Solar radiation actinic

FIGURE 1 Photodissociation of ozone in the near ultraviolet spectral region. Overlap of solar radiation actinic intensity, ozone absorption cross section, and 0( D) quantum yield to derive the 0( D) production frequency as a function of wavelength. [From Zellner, R., ed. (1999). Global Aspects of Atmospheric Chemistry, Steinkopff/Springer, Darmstadt, Germany.]... 

The prime driver of the chemical system of the earth s atmosphere is photochemical reactions caused by solar radiation. The atmosphere of the earth is divided into levels called the troposphere, stratosphere, mesosphere and thermosphere from nearest the ground to farthest according to the characteristics of the temperature gradient as shown in Fig. 3.1. The cause of the temperature inversion in the stratosphere, which characterizes the earth s atmosphere, is the formation of an ozone layer by the photolysis of oxygen, one of the major components of the atmosphere. In this chapter, the spectrum of solar radiation, actinic flux, and so on, that is necessary to calculate the photolysis rate of atmospheric molecules are explained. 

Chromophores (Ch) are transformed after absorption of the actinic solar radiation in excited singlet ( Ch ) and triplet (3Ch ) states (Rabek, 1996) (Eq. 3-11). Excited chromophores sensitize the formation of macroalkyls from the matrix polymer (Eq. 3-12a) and singlet oxygen from the ground state oxygen (Eq. 3-12b) and accelerate homolysis of POOH via an exciplex (Eq. 3-12c), Reaction scheme 3-3. 

Although the solar spectrum is very broad, extending from cosmic rays to radio waves (see Figure 2.1), the photochemical reactions are driven only by visible and ultraviolet (UV) radiation (actinic light - Figure 9.2). 

Herman and Mentall, 1982a Kylling et al., 1993 Minschwaner et al., 1995) as scattered radiation in many cases dominates the actinic flux and has an important effect on the photolysis of key chemical species (e.g., O3, NO2, HNO3). The measurement of scattered and absorbed solar radiation can provide information on the abundance of several chemical compounds (e.g., ozone, aerosols in the atmosphere) (see Box 4.2). 

In the calculation for atmospheric photodissociation reactions, how to calculate the effective solar intensity is a major issue, because not only direct irradiation from the sun, but light from all directions reflected and scattered by the ground surface, clouds, atmospheric molecules, and aerosols can contribute to photolysis. Furthermore, in the troposphere for example, only solar radiation that has not been absorbed by atmospheric molecules in the higher atmosphere, the stratosphere and above, can cause photolytic reactions. The spherically integrated solar intensity after considering those many atmospheric processes is called the actinic flux F (X) (photons cm s ), which means solar irradiation valid for photochemical effect. In atmospheric chemistry, jp is often used instead of kp for representing photolysis rate constant. Photodissociation rate constant in the atmosphere can be expressed using these parameters as... 

In the case of scattering by molecules and particles, being different fi om the absorption, solar radiation is not lost but the scattered light is also utilized effectively for the photolysis of atmospheric molecules. Thus, although the direct radiation intensity is attenuated, multiple scattered lights contribute to actinic flux (Sect. 3.5). 

The atmospheric lifetime of methyl nitrite with respect to reaction with OH radicals is estimated to be around 1 month. Methyl nitrite is dissociated quite rapidly by solar radiation, as it absorbs strongly in the actinic region the lifetime for photodissociation with an overhead Sun is about 8 1 min. (see table IX-M-1). Because of this rapid photolysis, other reactions such as with OH radicals are considered to be of minor importance as loss processes for this species from the atmosphere. 

Sources of H02 include the reactions of 02 with hydrogen atoms and formyl radicals, both of which are produced, for example, by the photodissociation of gaseous formaldehyde following absorption of solar actinic UV radiation. 

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