3.4. Photochemical reaction (Photolysis)

A photolysis reaction involves the breaking of a chemical bond in a molecule by an incident photon (Pilling and Seakin, 1995, Bozó, 2009, Mészáros, 1997). The reaction is written

(R3.6)

and the rate of this reaction is calculated as

,

(3.10)

where k is the photolysis rate coefficient.

Consider an elemental slab of air of vertical thickness dz and unit horizontal area. The slab contains [X]dz molecules of X (where [X] denotes the number density). A photon incident on a molecule of X has a probability of being absorbed, where A is the cross-sectional area of the molecule and is the absorption cross-section (units of cm2 molecule1) which defines the absorption characteristics of X. The molecules of X in the elemental slab absorb a fraction of the incoming photons. We define the actinic flux I as the number of photons crossing the unit horizontal area per unit time from any direction (photons cm−2 s−1) and the quantum yield qX (units of molecules photon) as the probability that absorption of a photon will cause photolysis of the molecule X. The number of molecules of X photolyzed per unit time in the slab is . To obtain the photolysis rate constant k, we divide by the number [X]dz of molecules of X in the slab:

.

(3.11)

Absorption cross-sections and quantum yields vary with wavelength. For polychromatic radiation, as in the atmosphere, equation (3.11) must be integrated over the wavelength spectrum:

.

(3.12)

where is the actinic flux distribution function.