Actinic flux spectral

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 spectral region near 120 nm, the solar hydrogen emission line at Lyman a represents an important source of ionization and dissociation. During quiet periods, it contains more energy than the rest of the spectrum at shorter wavelengths. The total flux of this line as well as its shape vary with solar activity. The actinic flux varies between a minimum of about 3.0 x 1011 photons cm 2s 1 for quiet solar activity and a maximum of about 6.0 x 1011 during high solar activity. 

The actinic flux is related but not equal to the irradiance. The relation between the two quantities has been illustrated carefully by Madronich (1987). Consider an atmospheric layer of infinitesimal vertical thickness dz, illuminated from above (Figure 4.8). The quantity of light incident on the top surface of the layer depends on the direction of incidence of the light, as defined by the spherical coordinates 0, <(>. This dependence is specified by the spectral radiance L(X, 0,4>), expressed in units of photons cm-2 s-1 nm 1 sr-1. The number of photons entering the layer (through ds, in time dt, from solid angle dm) is... 

To determine the spectral actinic flux in the layer in Figure 4.8 resulting from the spectral radiance L(X, 0,), we need to jump ahead a bit. The rate of photodissociation of a species A is written as... 

As indicated, the quantity on the RHS multiplying nA isjA. The spectral actinic flux is then the radiative quantity that drives the photodissociation, that is, the quantity that multiplies photodissociation rate coefficient. 

It is important to distinguish the actinic flux from the spectral irradiance. The spectral irradiance E(k) is the radiant energy crossing a surface (per unit surface area, time, and... 

The number of photons absorbed by a molecule A in a wavelength region X to X + dX is the product of its absorption cross section oA(X) (cm2 molecule-1), the spectral actinic flux I(X) (photons cm-2 s-1 nm-1), and the number concentration of A (molecules cm-3) ... 

Table 4.2 tabulates the solar spectral irradiance, normalized to a solar constant of 1367 Wm , Solar spectral actinic flux at the surface (0km), 20, 30, 40, and 50km is shown in Figure 4.11. 

For wavelengths below about lOOOnm (1 pm), the predominant absorbing species in the atmosphere are 02 and 03 (Figure 4.9). The spectral solar actinic flux at various altitudes, as shown in Figure 4.11, can, therefore, be computed fairly accurately by considering only the absorption by 02 and 03. The absorption cross sections of 02 and 03 are shown in Figures 4.12 and 4.13. All the absorption shown in Figure 4.12 and 4.13 leads to dissociation. 

FIGURE 4.11 Solar spectral actinic flux (photons cm-2 s 1 nm"1) at various altitudes and at the Earth s surface (DeMore et al. 1994). 

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. 

Photolysis reactions are central to atmospheric chemistry, since the source of energy that drives the entire system of atmospheric reactions is the Sun. The general expression for the first-order rate coefficient j for photodissociation of a species A is given by (4.39). Because the rate of a photolysis reaction depends on the spectral actinic flux / and because... 

The critical role of wavelength in photolysis can be seen by comparing Figure 4.11 and Table 4.4. From X = 305 nm to 320 nm, the absorption cross section drops by a factor of 10, and the quantum yield for O( D) formation drops from 0.9 to about 0.1. Since the cr<)) product changes rapidly with X, the actual rate of production of O (1D) is critically dependent on how I(X) varies with X. At the surface of the Earth, the spectral actinic flux increases by about an order of magnitude between X = 300 nm and X = 320 nm (see Table 4.3). 

TABLE 3.4 Estimated Ground-Level Spectral Actinic Fluxes /(/I) at 40° N Latitude... 

Note that the actinic flux is always related to a given altitude (depth z, Fig. 2.11) in the atmosphere (earth s surface at z = 0). It is important to distinguish the actinic flux 5 (A) from the spectral irradiance /(A), which refers to energy arrival on a flat surface with a fixed spatial orientation (Table 2.14) and which is the most common measured radiation quantity ... 

Ruggaber, A., R. Forkel and R. Dlugi (1993) Spectral actinic flux measurements and its ratio to spectral irradiance by radiation transfer calculations. Journal of Geophysical Research 98, 1151-1162... 

The direct physical measurement of the spectral actinic flux F(X) is not easy, although attempts have been made (Shetter and Muller 1999 Hofzumahaus et al. 1999). Generally, irradiance E(k) (radiation flux per unit area, W nm ) is measured by radiometers, and experiments to compare solar spectral intensity in the field with radiative transfer models have been made in order to convert the spectral irradiance E(X) to F(X). In these analyses, downward actinic flux Fd (A) is obtained by upper-hemispherical integration of observed spectral radiance L(k,9,4>) (radiation flux per solid angle, W sr m nm ), and F (A) is expressed as the sum of the flux of direct radiation Fq (X) and downward diffusive... 

Kazandzis, S., Bais, A.F., Balis, D., Zerefos, C., Blumthaler, M. Retrieval of downwelling UV actinic flux density spectra from spectral measurements of global and direct solar UV irradi-ance. J. Geophys. Res. 105, 4857-4864 (2000)... 

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