Reading:
Reactions of sunlight with atmospheric constituents - absorption, excitation, dissociation, ionization. These reactions are in the direction of breaking molecules and atoms up. In equilibrium, there are as many reactions combining the pieces back together - recombination. These types of reactions are ubiquitous - they occur in all planetary atmospheres. The reactions of oxygen and carbon dioxide are particularly important for Earth, Venus and Mars.
What causes the oxygen species - O3, O2, O and O+ - to have their distributions with altitude?
Not all photons are equal - the shorter the wavelength, the higher the energy - the greater the "damage". (Fig 2-1 of G&W)
Not all photons reach the ground
This reaction requires photons in the wavelength range of 1000-2000 A - which only penetrate to about 100 km. It is most efficient at about 1450 A.
O2 + photon -> O + O
The rate of dissociation depends on the number of O2 molecules, the flux of photons and the efficiency of dissociation. The efficiency just depends on quantum physics - which does not vary with altitude in the atmosphere. But the numbers of photons and molecules are highly altitude dependent.
Rate = Constant x n(O2) x F(1450 A)
Rate (z) = Constant x n(z) x F(z)
Units? Rate of reactions is in photodissociations per volume per second = photodissociations cm-3 sec-1
This comes from a product of a constant times n(O2) in cm-3 and Flux(1450) in cm-2 sec-1
So, what is the "Constant"? The units are [cm-3 sec-1] / [cm-3 cm-2 sec-1 ] = cm2 = area. The constant is an effective cross-sectional area of the molecule for dissociation by the photons.
Here are log plots -( this is for rate of photon absorption - but the principle is exactly the same for rate of O2 dissociation)
log a + log b = log c - constant
Thus, the product of a function that increases with altitude and a function that decreases with altitude produces a peak in the middle somewhere.
This is the production or source process for O - what is the loss process?
O + O -> O2
BUT - this is not so easy - if the O atoms have just a little too much energy when they collide, the molecule does not stay together but breaks up again. If there is a third party present - Mystery Molecule M - then Mr M can take away the energy - * means M has electrons in an excited state (hence, could radiate emission) or is moving faster.
O + O + M -> O2 + M*
This means you need a 3-body collision - isn't that really rare? If M is a common molecule - e.g. N2 - then no, it is not so rare.
Rate of recombination is proportional to the density of O squared times the density of M - if M is and abundant species - e.g. N2 which decreases exponentially with altitude - then the rate of recombination will be much stronger lower down - removing O at lower altitudes.
Rate = Constant x n(O)2 x n(M)
Fig 2-5
SO.... we get LAYERS of minor species - e.g. O atoms - due to two effects
(1) a source function that peaks in the middle of the atmosphere
(2) a loss function that is greater lower down - more recombination at higher density in lower atmosphere
If there were just photochemical equilibrium to worry about atmospheres would just be complicated. Add vertical transport - mixing and diffusion - and they get messy.
Mixing = homogeneous = uniform stiring
Diffusion = heterogeneous = seperation of species according to their mass
Low pressure = low density = long mean free path
High pressure = high density = short mean free path
So, in high density regions the atmosphere is well mixed (lots of collisions). In the low density regions the gases can separate out with different scaleheights. Since H = kT/mg - the gas species with low mass, m, will have large H and be more abundant at higher altitudes (i.e. not decrease in density with altitude as fast as the heavier species).
O + photon -> O+ + e-
CO2 + photon -> CO2++ e-
These reactions produce the ionospheres of Earth, Venus and Mars. But to balance these processes producing ions and electrons, there must be processes that remove them - by putting them back together again - recombination.
But recombination of ions and electrons runs into the same problem of carrying away the energy....
O+ + e- -> O
is a rare reaction - there is a tendency for the O to re-ionize - see Fig 2-5 above. But molecular recombination is much easier - about a million times easier - and usually leads to dissociation soon afterwards....
CO2++ e- -> CO2 -> CO + O
Again, the rate of this reaction will depend on the density of CO2+ times the density of electrons - and so will happen more frequently lower down in the ionosphere that at the top.
Again - you get a LAYER - a Chapman Layer - in this case of the ionized species CO2+ and e-. Below is the case for Venus:
OK - so these are pretty simple - one reaction, couple of species at a time. In reality, the atmosphere involves many species and many reactions.
Venus - first the neutrals
then the reaction rates for ions for low solar activity on the left - high solar activity on the right.
and finally the ion distributions... also for low solar activity on the left - high solar activity on the right.
