Actinic Flux and Photolysis Rate Constants

Photolysis rate constants of atmospheric molecules can be calculated by the formula given in Chap. 2, 

Meanwhile, the solar irradiance E X) (photons cm s nm ) mentioned in Sect. 3.1 is defined by 

Peterson (1976) developed the work of Leighton (1961) using new input parameters, and was further succeeded by Demerjian (1980) and Madronich (1987) giving the values of actinic flux with higher precision. Peterson (1976) calculated the 

Wavelength (nm) Power Actinic Flux Solar Zenith Angle (°)  

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There are several approaches to measuring actinic fluxes and photolysis rate constants. One approach is to measure the rate of decay of a species such as N02 directly, so-called chemical actinometry (e.g. see Madronich et al., 1983). Another approach is to measure the light intensity and convert this to an actinic flux. 

RELATIONSHIPS BETWEEN RADIANCE, IRRADIANCE, ACTINIC FLUX, AND PHOTOLYSIS RATE CONSTANTS... 

If a complete analysis taking into account the uncertainties in all the rate constants and initial concentrations cannot be made, perhaps a result of the complexity of the mechanism or lack of information on uncertainties for some rate constants, a partial analysis can still be useful. For example, an analysis taking into account only the uncertainties in photolysis rates will show whether discrepancies between observations and predictions can be explained by uncertainties in actinic fluxes and photolysis rate constants. 

Once the actinic fluxes, quantum yields, and absorption cross sections have been summarized as in Table 3.19, the individual products < .,v(A)wavelength interval can be calculated and summed to give kp. Note that the individual reaction channels (9a) and (9b) are calculated separately and then added to get the total photolysis rate constant for the photolysis of acetaldehyde. However, the rate constants for the individual channels are also useful in that (9a) produces free radicals that will participate directly in the NO to N02 conversion and hence in the formation of 03, etc., while (9b) produces relatively unreactive stable products. 

The absolute values of the absorption cross sections of HCHO have been somewhat controversial. This appears to be due to a lack of sufficient resolution in some studies as discussed in Chapter 3.B.2, if the spectral resolution is too low relative to the bandwidth, nonlinear Beer-Lambert plots result. The strongly banded structure means that calculations of the photolysis rate constant require actinic flux data that have much finer resolution than the 2- to 5-nm intervals for which these flux data are given in Chapter 3 or, alternatively, that the measured absorption cross sections must be appropriately averaged. One significant advantage of the highly structured absorption of HCHO is that it can be used to measure low concentrations of this important aldehyde in the atmosphere by UV absorption (see Sections A.ld and A.4f in Chapter 11.). 

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... 

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