大气圈地球化学及其环境效益

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

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

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

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

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

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 10-10 cm3 molecules s-1. Mean relative collision velocity

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

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)

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

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)

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:

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:

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∞.

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.

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

Photolysis reactions

Photolysis reactions

Photolysis reactions

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

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

Photolysis reactions Calculation of photolysis rates

Photolysis reactions Calculation of photolysis rates

Photolysis reactions Important atmospheric species that undergo photolysis