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3…Vertical Structure of the Atmosphere

3…Vertical Structure of the Atmosphere

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R. S. Mason

Fig. 5.4 Vertical structure of the atmosphere at a glance; a layers of air, and temperature,

b chemical mixing, c radio wave reflection levels. Values given are very approximate, and

depend on location and season

photochemistry the steady state average temperature at any point z, is determined

by the fact that at each point the average kinetic energy (KE) of air molecules must

be balanced by the potential energy (PE), i.e. the energy provided by the force due

to gravity. If the KE was greater than this PE, the molecules would be able to

escape, and be lost from the atmosphere; if it were less they would be pulled closer

to the Earth’s surface, until they were in balance. Therefore since gravity (and

therefore PE) decreases, the air temperature also decreases with increasing z, an

effect which is known as adiabatic cooling. It has a value of between 7 and

10 K km-1, depending on the amount of water vapour present. Condensation is an

exothermic process, therefore as the air cools, condensing water vapour makes an

input into the energy balance, diminishing the degree of cooling.

As discussed above, absorption of radiation by air molecules and photodecomposition is also exothermic, and O2 is the most abundant of the photochemically active gases present in the atmosphere. At high z, Iz is high, but nz is low, and

therefore Rz is low (see Sect. 5.3). At low z, nz is high, but Iz is very low because of

its absorption already in the upper atmosphere, and again Rz is low. Therefore Rz

reaches a maximum at the intermediate altitudes, i.e. in the stratosphere where the

solar heating effect more than offsets the adiabatic cooling effect. The result is

that, whilst air temperature decreases with increasing z throughout the tropo- and

meso-spheres, it increases throughout the stratosphere. The regions of temperature

inversion at the boundaries are called the tropo-, strato- and meso-pauses.

5 Atmospheric Photochemistry


Much of the heat generated is manifested in the kinetic energy of the photodecomposition products which are expelled from repulsive excited state potential

energy surfaces (Fig. 5.3), following UV absorption. This excess kinetic energy is

rapidly dispersed between all particles of the gas mixture and equilibrated across

all energy modes (vibration and rotation) as a result of collision. However, above

about 85 km, the air is so thin that the collision frequency is too low for the excess

energy to equilibrate. The region therefore becomes dominated by the particles

with high kinetic energy but very low molecular temperature, the disparity

increasing with z. It is from this region, the thermosphere, that atoms and molecules (particularly H2) escape the Earth’s atmosphere. Since there is not thermal

equilibrium, the notion of ‘temperature’ in this region is not strictly appropriate.

Despite direct absorption of solar radiation, and scattering, it is clear from Fig.

5.1 that the air is transparent to most of the incoming radiation, which therefore

reaches and heats up the Earth’s surface. The air is therefore heated mainly by

contact with the surface which causes convection, thus leading to vertical mixing

of the air. The mechanism is that a package of air heated above ambient at the

surface expands and rises due to becoming less dense than surrounding air, setting

up a convection current. The air packet would keep rising until it reaches surrounding air which is of the same density; as it rises, it expands due to the fall in

pressure, so it cools, but it must lose energy to its surroundings faster than the rate

of change of adiabatic cooling in order for the gas to equilibrate with the surrounding air and hence become stationary. It therefore tends to keep rising, unless

it first hits a region of air which is already less dense than it ought to be if subject

only to adiabatic cooling. The convection package therefore tends to rise until it

hits the tropopause, where it meets air whose density is lower than demanded by

the gravitational change, because of the solar heating effect. The tropopause

therefore acts as a cap on the vertical movement of air from the troposphere to the

stratosphere, making the two air masses quite distinct. Convection currents

approaching the tropopause therefore change direction to the horizontal; this air

cools, causing it to sink; the high pressure air sinking to the Earth’s surface, turns

and rushes into the low pressure region at the base of the rising convection column

of air, thus setting up the gigantic vertical cyclic motions of air known as the

Hadley cycles. When combined with the horizontal motions caused by the rotation

of the Earth, this acts as a very efficient mixing mechanism (and of course the

phenomenon we know as weather).

In contrast, convection currents are not possible in the stratosphere, because of

the positive temperature gradient. Mixing there is caused by differential solar

heating of the stratospheric gas across the latitudes and is known as advection. Air

heated at the equator expands and moves towards the poles. Displaced air tends to

rise at the poles and comes back down to replace the sinking heated air at the

equator. An important point is that air on either side of the tropopause, moves

horizontally and parallel, which makes mixing across the boundary difficult. The

stratospheric air mass is therefore, partially isolated from that of the troposphere.

Thus, whereas effective mixing within the troposphere may take only a few days or

weeks, effective mixing across the boundary may take many months.


