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o 22

f 23 24

120 160 200 240 280 320 360 Wavelength (nm)

FIGURE 12.15 Absorption cross sections for 02 and 03 from 120 to 360 nm, showing the window from ~ 185 to 210 nm (adapted from Rowland and Molina, 1975).

Li et al., 1995b). The F02 reaction with 03 is slow (Li et al., 1995b; DeMore et al., 1997), as are the reactions with the organics (Sehested et al., 1994; Li et al, 1995b). While fluorine is cycled between F, F02, and FO, fluorine atoms are removed efficiently by their fast reactions with CH4 and HzO to form HF,

kj'£¡8K = 6.7 X f0~" cm3 molecule 1 s_1 (DeMore et al., 1997), F + H20 -> HF + OH, (25)

¿29xk = 1.4 x 10"" cm3 molecule"1 s"1 (DeMore et al., 1997), which, unlike HC1, does not itself react with OH (indeed, reaction (25) is exothermic). The HF formed in these reactions also appears to be unreactive on ice surfaces typical of polar stratospheric clouds (Hanson and Ravishankara, 1992a).

While the FOz reactions with NO and NOz are moderately fast, with room temperature rate constants of the order of 10"12 and 10"13 cm3 molecule"1 s"1, respectively (Sehested et al., 1994; Li et al., 1995b), the concentrations of NO and N02 are sufficiently small that they do not represent major atmospheric loss processes for F02. It is interesting, however, that the FOz + NO reaction proceeds by transfer of the F atom to form FNO (which photolyzes) rather than by transfer of an oxygen atom, which is more common for

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