT12 3++) =++( 123 )pV nRT
pV nRT
pV nRT11
22
33=
=
=pV=nRTeqn. 4
can be rewritten:
eqn. 5
or even:
eqn. 6
where cis some unit of the amount of
material per unit volume (g m-^3 or molecules
cm-^3 ). The barometric law predicts that
pressure and concentration of gases in the
atmosphere decline at the same rate with
height.
The relationship between partial pressure
and gas-phase concentration explains why
concentrations in the atmosphere are
frequently expressed in parts per million
(ppm) or parts per billion (ppb) (see
Table 3.1). This is done on a volume basis so
that 1 ppm means 1 cm^3 of a substance is
present in 10^6 cm^3 of air. It also requires that
there is one molecule of the substance
present for every million molecules of air, or
one mole of the substance present for every
million moles of air. This ppm unit is thus a
kind of mole ratio. It can be directly related
to pressure through the law of partial
pressure, so at one atmosphere (1 atm)
pressure a gas present at a concentration of 1
ppm will have a pressure of 10-^6 atm.ccz=- 0 exp(zH)nnz=- 0 exp(zH)ppz=- 0 exp(zH)