Actinic flux estimates

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. 

The actinic flux F( A), describing the intensity of light available to the molecule for absorption, depends on many factors, including geographical location, time, season, presence or absence of clouds, and the total amount of 03 and particles in the air which scatter light as it passes through the atmosphere. At the earth s surface, however, the actinic flux estimates and associated data of Madronich (1998) in Table 3.7 are commonly used to estimate rates and lifetimes of species with respect to photolysis under cloudless conditions. 

To estimate the solar flux available for photochemistry in the troposphere then, one needs to know not only the flux outside the atmosphere but also the extent of light absorption and scattering within the atmosphere. We discuss here the actinic flux F(A) at the earth s surface the effects of elevation and of height above the surface are discussed in Sections C.2.d and C.2.e. 

There are a number of estimates of the actinic flux at various wavelengths and solar zenith angles in the literature (e.g., see references in Madronich, 1987, 1993). Clearly, these all involve certain assumptions about the amounts and distribution of 03 and the concentration and nature (e.g., size distribution and composition) of particles which determine their light scattering and absorption properties. Historically, one of the most widely used data sets for actinic fluxes at the earth s surface is that of Peterson (1976), who recalculated these solar fluxes from 290 to 700 nm using a radiative transfer model developed by Dave (1972). Demerjian et al. (1980) then applied them to the photolysis of some important atmospheric species. In this model, molecular scattering, absorption due to 03, H20, 02, and C02, and scattering and absorption by particles are taken into account. 

TABLE 3.7 Actinic Flux Values F( A) at the Earth s Surface as a Function of Wavelength Interval and Solar Zenith Angle within Specific Wavelength Intervals for Best Estimate Surface Albedo Calculated by Madronich (1998) ... 

FIGURE 3.21 Calculated actinic flux centered on the indicated wavelengths at the earth s surface using best estimate albedos as a function of solar zenith angle (from Madronich, 1998). 

TABLE 3.10 Percentage Increase in the Calculated Actinic Flux at a Surface Elevation of 1.5 km Using Best Estimate Albedos as a Function of Solar Zenith Angle and Selected Wavelengths"... 

Use the data in Tables 3.7 and 3.11 to calculate the ratio of the actinic flux at the earth s surface for an 80% surface albedo compared to the best estimate albedo at solar zenith angles of 0 and 78° for the following wavelength regions 298-300, 318-320, and 400-405 nm. Comment on the expected effects on photochemistry in the boundary layer. 

Absorption of sunlight induces photochemistry and generates a variety of free radicals that drive the chemistry of the troposphere as well as the stratosphere. This chapter focuses on the absorption spectra and photochemistry of important atmospheric species. These data can be used in conjunction with the actinic fluxes described in the preceding chapter to estimate rates of photolysis of various molecules as well as the rate of generation of photolysis products, including free radicals, from these photochemical processes. 

Sasha Madronich generously not only reviewed the section on atmospheric radiation, but provided his unpublished calculations of actinic fluxes at different altitudes in a form useful to the atmospheric chemistry community for estimates of photolysis rates from the troposphere through the stratosphere. A number of colleagues reviewed chapters or portions of chapters, and their insightful comments and suggestions are... 

Demerjian, K.L., K.L. Schere, and J.J. Peterson. 1980. Theoretical estimates of actinic flux and pho-tolytic rate constants of atmospheric species. Adv. Environ. Sci. Technol. 10 369-392. 

In Table 3 the mean of the all calculated photolysis frequencies for the various compounds obtained from the analysis of the actinic flux measurements performed using EUPHORE chamber facilities is compared with results from other similar studies. In the estimates a quantum yield of unity has been assumed. 

At wavelengths X > 242 nm, the atmosphere is transparent with respect to 02. Ozone is the dominant absorber in the range 240-320 nm. Table 4.3 gives estimated surface-level spectral actinic flux at 40°N latitude on January 1 and July 1. 

TABLE 4.3 Estimated Ground-Level Actinic Flux I(X) at 40° N Latitude... 

Shot-noise-limited performance can be conveniently recognized by noting how the S/N of a system changes with light level. The S/N should vary quadratically with intensity if it is shot-noise-limited. Once a measurement is determined to be shot-noise-limited, the quantification of the noise amplitude provides a simple, direct, and effective way to estimate the actinic flux experienced by a sample during a... 

Figure IX-C-22. Calculated photolysis frequencies as a function of solar zenith angle for methylglyoxal photodecomposition in the lower troposphere clear-sky conditions are assumed with an ozone column of 350 DU. The large differences between the EUPHORE estimates and the y-values calculated from f, a, and actinic flux data may reflect the removal of methylglyoxal by HO2 radicals that could not be eliminated in the EUPHORE experiments.

The data of Laufer and Keller (1971) that are shown in figure IX-E-14 provide data applicable to the wavelength range of the actinic flux available in the troposphere. We have interpolated between the points of the given 5 nm interval data to derive estimates for each nm of wavelength needed for photolysis frequency estimates these are given in table IX-E-3. 

Photolysis frequencies for those compounds that show some overlap of the actinic flux with the long-wavelength absorption bands have been estimated by Calvert et al. (2008) using the cross section data of table IX-G-1 (298 K) and an assumed quantum yield of photodissociation of unity over the entire absorption region. These data are shown in figure IX-G-3. 

We have used the cross sections of tert-butyl nitrite given in table IX-H-4, an assumed total quantum yield of photodecomposition of unity, independent of wavelength, and the appropriate actinic flux data (cloudless skies, vertical ozone column of 350 DU) to estimate the -values for tert-butyl nitrite photodecomposition within the lower troposphere. These data are summarized in figme IX-H-16. The data of figure IX-H-16 give a photochemical hfetime for tert-butyl nitrite of about 4.3 min. with an overhead Sun. 

Figure IX-L-20. Approximate -values for the rate of formation of excited singlet state molecules, estimated for 2-hydroxynaphthalene (dashed curve) and 3-hydroxyphenanthrene (solid curve) these represent approximate maxima in yXTotal). The cross sections used in the calculation are from Ldng (1961-1971) as measured in ethanol solutions actinic flux data are for a cloudless sky in the lower troposphere with an overhead ozone column of 350 DU.

Wegner (1976) compared his rates of labeled actin incorporation to those estimated for a nontreadmilling process, and he introduced the parameter 5 to evaluate the commitment of a particular system to biased flux ... 

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