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200 250 300 350 400 450 500 Wavelength (nm)

FIGURE 16.5 Typical spectral distribution from a sunlamp (reprinted with permission of North American Philips Lighting Corporation).

(4) Xenon lamps High-pressure xenon lamps provide the most faithful artificial reproduction of the solar energy distribution at the earth's surface in the wavelength region 290-700 nm. Figure 16.6 compares the output of an unfiltered xenon lamp to the zero air mass solar spectral irradiance (Winer et al., 1979). Unlike black lamps, xenon lamps have substantial intensity in the critical region around 300 nm; the region < 290 nm can be filtered out to match the solar energy distribution at the earth's surface using Pyrex of varying thickness (Fig. 16.10). fn contrast to black lamps, the xenon lamp has maximum intensity at wavelengths above 400 nm.

The Xe lamp also has a series of peaks in the 800- to fOOO-nm region that do not appear in sunlight. This relatively low-energy radiation does not cause significant photochemistry in the troposphere. However, if desired, the intensity of these peaks can be decreased with the use of appropriate filters.

(5) Measurement of light intensity In chamber studies, the spectral distribution of the irradiation source must be measured periodically (e.g., using a calibrated monochromator-photomultiplier combination) because the lamp and the windows in the chamber "age" (i.e., change with time). In addition to the spectral energy distribution of the lamp, the total absolute light intensity also must be measured. In particular, the intensity of the region below 430 nm where the most important photochemistry (e.g., of N02, 03, and HCHO) occurs is of greatest interest. Both of these calibrations are tedious and must be carried out with care. However, they are sufficiently critical to data interpretation that they are carried out frequently. For example, measurement of the total absolute light intensity is typically carried out after every four or five runs, and in some cases where knowledge of the light intensity is essential, after every run.

The photolysis rate for NOz might be expected to be a good (although nonspecific) indicator of the intensity in the region <430 nm since it absorbs strongly (see Chapter 4.C) and is also one of the major photochemi-cally active species in VOC-NOx systems. Thus a standard procedure in smog chamber studies is to measure the rate of photolysis of N02 (&,) as a relative measure of the total light intensity:

200 250 300 350 400 450 500 Wavelength (nm)

FIGURE 16.5 Typical spectral distribution from a sunlamp (reprinted with permission of North American Philips Lighting Corporation).

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