1. ATS 621 Fall 2012
Lecture 9

2. Next topic: Photochemistry (Ch 3 notes)

3. Can these energies break chemical bonds?
weak O2-O bond in ozone (~ 100 kJ/mol)
moderately strong C-H bond in formaldehyde (~368 kJ/mol)
Corresponds to visible and shorter l
But the shortest l available in troposphere depends on screening by atmosphere

4. EXAMPLE: PHOTODISSOCIATION OF OXYGEN
Estimate the wavelength of light at which photodissociation of O2 into
2 ground-state oxygen atoms:
O2 + hv  O + O
The enthalpy for this reaction is H=498.4 kJ/mol (endothermic)
Using:
So O2 cannot photodissociate at wavelengths longer than about 240 nm

5. What l and energy reach the surface?

6. Vertical distribution of absorption

7. LIGHT: REFLECTING, SCATTERING AND ABSORBING
GASES:
GHGs are absorbing in the IR(as seen for example for CO2)
Gases can scatter in the UV/visible  Rayleigh scattering
AEROSOLS:
Absorption depends on composition (eg. black carbon)
Scattering explained by Mie Theory  reduction in visibility

8. Intensity of light as a function of height
As radiation passes through the atmosphere, it is attenuated by scattering and absorption by gases and particles. This attenuation can be expressed using the Beer-Lambert Law:

9. MAJOR PHOTODISSOCIATING SPECIES
A photon (hv) is a reactant.
Reminder: λ> 290 nm only in the troposphere!
NO2 + hv  NO + O < 420 nm
O3 + hv  O(3P) + O < < 1200 nm
O(1D) + O2 < 315 nm
HNO2 + hv  NO + OH < 400 nm
H2O2 + hv  2OH
NO3 + hv  NO + O2 NO3 “stores” NOx at night
NO2 + O
HCHO + hv  HCO + H
CO + H2 dominant path for > 320 nm

10. Number of photons absorbed by species X is:
The rate of a photolytic chemical reaction depends upon the number of photons (of a specified wavelength) that are absorbed by the species reacting (and on the outcome of that energy absorption)
ACTINIC RADIATION: the integrated radiation (photon flux) from all directions to a sphere (sum of direct, scattered, reflected light)
ACTINIC FLUX is J(l)
Number of photons absorbed by species X is:
()=absorption cross section
[cm2/molecules]
J()=actinic flux
[photons/cm2/s]
Fa()=number of photons abs by X
[photons/cm3/s]
[X] is in units of number density

11. Variations in actinic flux
Actinic flux depends on altitude, and will be modified by SZA (thus time, season, latitude), as well as by scattering and absorption by gases and particles. The absorption is mostly from stratospheric O3 (the ozone column).
Must also consider not only direct solar, but also scattered/ reflected radiation  need surface albedo estimates
Computational codes developed to calculate actinic flux for different constituent profiles, angles, locations

13. COMPUTING RATES OF PHOTOLYSIS
Molecule is excited into an electronically excited state by absorption of a photon:
A + hv  A*
The excited molecule may release the absorbed energy by any of:
1. Dissociation A*  B1 + B2
2. Direct Reaction A* + B  C1 + C2
3. Fluorescence A*  A + hv
4. Collisional deactivation A* + M  A + M
5. Ionization A*  A+ + e-
The relative efficiency of each of these is described by the quantum yield (i): number of excited molecules of A* undergoing a process (i) to the total number of photons absorbed. By Stark-Einstein Law, I = 1
You may come across the “overall quantum yield of a stable product A” (A) which is defined as the number of molecules of A formed over the number of photons absorbed. A > 1 for a chain reaction

14. COMPUTING RATES OF PHOTOLYSIS (CONT’D)
Example: Two photochemical processes for formaldehyde to produce (H+HCO) or(H2+CO) thus, ()= H+HCHO + H2+CO To compute the rate of disappearance of HCHO according to 1st rxn use only H+HCHO

15. COMPUTING RATES OF PHOTOLYSIS (CONT’D)
The total rate of photolysis of X:
Rate [molecules/cm3/s]
j = photolysis rate constant [s-1]
Note, the use of j distinguishes the photolysis rate constant from other rate constants (k).

18. GENERATION OF OH
OH radical drives the daytime chemistry of both polluted and clean atmosphere
What is the source of OH (and other radicals)?
Major source:
O3 + hv ( 320 nm)  O(1D) + O2
O(1D) + H2O  2OH
Other sources:
HONO + hv (< 400 nm)  OH + NO
nitrous acid (photodissociation)
H2O2 + hv (< 370 nm)  2OH
hydrogen peroxide
HO2 + NO  OH +NO2
(sources and sinks of HO2 effectively sources and sinks of OH  HOx)

19. BASIC PHOTOCHEMICAL CYCLE OF NO2, NO AND O3
NOx is released in combustion processes, also saw that there are natural sources, such as lightning. The following is a “fast” photochemical cycle with no net consumption or production of species:
NO2 + hv  NO + O (1)
O + O2 + M  O3 + M (2)
O3 + NO  NO2 + O2 (3) O3 hv
NO2 NO O3
What is steady state [O3]?
SS for O: R2=R1  [O] depends on [NO2]
SS for NO2: R1=R3
SS for NO: R3=R1
SS for O3: R3=R2  sub R1 for R2 from above:
This is the photostationary state relation. The steady state concentration of ozone is controlled by the ratio of NO2 to NO here.
However, 3 rxn cycle is incomplete for predicting ozone concentrations  don’t forget carbon compounds!

20. Stratospheric Chemistry

21. THE MANY FACES OF ATMOSPHERIC OZONE
In stratosphere: UV shield
Stratosphere:
90% of total
In middle/upper troposphere: greenhouse gas
Troposphere
In lower/middle troposphere: precursor of OH,
main atmospheric oxidant
In surface air: toxic to humans and vegetation

22. The photochemical mechanisms that give rise to the ozone layer were discovered by the British physicist Sidney Chapman in 1930.

23. CHAPMAN MECHANISM FOR STRATOSPHERIC OZONE (1930)
Odd oxygen family [Ox] = [O3] + [O]
slow R2 R1 O2
O O3
fast R3 R4
slow

24. O3 + X  XO + O2 O + XO  X + O2 Net: O3 + O  2 O2 X is a catalyst
Chapman got it almost right… CATALYTIC CYCLES FOR OZONE LOSS: General Idea
O3 + X  XO + O2
O + XO  X + O2
Net: O3 + O  2 O2
X is a catalyst
The catalyst is neither created nor destroyed…but the rate for the catalytic cycle [odd-O removal in this case] depends on catalyst concentrations

25. Extras

26. STRATOSPHERIC OZONE HAS BEEN MEASURED FROM SPACE SINCE 1979
Method: UV solar backscatter
Last Saturdays’s ozone layer…
Notice the Antarctic ozone hole l1 l2
Ozone layer
Scattering by
Earth surface
and atmosphere
Ozone
absorption
spectrum l1 l2