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ABSORPTION
• Beer’s Law
• Optical thickness
• Examples
BEER’S LAW
Note: Beer’s law is also attributed to Lambert and Bouguer, although, unlike Beer, they did not recognize that it applies
only to monochromatic radiation.
Absorption occurs in the atmosphere for both solar and terrestrial radiation. Beer’s law
describes the absorption of a monochromatic bean of radiation passing through a gas.
[Other conditions for the validity of Beer’s law are that T, p are constant along the path, and
that the radiant energy density is not “too high,” otherwise non-linear effects will occur.]
Beer’s law in differential form can be written in terms of the (spectral) absorptivity, da:
 dL
  a ds
(30.1)
L
where a is called the mass absorption coefficient (m3kg-1 m-1 ), that is, it is the
absorption cross-section per unit mass. One can think of the mass absorption coefficient in the
following manner. Suppose one were to remove one kilogram of the absorbing gas and replace
it with an ensemble of blackbodies with total cross-sectional area, normal to the beam, equal
da 
to a . Then the absorption would remain the same as if one had not removed the gas.
The shape of individual spectral lines may be approximated by the Lorentz line profile:
 a 
S
L
 (  0 ) 2   L 2
(30.2)

S    a d
is called the line strength.
0
The volume absorption coefficient is simply the product of the mass absorption coefficient and
the gas density:
 a   a
(m1 )
(30.3)
This can be interpreted in two ways. The first is analogous to the interpretation of the mass
absorption coefficient as given above. That is, it is the absorption cross-section per unit

volume. The second interpretation
is that it is the absorptivity per unit length along the path
of the beam.
The optical path, u, is defined to be the absorber mass per unit area along the path, that is:
1
du  ds  u   ds
(kgm2 )
(30.4)
0
The optical path is a density-weighted path length. For a vertical path, the optical path for
water vapour is simply the precipitable water.
OPTICAL THICKNESS
Optical thickness, a, is the product of the optical path and the mass absorption coefficient
(alternatively, the product of the path length and the volume absorption coefficient). It is
dimensionless.
1
d a   a ds   a    a ds
(30.5)
0
Note: optical thickness along a vertical path is known as the optical depth.
Beer’s law can be expressed particularly simply in terms of optical thickness, viz:
dL
  d a 
L
(30.6)
Eq. 30.6 can be integrated to give the integral version of Beer’s law:
L  L 0 exp(  a )
where L0 is the incident radiation.
The transmissivity of a gas over path length, l, is therefore;
(30.7)
ta 
L
 exp( a )
L 0
(30.8)
Eqs. 30.7 and 30.8 demonstrate clearly that the effect of absorption over a finite path is an
exponential attentuation of the beam. The absorptivity over a finite path is:

a  1  exp(  a )
Clearly, the absorptivity approaches unity with increasing optical thickness.
(30.9)
EXAMPLES
1)
THE VERTICAL PROFILE OF ABSORPTION (Chapman Profile; see Wallace and Hobbs)
Where does the maximum radiant heating occur in the atmosphere? There is no general answer to
this question since the absorption profile (and hence the heating profile) depends upon the total
optical thickness (depth) along the path, and also upon the profile of absorber concentration.
In order to simplify the problem and come up with an answer to the question, we will consider
here an isothermal atmosphere, in hydrostatic equilibrium, with a constant absorber mixing
ratio, r=/a, where  and a are, respectively, the absorber density and the air density.
We already know that pressure and air density vary exponentially with height in an isothermal
atmosphere. If the mixing ratio of the absorber is constant, then its density must also vary
exponentially with height, viz:
 z 

 H
   0 exp  
(30.10)
where H=RT/g is the scale height of the atmosphere.
Consider a downward beam passing through such an atmosphere. Since the heating rate is
determined by dL/dz, we use Beer’s law:
dL
 L a   L exp( a ) a 
dz
(30.11)

    dz
where a z a
. If we assume the mass absorption coefficient to be independent of
height, and  given by Eq. 30.10, then it is straightforward to integrate and show that:
 a  H a 
(30.12)
that is, the optical thickness increases in proportion to the density, as one moves down
through the atmosphere.
Solving Eq. 30.12 for a and substituting into Eq. 30.11 leads to:
dL L
 a

 a e
dz
H
(30.13)
Eq. 30.13 is known as the Chapman profile of absorption (or equivalently of heating rate).
The physical explanation for the maximum in heating rate in the middle of the atmosphere is
simple. Eq. 30.11 states that the absorption is proportional to the product of the absorber
density and the incident radiance. At high altitudes the radiance is high but the density low.
at low altitudes, the density is high but the radiance is low because of absorption higher up.
Hence the absorption must be low at both high and low altitudes in the atmosphere. The
maximum in absorption must therefore occur at some intermediate altitude. It is
straightforward to show by setting:
d  dL 

0
dz  dz 
(30.14)
that the maximum absorption occurs at an optical thickness of unity. It turns out that, even
when the assumptions of constant temperature and constant mass absorption coefficient are
relaxed, the maximum absorption still occurs around unit optical depth. Unit optical depth is
also known as the penetration depth, that is the depth at which the radiance is diminished by
a factor of 1/e.
Absorption of ultraviolet radiation by ozone can also be used to explain the temperature
maximum at the top of the stratosphere (at an altitude of about 50 km).