1. Atmospheric chemistry
Lecture 2:
Photochemistry & kinetics
Dr. David Glowacki
University of Bristol,UK

2. Quick review of yesterday
We discussed atmospheric structure
Temperature & pressure gradients, as well as Coriolis forces are related to atmospheric transport
Today…
We’ll gain some insight into the relationship between atmospheric structure and atmospheric chemistry
Atmospheric chemistry depends on sunlight, temperature, and pressure; Today we’ll learn about
Photochemistry
Chemical kinetics

3. The Atmosphere is a low temperature chemical reactor
Important Chemistry: UV
Stratosphere
O3 layer
UV absorption by O3
visible
Troposphere
Tropopause
-70oC
14 km
IR absorption by
Greenhouse gases
(H2O, CH4, CO2)
Surface emissions resulting in O3 and aerosol formation, and acid rain
Regional and global
biogenic emissions
(CH4)
Urban Anthropogenic emissions
Surface O3

4. Atmospheric Chemistry starts with sunlight
E = hv
v = c/l l
Energy/kJ mol-1
Red
700
170
Orange
620
190
Yellow
580
210
Green
530
230
Blue
470
250
Violet
420
280
Near UV
Far UV
200-50
visible
Breaking chemical bonds requires energy
Sunlight has energy
If sufficient energy is deposited in the bond, then it will break
O3 has a bond energy of ~105 kJ mol-1

5. Photoexcitation gives excited molecules, A*
Photoexcitation may result in a number of processes:
Initial photoexcitation
Dissociation
Fluorescence
Collisional relaxation
Ionization *
Photochemistry depends on temperature, pressure, and the wavelength of the absorbed light

6. Photoexcitation kinetics
The rate of formation of A* is written:
where jA is the photochemical rate constant
Competition between subsequent processes is determined by the quantum yield, ϕ, for each process where:
Dissociation yield =Φ1
Fluorescence yield =Φ2
Collisional relaxation yield =Φ3
Ionization =Φ4

7. Understanding the photolysis rate
Quantum yield: efficiency at which absorbed photons result in the molecular process of interest
absorption cross section: number of photons absorbed by a molecule at a particular wavelength
Spectral actinic flux: density of photons in the atmosphere at a particular wavelength
need to integrate over the entire wavelength range

8. Understanding photolysis rates
Atmospheric actinic flux
O3 absorption cross section
Photochemical processes depend on:
temperature (absorption cross sections & quantum yields)
Pressure (collisional relaxation)
Altitude (actinic flux)

9. Atmospheric absorption of light
Gases absorb light
The absorption of light depends on the concentration of the gas, N, its absorption cross section, σ, & the path length, l,through the gas
May be described by the Beer-Lambert law

10. Atmospheric absorption of light
The Beer Lambert law:
Explains the altititude dependence of actinic flux
Is often used to measure atmospheric trace gas concentrations
DOAS (differential optical absorption spectrometry)
FTIR spectrometry

11. Chemical Kinetics

12. Kinetics depends on the potential energy surface (PES)
What molecules do is determined by their potential energy landscapes – energy as a function of coordinates
Stable molecules are minima on a PES
Potential energy surfaces (PES) are multidimensional, but we usually think about their motion projected in one dimension
T dependence of reaction rate coefficients well described by the Arrhenius equation:

13. First order Unimolecular kinetics

14. Mechanisms with more than one chemical reactions: exact solutions
Coupled chemical reactions, often result in mechanisms of the sort:
For this system we can
write three rate equations,
one for each species:
A B k1
B C k2
In matrix form:

15. Chemical Mechanisms with Coupled Chemical Reactions: Coupled differential Equations
Analytic solutions exist for this eigenvalue problem to solve for concentration vs. time
If the initial concentration of every species but [A] is zero, the solutions are
Concentration vs time when k2/k1=0.5
B changes a lot;
Not low or constant
Concentration vs time when k2/k1=10
B doesn’t change much
Low and ~constant

16. Chemical Mechanisms with Coupled Chemical Reactions: Steady State Approximation
Consider again the following mechanism:
Steady state approximation: assume the
rate of change of intermediate B is zero
A B k1
B C k2
Approximate Steady state solution
Equivalent when k2 >> k1
making [B] low & ~constant
Exact solution

17. Chemical Lifetimes A B B C
Often we are interested in the average lifetime of a molecule before it reacts away
Lifetime has units of time
The interplay between chemical lifetimes and atmospheric mixing processes determines much of atmospheric chemistry
A B k1
B C k2

18. Collision Theory Molecules are constantly moving
Molecular gases are constantly colliding with each other with a T & P dependent collision frequency
Each collision has a particular amount of energy associated with it
This energy may lead to chemical reaction
Threshold
energy

19. Bimolecular Kinetics
Atmospheric chemistry involves both unimolecular and bimolecular processes
Bimolecular kinetics depend on pressure, [M]
A reasonable model for a bimolecular reaction is

20. Visualizing bimolecular pressure dependence:
O + O2 + M  O3 + M M O OO
M = O2 or N2 O3
O + O2 reaction coordinate

21. Bimolecular Kinetics: The Low & High pressure Limits
The total bimolecular process:
We want to know the rate
of AB formation
Write rate equations for AB*
Assume AB* is in steady state
Solve for AB* and plug into
the first equation

22. Bimolecular Kinetics: The Low & High pressure Limits
Low Pressure Limit
[M] is very small
k4 >> k5[M]
k5[M] goes to zero
Overall reaction rate depends linearly on [M]
High Pressure Limit
[M] is very large
k4 << k5[M]
k4 goes to zero
Overall reaction rate is independent of [M]
Instantaneous stabilization

23. T & P dependent kinetic effects
Laboratory measurements of rate coefficients give rise to T & P dependences which are well described by the kinetic master equation

24. Quick Summary Atmospheric chemistry dominated by photolysis
Molecular motion on a potential energy surface (PES) determines reactivity
In the atmosphere, simple reactions combine to form kinetic networks (i.e., coupled sets of important reactions)
The steady state approximation is a useful simplification for short lifetimes
Chemical reactions depend on both pressure & temperature, and are determined through a combination of experimental & theoretical approaches