SOME GEOLOGICALLY IMPORTANT ELEMENTS - Chemical Fundamentals of Geology and Environmental Geoscience - page 165

Box 9.7 Molecular spectra: greenhouse gases

We saw in Chapter 6 that atoms emit and absorb light at
characteristic wavelengths as a result of electrons jumping
between bound (i.e. quantized) energy levels within the
atom. Molecules too can absorb and emit their own charac-
teristic electromagnetic spectra, although the mechanisms
and wavelength range involved are entirely different.
at room temperature a gas molecule like CO 2 is not a
rigid body but is constantly stretching and flexing. the pre-
cise molecular geometries implied by Figures 7.4 and 7.5
merely indicate the mean positions of atoms, about which
they constantly oscillate in a variety of ways (and at var-
ious frequencies ω 1 , ω 2 , ω 3 …) as illustrated in Figure 9.7.1.
Wave-mechanical analysis of such internal molecular vibr-
ations shows they too are quantized, giving rise to a series
of discrete vibrational energy levels . a molecule can switch
between modes of vibration (energy levels) by absorbing or
emitting a photon whose energy is equivalent to the differ-
ence in energy Δ E between the molecule's initial and final
vibrational states.
a pure gas - whether diatomic like O 2 or triatomic like h 2 O
or CO 2 - will therefore emit or absorb radiant electromagnetic
energy at a series of discrete wavelengths (or photon ener-
gies) characteristic of that molecule. a gas mixture like air will
combine the spectra of all of the constituent gases. the
photon energies involved are, however, much lower than
those associated with electronic transitions in atoms, and

correspond to wavelengths in the infrared (Ir) range. Molecular
rotation is likewise quantized, leading to further characteristic
absorption and emission lines at Ir wavelengths.
the earth's climate results from a balance between
(a) incoming radiant energy received from the Sun and (b)
thermal energy radiated into space by the earth and its
atmosphere. the earth's mean surface temperature of
288 K = 15 °C confines it to radiating energy in the infrared
(Ir) wavelength range. as outgoing radiant energy passes
through the atmosphere, some wavelengths are absorbed by
the vibrational and rotational transitions in atmospheric
gases, 6 inhibiting the earth's ability to radiate heat into space
and upsetting the balance between incoming (solar) and out-
going energy fluxes. this atmospheric spectral 'blanket' - the
so-called 'greenhouse effect' - significantly warms the earth's
climate, and the most strongly absorbing species like CO 2 ,
N 2 O, h 2 O and Ch 4 are thus known as 'greenhouse gases'.
Were it not for the greenhouse gases naturally present in
the atmosphere, the earth's mean surface temperature
would be colder, about 255 K = −18 °C. Life on earth thus
benefits from a natural greenhouse effect of around +33 °C.
Current concern about climate change relates to the add-
itional anthropogenic (or 'enhanced') greenhouse effect
caused by mankind's accumulated emissions into the
atmosphere of CO 2 (from fossil fuel burning and the destruc-
tion of tropical rainforests) as well as of N 2 O, Ch 4 and CFCs.
Greenhouse gases vary considerably in their atmospheric
lifetimes and their impact on global warming (table 9.7.1).

ω 1

Table 9.7.1 Global warming potentials of greenhouse
gases. h 2 O is not shown in the table because - although
it is the most abundant greenhouse gas - its atmospheric
concentration varies on a short timescale, is poorly
mixed, and is little affected by human activity

ω 2

Name Formula Atmospheric

lifetime/years

Global warming
potential *

ω 3

Over 20
years

Over 100
years

Figure 9.7.1 Symmetric and asymmetric modes of
vibration of a linear triatomic molecule (e.g. CO 2 ). the
darker atoms represent mean atomic positions while the
fainter ones indicate one direction of oscillation. the
three fundamental frequencies of vibration ω 1 , ω 2 and ω 3
are shown. an animation of the modes of vibration of a
non-linear triatomic molecule such as h 2 O can be seen at
www1.lsbu.ac.uk/water/vibrat.html.

Carbon dioxide CO 2 Centuries § 1 1

Methane Ch 4 12 84 28

Nitrous oxide N 2 O 121 264 265

CFC-12 CCl 2 F 2 100 10,800 10,200

hCFC-22 ChClF 2 12 5280 1760

* GWp is a dimensionless measure of how much heat a given
mass of a greenhouse gas traps in the atmosphere relative to
the same mass of carbon dioxide . Values from IpCC (2013)
tables 8.7 and 8.a.1.
§ Defining the atmospheric lifetime of CO 2 is a complex question.
See archer et al. (2009).

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the energy absorbed either raises atmospheric temperature
or is radiated in other directions (including downwards).