R. S. Mason

‘Fast’ reacting species (e.g. most pollutants) therefore never reach the stratosphere; only slow reacting species do so. An important example of the latter are of

course the chlorofluorocarbons (CFCs) which are immune to both chemistry and

photochemistry in the tropopause, and which therefore eventually congregate in

the upper atmosphere, where they become subjected to much higher energy

radiation, and photodecomposition, when their subsequent reaction can cause

significant damage to the ozone layer.

The air at the top of the stratosphere is similarly capped by the temperature

inversion of the stratopause, above which efficient vertical mixing again becomes

possible. The mesosphere, where air pressure is still high enough for convection

forces to operate, merges into the very low pressure region of the thermosphere,

where gas molecules move more or less independently (movement is dominated by

molecular transport in this region) and the kinetic ‘temperature’ resulting from

photodecomposition is high.

There is therefore an effective temperature inversion at the mesopause

(*80 km) which acts as a barrier to transport between the two layers of the mesoand thermo-spheres.

5.4 Photochemistry of the Thermosphere

Very short wavelength solar radiation, EUV: 80 B k B 103 nm and X-rays:

1–10 nm, causes photo-ionisation (see Table 5.3), but is rapidly attenuated;

restricting ionisation to z [ 60 km: a region which is therefore also known as the

ionosphere (but it encompasses both the thermosphere and part of the mesosphere,

see Fig. 5.4).

The ions dominating during the daytime are therefore N+, O+, N2+, O2+ and NO+.

The charge density (cations and electrons) reaches values in the region of [106

cm-3, which at this level has a mixing ratio of ca. 1 in 1011. This compares with

natural charge densities of *103 cm-3 at ground level (caused mainly by ionising

radiation emitted from radioactive materials), which is a mixing ratio of\1 in 1016.

For comparison, an electrical plasma, such as that produced in a low power electrical discharge lamp has charge densities in the region of 1011 cm-3.

The chemistry that ensues is the reaction between ions and molecules, and ion–

electron recombination, which is relatively fast for molecules, but slow for atomic


Table 5.3 Photoionisation reactions in the ionosphere; IE is the ionisation energy, AE is the

appearance energy for dissociative photoionisation, and the energies are also expressed in terms

of the equivalent photon wavelength k

hm ? O2

?O2+ ? eIE = 12.07 eV

k = 102 nm

?O+ ? O ? eAE = 18.78


?N2+ ? eIE = 15.58


hm ? N2

AE = 24.3


?N+ ? N ? eetc.

5 Atmospheric Photochemistry


Oỵ ỵ N2 ! N ỵ NOỵ


Oỵ ỵ O2 ! O ỵ Oỵ




2 ỵ O ! N ỵ NO



2 ỵ O2 ! N2 ỵ O2



2 ỵ e ! O ỵ O


NOỵ ỵ e ! N ỵ O


The *designation represents an excited state. There are, of course, many other

reactions involving the less abundant species.

The free electrons which also congregate in this region are responsible for

reflecting radio waves beamed from the Earth’s surface. The optimum value of

z for this occurrence depends on radio frequency, and a plot of reflection altitude

versus frequency shows peaks. The higher frequencies penetrate to higher altitudes; e.g. there is a sharp peak at *6.1 MHz, which penetrates up to about

750 km, and which early in the history of upper atmosphere research was used to

designate the region where z [ 750 km, as the F ‘layer’. Likewise ‘layers’ E and D

were also defined by lower frequency peaks. Although we now realise that they are

not specific layers of air, partially isolated by their mixing properties, as we

recognise today for the troposphere etc., the F, E and D regions are, however, still

referred to in the literature as convenient markers for regions of the upper atmosphere. The E region stretches up from about 120 km, and the lower boundary of

the D region is at the lower limit of the ionosphere at *60 km. It therefore bridges

the mesopause at 80 km (see Fig. 5.4).

In terms of ionic chemistry, O+ dominates the F region, which is converted via

reactions (5.8) and (5.9) to O2+ and NO+, and it is the latter molecular ions which

dominate the E region. In the lower part of the D region, i.e. the mesosphere,

which is the coldest part of the atmosphere, clustering results in the principal end

product of ion–molecule reactions in air, via a complex series of reactions, which

are clusters of water molecules bound to H+, i.e. H+(H2O)n where n goes up to

values [10. These molecular ions are eventually removed by ionelectron

recombination (5.17).


2 ỵ H2 O ! O2 ỵ H2 O


H2 Oỵ ỵ H2 O ! H3 Oỵ ỵ OH


H3 Oỵ ỵ H2 O ! H2 Oị2 Hỵ ỵxH2 O etcị ! H2 Oị2ỵx Hỵ

H2 Oị2ỵx Hỵ ỵ e ! 2 ỵ xịH2 O ỵ H



whereas chemical mixing in the tropo- and mesospheres is driven mainly by

turbulent convection processes, and in the stratosphere is driven by advection, the

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