Co

400 450 500 550 600 650 700 750 Wavelength (nm)

FIGURE 4.8 Absorption cross sections of 03 in the Chappuis band at room temperature (adapted from Burkholder and Talukdar, 1994).

in the troposphere at 290 nm; thus 03 in the stratosphere strongly absorbs A < 290 nm, limiting the UV reaching the earth's surface.

The Huggins bands are in the 300- to 360-nm region, whereas the 440- to 850-nm region represents the Chappuis bands. As seen in Figs. 4.7 and 4.8, both absorptions are much weaker than the Hartley bands.

Table 4.3 shows the ozone absorption cross sections (base e) at 298 and at 226 K measured by Molina and Molina (1986), with which recent studies are in agreement, particularly below 240 nm (e.g., see Malicet et al., 1995; DeMore et al., 1997). The cross sections below 275 nm are not very sensitive to temperature over the 226-298 K temperature range, typically < 1%, but the temperature dependence becomes significant above 275 nm. Table 4.4 shows the temperature dependence of these absorption cross sections at longer wavelengths, averaged over the spectral regions shown.

2. Photochemistry

The photolysis of 03 produces molecular oxygen and atomic oxygen, either or both of which may be in electronically excited states, depending on the excitation energy. Table 4.5 shows the wavelength threshold below which each combination of products may be formed. For example, the primary photochemical process

in principle requires light corresponding to A = 310 nm. (However, as discussed below, production of O('D) has been observed experimentally out to at least 336 nm.)

In addition to energetic considerations, however, there are other factors such as spin conservation that also determine the importance of various sets of products. As discussed in Chapter 3.A, since the ground state of 03 is a singlet, dissociation into either two singlet states (e.g., reaction (5)) or into two triplet states is expected to predominate. However, as discussed shortly, both hot-band absorption by rovibra-tionally excited 03 and by a spin-forbidden process are believed to contribute significantly to the atmospheric photochemistry of 03.

The most important aspect of 03 photochemistry for the troposphere is the yield and wavelength dependence of 0('D) production in reaction (5) since it is a source of hydroxyl free radicals via its reaction with water vapor:

kh = 2.2 X 10"10 cm3 molecule"1 s"1

(DeMore et al, 1997). Only about 5% of the 0('D)-H20 interaction results in deactivation of the Oi'D) to the ground state 0(3P), and < 1% in the alternate set of products H2 + 02 (DeMore et al., 1997).

The very fast reaction of O('D) with water vapor occurs in competition with its deactivation by air:

k-j = 2.6 X 10"" cm3 molecule"1 s"1 for M = N2, = 4.0 X 10"" cm3 molecule"1 s"1 for M = 02

(DeMore et al., 1997). In 1 atm of air at 50% relative humidity and 298 K, approximately 10% of the O('D) produced via reaction (5) reacts with water vapor to form hydroxyl radicals.

The wavelength dependence of the quantum yield for Oi'D) production has been somewhat controversial. The recommended value between 290 and 305 nm is <j>[O('D)] = 0.95, decreasing slightly at shorter wavelengths to values in the range 0.85-0.90 (see review in DeMore et al., 1997). Talukdar et al. (1998) reported yields of O('D) of 0.89 ± 0.04 (2 a), independent of temperature from 203 to 320 K, consistent with the production of small amounts of 0(3P) in this region (e.g., Brock and Watson, 1980). The falloff in the quantum yield at shorter wavelengths is unusual and not well understood in terms of the spectroscopy and electronic states of 03 (e.g., Steinfeld et al., 1987; Wayne, 1987). Wayne (1987) suggests that the upper electronic states initially populated on light absorption have a curve crossing with a repulsive state that correlates with the ground-state products 0(3P) + 02CX~) and that the relative positions of the states are such

400 450 500 550 600 650 700 750 Wavelength (nm)

FIGURE 4.8 Absorption cross sections of 03 in the Chappuis band at room temperature (adapted from Burkholder and Talukdar, 1994).

4. PHOTOCHEMISTRY OF IMPORTANT ATMOSPHERIC SPECIES TABLE 4.3 Ozone Absorption Cross Sections (Base e)u

length (nm)

102"<r (cm2

molecule 1 )

Wavelength (nm)

102"(t (cm2

molecule 1 )

T = 226 K

T = 298 K

T = 226 K

T = 298 K

Was this article helpful?

0 0

Post a comment