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Source: Demerjian et al. (1980).

Source: Demerjian et al. (1980).

those of Peterson (1976) and Demerjian et al. (1980), since they are relative values, they are not expected to differ significantly from those that would be derived with the Madronich (1998) actinic flux values.)

e. Effect of Height above Earth's Surface on F(K)

Figure 3.24 shows the relative changes in the total actinic flux as a function of altitude from 0 to 15 km at solar zenith angles of 20, 50, and 78° and at wavelengths of 332.5 (part a), 412.5 (part b), and 575 nm (part c), respectively. Again, since these are relative changes, these results of Peterson (1976) and Demerjian et al. (1980) are not expected to be significantly different from those that would be obtained with the Madronich (1998) actinic flux estimates.

At the largest solar zenith angle shown, 78°, all of the curves show a decrease in the actinic flux from 15 km to lower altitudes. This occurs because at these large values of 6 and hence long path lengths through the atmosphere, backscattering of the light increases as it passes through the atmosphere.

The calculated actinic flux typically increases significantly in the first few kilometers. This is partly due to scattering of light by particulate matter and to light absorption by tropospheric 03 close to the surface. The effect of 03 can be seen by comparing the total fluxes at 332.5 nm (Fig. 3.24a), where 03 absorbs, to those at 575 nm (Fig. 3.24b), where it does not.

Peterson and co-workers have examined the percentage increase in total actinic flux, going from the surface to ~ 1 km; they estimate that at short wavelengths (A < 310 nm), the increase is > 37.5% for all zenith angles. This increase in flux with altitude at short wavelengths could be particularly significant in photochemical smog formation. Thus pollutants trapped in an inversion layer aloft may be exposed to higher actinic fluxes than at ground level and photolyze more rapidly, hastening the formation of various secondary pollutants. The increased actinic flux with altitude close to the earth's surface is the basis for their suggestion that the presence of increased 03 in, or close to, the inversion layer may be at least partially the result of the height dependence of F( A).

These predictions have been borne out experimentally in studies in which the rate of photolysis of N02 was measured from the surface to ~ 7.6-km altitude and found to increase with height by more than 50% (Kelley et al., 1995; Volz-Thomas et al., 1996).

f Sensitivity of Calculated Actinic Fluxes to Input Values for Surface Albedo and Ozone and Particle Concentrations

As discussed earlier, the net actinic flux incident on a volume of air is sensitive to a number of parameters,

TABLE 3.9 Tabulation of Solar Zenith Angles (deg) as a Function of True Solar Time and Month

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