300 350 400 450 500 550 X (nm)

FIGURE 16.4 Relative spectral distributions for a typical black lamp and the solar spectrum at zenith angles of 0 and 80° normalized to the same N02 photolysis rate constant (adapted from Carter et al., 1984).

spectrum at two zenith angles, where all curves have been normalized to the same NOz photolysis rate constant /c, (Carter et al., 1984). (See later for a discussion of k{, which is a measure of the total light intensity.) Superimposed on the broad emission continuum from the phosphor are the low-pressure mercury lines at 313, 334, 365-366, 405-408, and 436 nm.

While such lamps provide good light intensity in the 340- to 400-nm region (and at the 313-nm mercury line) where important atmospheric photochemistry occurs, their spectral distribution is very different from that of the sun. Specifically, much of the intensity resides in the sharp mercury lines, and the output is poor in the critically important actinic UV region from 290 to 340 nm. In addition, the intensity falls off at A > 375 nm, whereas the intensity of solar radiation is increasing significantly in this region. Such differences can significantly alter the photochemistry of important species such as 03, N03, and HCHO, even if the lamp output is normalized to give the same N02 photolysis rate constant (A:,) as the sun.

(3) Sunlamps A sunlamp is similar to a black lamp, except that a different type of phosphor is used and the lamp envelope transmits UV. Figure 16.5 shows a typical spectral distribution from a commercial sunlamp. The wavelength corresponding to maximum power is shifted to lower wavelengths (~ 310 nm), compared to black lamps, and there is significant intensity down to ~270 nm. However, the intensity decreases rapidly above ~330 nm. The mercury lines can again be seen superimposed on the phosphor fluorescence.

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