Atmospheric Photolysis
Determining the rate at which a molecule is photolysed requires a knowledge of the wavelength dependence of the photolysing radiation, and a quantitative knowledge of the spectroscopy and photochemistry of the molecule of interest. The intensity of radiation is known as the Actinic Flux (I(z, λ)), and is wavelength and altitude dependent, as illustrated
The actinic flux is determined by the solar spectrum and the amount of radiation that has been absorbed by O2 (primarily below 200 nm) and O3 (between 220 and 290 nm). The spectrum of the molecule is described by the (wavelength-dependent) Absorption Cross Section (σ(λ)),. And we also take account of the photochemistry by including the Quantum Yield (φ(λ)), although this is often assumed to be unity. The Photolysis Rate Constant, J, is then given by
The magnitude of J is determined by the overlap between the actinic flux and the cross section (and quantum yield, when this varies with wavelength), as illustrated.
Ozone Predictions and Catalytic Cycles
By considering the interaction between solar radiation and O2 and other molecules, and taking into account the solar zenith angle and transport, it is possible to create a model of the atmosphere that describes the ozone layer. Models can predict the position of the ozone maximum quite well, but predicts ozone concentrations that are a factor of two too high, when nothing but oxygen-only chemistry is used.
Something is destroying ozone, but it was not immediately clear to scientists what the problem was. The problem is that there are many things that can destroy ozone, but they are not present in very high concentrations. For example, Cl atoms will react with ozone, but [O3]/[Cl] is typically 10 000. The problem is solved when catalytic cycles are considered. Cl atoms can destroy O3 and then be regenerated in cycles such as:
Cycle 1
Cl + O3 → ClO + O2 (3.6)
ClO + O → Cl + O2 (3.7)
Net O + O3 → O2 + O2 (3.3)
The net result of the cycle is reaction (3.3), the only reaction that destroys odd oxygen in the oxygen-only scheme. The impact of Cl atoms is to increase the rate of this process by reducing the activation barrier. Typically, one Cl atom can destroy ca. 10 000 O3 molecules. In this cycle, Cl could be replaced by NO, H, OH, Br or I.
Various other cycles can be written. At high altitudes
Cycle 2
O + HO2 → OH + O2 (3.8)
O + OH → H + O2 (3.9)
Net O + O + M → O2 + M (3.5)
At low altitudes,
Cycle 3
OH + O3 → HO2 + O2 (3.10)
O3 + HO2 → OH + 2O2 (3.9)
Net O3 + O3 → 3O2 (3.11)
Null Cycles, Holding Cycles and Reservoir Compounds
All atmospheric reactions occur in competition with other reactions. One cycle does not describe exactly what is happening to all of the Cl atoms in the atmosphere — if this were the case, all atmospheric ozone could be destroyed by one Cl atom.
Cycle 5 :Cycle 6
NO + O3 → NO2 + O2 (3.12) : NO + O3 → NO2 + O2 (3.12)
NO2 + O → NO + O2 (3.13) : NO2 + hν → NO + O (3.14)
O + O3 → O2 + O2 (3.3) : O3 + hν → O2 + O (3.4)
Cycle 5 is an ozone-destroying cycle of the type we have already seen, but Cycle 6 simply recycles odd oxygen. Cycle 6 is a Null Cycle. It is important because NOx cannot destroy O3 if it is tied up in a null cycle.
NOx species can also be involved in Holding Cycles. In cycles of this type, NOx is tied up in Reservoir Compounds.
Cycle 7
NO3 + NO2 + M N2O5 + M (3.15,-3.15)
N2O5 or dinitrogen pentoxide is an example of a reservoir compound.
Other Families (ClOx, HOx)
For the NOx species, null cycles, holding cycles and reservoir compounds exist without interaction with other species. This is not the case for the ClOx and HOx families. Null cycles can be written, that also involve NOx
Cycle 8 :Cycle 9
OH + O3 → HO2 + O2 (3.10) : Cl + O3 → ClO + O2 (3.6)
NO + HO2 → NO2 + OH (3.16) : NO + ClO → NO2 + Cl (3.17)
NO2 + hν → NO + O (3.14) : NO2 + hν → NO + O (3.14)
O3 + hν → O2 + O (3.4) : O3 + hν → O2 + O (3.4)
It turns out that the detailed kinetics of reactions (3.16) and (3.17) have a very marked impact on models of stratospheric ozone.
Holding cycles for HOx and ClOx can also be written
Cycle 10
OH + NO2 + M → HNO3 + M (3.18)
HNO3 + hν → OH + NO2 (3.19)
Typically 50 % of the stratospheric load of NOx exists as HNO3.
Cycle 11
Cl + CH4 → HCl + CH3 (3.20)
OH + HCl → H2O + Cl (3.21)
Typically 70 % of the stratospheric load of ClOx exists as HCl.
Other reservoir compounds are, HOCl, HO2NO2 and ClONO2.
Sources of Stratospheric Trace Constituents
NOx
Natural: N2O formed from incomplete microbiological nitrification (NH4+ → NO3-) or dentrification (NO3- → N2).
N2O + O(1D) → NO + NO
Polluted Direct injection of NO by SST (Concorde).
HOx
Natural: CH4 from enteric fermentation in ruminants (cattle)
CH4 + O(1D) → OH + CH3
Polluted: Direct injection of H2O by SST.
H2O + O(1D) → OH + OH
ClOx
Natural: Emission of CH3Cl from oceans, and burning vegetation.
CH3Cl + hν → CH3 + Cl → → ClO
CH3Cl + OH → H2O + CH2Cl → → ClO
Polluted: Emission of CFCs in the troposphere, followed by photolysis
(Similarly for BrOx and IOx.)
although the concentration of O3 varies through the stratosphere, the total ozone column abundance = 300 Dobson Units (which means at STP, the ozone layer would be 3 mm thick), and the molecular concentration at one atmosphere is about 2.5 × 1019 molecule cm-3.
Note that NOx refers to NO and NO2. NOy refers to NOx plus, N2O5, HNO3, HO2NO2, PAN (but not N2O).
