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Angle etal., 1992

Fields and meadows

0.03-0.04"

Eck etal., 1987

Desert

0.06-0.09"

Eck etal., 1987

Salt flats

0.57-0.65"

Eck etal., 1987

" Minimum reflectivities at 370 nm. h Measured with respect to N02 photolysis.

" Minimum reflectivities at 370 nm. h Measured with respect to N02 photolysis.

using the Beer-Lambert law, if the concentrations and absorption coefficients of all absorbing species are known.

2. Calculating Photolysis Rates in the Atmosphere a. Photolysis Rate Constant I I, Radiance [L(\)l, Actinic Flux [F(X)], and Irradiance [£(X)]

The rate of photodissociation of a molecule, A, upon light absorption,

can be described as a first-order process (see Chapter 5) with a rate constant, kp, known as the photolysis rate constant:

In effect, kp takes into account the intensity of available light that the molecule can absorb, the intrinsic strength of light absorption in that region by A, i.e., the absorption cross section a, and the quantum yield for photodissociation, (f>.

The light available to a molecule in air for absorption and photodissociation includes both direct and scattered and reflected radiation coming from all directions as described earlier and depicted in Fig. 3.16. The term actinic flux or spherically integrated actinic flux, denoted by F( A), is used to describe the total intensity of this light and is the quantity of interest in calculating kv.

However, in practice, available light intensity is often measured using flat-plate devices such as the one

FIGURE 3.17 Typical device (Eppley Laboratories Model 8-48) used to measure solar irradiance. The detector consists of a differential thermopile with the hot junction receivers blackened with flat black coating and the cold junction receivers whitened with BaS04 (photo supplied courtesy of G. L. Kirk, Eppley Laboratories).

FIGURE 3.17 Typical device (Eppley Laboratories Model 8-48) used to measure solar irradiance. The detector consists of a differential thermopile with the hot junction receivers blackened with flat black coating and the cold junction receivers whitened with BaS04 (photo supplied courtesy of G. L. Kirk, Eppley Laboratories).

shown in Fig. 3.17. Here light of a given wavelength that comes from all directions in a hemisphere and crosses the surface is measured. The net flux crossing the surface per unit area and time is known as the irradiance, E( A), and represents the flow of light across a flat plane rather than the total light coming from all directions that a molecule actually encounters in the atmosphere. Although actinic flux and irradiance are clearly related, they are not identical; for a detailed treatment of the actinic flux, irradiance, and radiance (defined next), see Madronich (1987).

An expression for the number of A molecules dissociating per unit volume per unit time is developed in Box 3.2. Comparing Eq. (NN) in Box 3.2 to Eq. (CC), the photolysis rate constant kp must be given by kp = f (f>(A)a(A)f L(A,&, cf>)d(odA

where F(A) is the spherically integrated actinic flux, Eq. (FF). We again stress that cr(A) is the absorption cross section to the base e, arising from the use of the differential form of the Beer-Lambert law to obtain Eq. (KK).

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.

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