1. 大气圈地球化学及其环境效益

2. 6.2 Chemical Kinetics
中国科学技术大学 环境地球化学概论

3. Thermal and Photolysis Reactions
Atmosphere reactions: 2 types
1. Thermal reactions: in which the collision of molecules or the interval vibrations of molecules causes a reaction. There reactions are
Decomposition reactions
Combination reactions
Disproportion reactions

4. Thermal Reactions Rates
Bimolecular reactions
A + B  C + D
The reaction rate is expressed as
The concentrations are expressed as number densities
so that the product [A][B] is proportional to the frequency of collisions.
The temperature dependence for the reaction rate constant k,
A: the A-factor or the pre-exponential factor
Ea: activation energy or the threshold energy for reaction
R: the gas constant
T: the gas temperature

5. Molecular cross-section
Thermal Reactions Rates
Bimolecular reactions
A is
related to
Molecular cross-section
Steric factor
Mean
relative collision
velocity

6. Molecular cross-section
Thermal Reactions Rates
Bimolecular reactions
The maximum possible value of the rate constant of a bimolecular reaction is achieved if every molecular collision between A and B results in reaction. This is called the gas-kinetic collision rate.
Molecular cross-section

7. Thermal Reactions Rates
Bimolecular reactions
The corresponding value of the second order rate constant k at 298K for molecules of interests in atmospheric chemistry is in the range of cm3 molecules s-1.
Mean
relative collision
velocity

8. Thermal Reactions Rates
Bimolecular reactions
Most reactions have rate constants less than this;
The activation energy Ea must be overcome for the reaction to proceed.
Molecules that are geometrically complex may have to be aligned properly at the point of collision for reaction to take place and perfect alignment is not achieved in every collision.
Steric factor

9. Thermal Reactions Rates
Three-body reactions
A + B + M  AB + M
A three-body reaction involves reaction of two species A and B to form one single product AB.
This third requires a third body M to stabilize the excited product AB* by collision.
The third body M is any inert molecule that can remove the excess energy from AB* and eventually dissipate it as heat. (N2, O2 in the atmosphere)

10. Thermal Reactions Rates
Three-body reactions
A + B + M  AB + M
Example 1:
O +O +M  O2 +M
The elementary steps of a third body reaction are:
A + B  AB* (R1) AB* + M  AB + M* (R3)
AB*  A + B (R2) M*  M + heat (R4)
Example 2: OH + NO2 + M  HNO3 + M
Example 3: O + O2 + M  O3 + M

11. Thermal Reactions Rates
Three-body reactions
A + B + M  AB + M
The rate of a three-body reaction is defined as the formation rate of AB:
In the atmosphere, [M] is simply the number density of air.
Preudo-study-state approximation (PSSA)

12. Thermal Reactions Rates
Three-body reactions
Preudo-study-state approximation (PSSA)
When an intermediate (e.g., AB*) has a very short lifetime and reacts as soon as it is produced, the rate od generation of AB* is equal to the rate of disappearance.
PSSA is fundamental way to deal with such reactive intermediates when deriving the overall rate of a chemical reaction mechanism.
PSSA gives:

13. Thermal Reactions Rates
Three-body reactions
Preudo-study-state approximation (PSSA)
When an intermediate (e.g., AB*) has a very short lifetime and reacts as soon as it is produced, the rate od generation of AB* is equal to the rate of disappearance.
PSSA is fundamental way to deal with such reactive intermediates when deriving the overall rate of a chemical reaction mechanism.
PSSA gives:

14. Thermal Reactions Rates
Three-body reactions
The formation rate of AB depends on the concentration of M, i.e.,
Pressure-dependet:
Low-pressure limit case: R ∝ [M]
High-pressure limit case:
R independent of [M]
k0 = k3k5/k4 is referred as the low-pressure limit rate constant.
k3 is referred as the high-pressure limit rate constant k∞.

15. Photolysis reactions Photolysis reactions:
Sunlight drives the chemistry of the atmosphere.
These reactions that involve the breaking of a chemical bond by an incident photon.

16. Photolysis reactions
Figure 1. Some of the photolysis reactions that occur at various altitudes in the atmosphere
(Source: Atkins, Physical Cheistry, pp820)

17. Photolysis reactions

18. Photolysis reactions

19. Photolysis reactions

20. Photolysis reactions Actinic flux
The actinic flux at the earth’s surface is affected by the extent of light absorption and scattering in the atmosphere, the zenith angle, the extent of surface reflection, and the presence of clouds.
Definition of solar zenith angle θ at a point on the earth surface

21. Photolysis reactions Actinic flux
Estimation of the actinic flux: use a radiative transfer model.
Figure 3. Calculated actinic flux

22. Photolysis reactions
Calculation of photolysis rates

23. Photolysis reactions
Calculation of photolysis rates

24. Photolysis reactions
Important atmospheric species that undergo photolysis