Halogen Cycles

At the time of the first tropospheric measurements of anthropogenic halogenated hydrocarbons in the early 1970s, the quantities of the chlorofluorocarbons in the atmosphere were found to be approximately equal to the total amounts ever manufactured. In 1974 Molina and Rowland (Molina and Rowland 1974; Rowland and Molina 1975) realized that chlorofluorocarbons (CFCs), manufactured and used by humans in a variety of technological applications from refrigerants to aerosol spray propellants, have no tropospheric sink and persist in the atmosphere until they diffuse high into the stratosphere where the powerful UV light photolyzes them. The photolysis reactions release a chlorine (CI) atom, for example, for CFC13 (CFC-11) and CF2C12 (CFC-12):

CFC13 + hv —> CFC12 + CI CF2C12 +hv —> CF2C1 + CI

To photodissociate, CFCs need not rise above most of the atmospheric 02 and 03; they are photodissociated at wavelengths in the 185-210 nm spectral window between 02 absorption of shorter wavelengths and 03 absorption of longer wavelengths. The maximum loss rate of CFC13 occurs at about 23 km, whereas that for CF2C12 takes place in the 25-35 km range. As with N20, the bulk of the removal of CFC13 and CF2C12 is confined to the tropics, reflecting larger values for photolysis rates in this region.

The only continuous natural source of chlorine in the stratosphere is methyl chloride, CH3C1 (see Chapter 2). The tropospheric lifetime for CH3C1 is sufficiently long, 1.5 years (Table 2.15), so some amount of CH3C1 is transported through the tropopause. In the stratosphere, as in the troposphere, CH3C1 is removed primarily by reaction with the OH radical. At higher altitudes in the stratosphere a portion of CH3C1 is photolyzed. Regardless of the path of reaction, the chlorine atom in CH3C1 is released as active chlorine.

5.5.1 Chlorine Cycles

The chlorine atom is highly reactive toward 03 and establishes a rapid cycle of 03 destruction involving the chlorine monoxide (CIO) radical:

C10r cycle 1: CI + 03 -U CIO + 02 kt = 2.3 x 10"11 exp(-200/T) (reaction 1)

CIO + O —> CI + Oz k2 = 3.0 x 10"11 exp(70/T) (reaction 2) Net: 03 + O —> 02 + 02

Let us estimate the lifetime of the CI atom against reaction 1 and the lifetime of CIO against reaction 2. To do so we need the concentrations of 03 and O; these are related by (5.7). At 40 km, for example, where T = 251 K, the 03 concentration can be taken as

CIO,

FIGURE 5.13 The stratospheric CIO, family.

0.5 x 1012 molecules cm 3, and the ratio [0]/[03] was shown to be about 9.4 x 10 4. The two lifetimes at 40 km are xci =^7^ = 0.2 8

tao=m=53s

Subsequent reactions of the CFC12 and CF2C1 radicals lead to rapid release of the remaining chlorine atoms.

CI and CIO establish the C10x chemical family (Figure 5.13). The rapid inner cycle is characterized by CIO formation by reaction 1 and loss by reaction with both O and NO. If CIO reacts with O, the catalytic 03 depletion cycle above occurs. If CIO reacts with NO, the following cycle takes place:

CIO + NO CI + N02 k3 = 6.4 x 10"12 exp(290/T) (reaction 3) N02 + hv —y NO + O Net: 03 + hv —► O + 02

Because the O atom rapidly reforms 03, this is a null cycle with respect to 03 removal. The rate of net 03 removal by the C10t cycle of reactions 1 and 2 is

Within the CIO* chemical family we can compare the relative importance of reactions 2 and 3:

At 40 km, T = 251 K, [0]/[03] ^ 9.4 x 10"4, [03] ^ 0.5 x 1012 molecules cm"3, and [NO] = 1 x 109 molecules cm"3. The rate coefficient values are feCio+o = 4 x 10"11 cm3 molecule"1 s"1 and &Cio+o = 2 x 10 " cm3 molecule"1 s"1. Thus feo+o[0]

The steady-state ratio [C1]/[C10] within the CIO, family is given by

At 40km

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