Photochemical Effects of Radiation

Absorption of ultraviolet and visible photons by atmospheric molecules can induce transitions into electronically excited states, which may then photodissociate (see Chapter 2). The dissociation products play a crucial role in the photochemistry of the atmosphere. 

In atmospheric problems, it is necessary to estimate the quantitative value of the photodissociation rate of a molecule A. This is given by  

For a wavelength interval dA, the photodissociation coefficient of molecule A is proportional to the actinic flux qvdA, the absorption cross section ta(A), and the photodissociation quantum efficiency ca(A). The actinic flux q dA represents the number of photons per unit area and 

The spectral distribution of the absorption cross section and the quantum efficiency of atmospheric molecules is measured in the laboratory and can vary with temperature. In most cases, the quantum efficiency e is nearly unity but may be less, especially near the dissociation limit. This limit (maximum wavelength or minimum frequency) corresponds to the minimum energy required to dissociate the molecule (see, e.g., Eq. (2.18) in the case of O2 photolysis). 

In most cases, the spectral variation of the cross section is sufficiently gradual that one can integrate Equation (4.113) by simple methods. In a few cases (Schumann Runge bands of O2 and 5 bands of NO) the spectrum contains a multitude of narrow lines and more sophisticated methods are needed. A more detailed discussion occurs in Section 4.7.3. 

---

In order to study the photochemical effects of radiation, we define the actinic flux (r) as the amount of energy (W m 2Hz 1) penetrating a unit volume of space at point r with... 

---