[Os[O

Even though O3 is present at roughly 1000-fold greater concentration than N02, the large value of the N02 + O rate coefficient compensates for this difference to make the N02 + O reaction almost 5 times more efficient in odd oxygen destruction than O + 03.

The cycle shown above is most effective in the upper stratosphere, where O atom concentrations are highest. An NO, cycle that does not require oxygen atoms and therefore is more important in the lower stratosphere is:

The nitrate radical N03 formed in reaction 2, during daytime, rapidly photolyzes (at a rate of about 0.3 s"1). There are two channels for photolysis:

FIGURE 5.9 NO, N02, HN03, N205, and C10N02 mixing ratios as a function of altitude (WMO 1998). All species except N205 measured at sunset; N205 measured at sunrise. Lines are the result of a calculation assuming photochemical steady state over a 24-h period. [Adapted from Sen et al. (1998).]

The photolysis rate by channel a is about 8 times that for channel b. Channel b leads to an overall loss of odd oxygen and is the reaction involved in cycle 2. At night, N03 formed by reaction 2 does not photodissociate and participates in an important series of reactions that involve stratospheric aerosol particles; we will return to this in Section 5.8.

Figure 5.9 shows mixing ratios of NO, N02, HN03, N205, and C10N02 (this compound to be discussed subsequently) measured at 35°N. At 25 km, the predominant NOj, species is HN03, whereas from 30 to 35 km, N02 is the major compound. Above 35 km, NO is predominant. We will consider the source of HN03 shortly.

5.4 HO* CYCLES

The HOA family (OH + H02) plays a key role in stratospheric chemistry. In fact, the first ozone-destroying catalytic cycle identified historically involved hydrogen-containing radicals (Bates and Nicolet 1950).

Production of OH in the stratosphere is initiated by photolysis of 03 to produce O('D), followed by

O('D) + H20 —> 2 OH ¿0(1D)+H20 = 22 x 10~10 cm3 molecule_1 S_1

0(1D) + CH4 —> OH + CH3 /t0(id)+ch4 = 1-5 x 10"10 cm3 molecule"1 s 1

Whereas the troposphere contains abundant water vapor, little H20 makes it to the stratosphere; the low temperatures at the tropopause lead to an effective freezing out of water before it can be transported up (a "cold trap" at the tropopause). Mixing ratios of H20 in the stratosphere do not exceed approximately 5-6 ppm. In fact, about half of this water vapor in the stratosphere actually results from the oxidation of methane that has leaked into the stratosphere from the troposphere. Between 20 and 50 km the total rate of

OH Mixing Ratio, ppt

OH Mixing Ratio, ppt

Solar Zenith Angle, degrees

FIGURE 5.10 (a) OH mixing ratio versus altitude (pressure) and (b) OH concentration as a function of solar zenith angle (WMO 1998). Solid curve is prediction by Wennberg et al. (1994).

Solar Zenith Angle, degrees

FIGURE 5.10 (a) OH mixing ratio versus altitude (pressure) and (b) OH concentration as a function of solar zenith angle (WMO 1998). Solid curve is prediction by Wennberg et al. (1994).

OH production from the two reactions above is ~ 2 x 104molecules cm 3 s About 90% is the result of the O('D) + H20 reaction; the remaining 10% results from 0(1D) +CH4.

Figure 5.10 shows the OH mixing ratio as a function of altitude at three different latitudes and the OH concentration as a function of solar zenith angle. The strong solar zenith angle dependence reflects the photolytic source of OH. Figure 5.11 shows OH, H02, and total HOx versus altitude.

OH and H02 rapidly interconvert so as to establish the HOt chemical family (Figure 5.12). A reaction that is important in affecting the interconversion between OH

5 10 15

FIGURE 5.11 OH, H02, and total HOx versus altitude, as compared with different model predictions (Jucks et al. 1998). Concentration profiles of OH and H02 measured during midmorning by satellite on April 30, 1997 near Fairbanks, Alaska (65°N). Model predictions: model A—JPL 1997 kinetics; model B—rate of O + H02 decreased by 50%; model C—rate of O + OH decreased by 20% and rate of OH + H02 increased by 30%; model D—rates of O + H02 and OH + H02 decreased by 25%.

5 10 15

FIGURE 5.11 OH, H02, and total HOx versus altitude, as compared with different model predictions (Jucks et al. 1998). Concentration profiles of OH and H02 measured during midmorning by satellite on April 30, 1997 near Fairbanks, Alaska (65°N). Model predictions: model A—JPL 1997 kinetics; model B—rate of O + H02 decreased by 50%; model C—rate of O + OH decreased by 20% and rate of OH + H02 increased by 30%; model D—rates of O + H02 and OH + H02 decreased by 25%.

and H02 in the HOt cycle is

HOz + NO —► N02 + OH k = 3.5 x 10"12 exp(250/r) The N02 formed in this reaction photolyzes

followed by

HN03 HOC1

HOBr

FIGURE 5.12 The stratospheric HO* family. The upper panel shows only the reactions affecting the inner dynamics of the HOx system. The lower panel includes additional reactions that affect OH and H02 levels; these reactions will be discussed subsequently.

HN03 HOC1

HOBr

FIGURE 5.12 The stratospheric HO* family. The upper panel shows only the reactions affecting the inner dynamics of the HOx system. The lower panel includes additional reactions that affect OH and H02 levels; these reactions will be discussed subsequently.

As a result, if a molecule of H02 reacts with NO before it has a chance to react with either O or 03, the result is a do-nothing (null) cycle with respect to removal of 03. Each time a molecule of H02 follows the bottom path and reacts with O or 03, two molecules of odd oxygen are removed:

Regeneration of H02 occurs by

OH + 03 —> H02 + 02 The resulting catalytic ozone-depletion cycles are

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