W lexp[afci

The characteristic approach time of this solution to its steady state is

Applying this to (5.11), we find the characteristic time needed to establish the 03 steady state is given by xss = I f-MMM1/2 (5.17)

Although j0l and y'o, decrease as altitude decreases, the exponential increase of [M] with decreasing altitude exerts the dominate influence on xg3. Thus, we expect this time scale to increase at lower altitudes.

Let us estimate Tq, as a function of altitude, at a solar zenith angle of 0°:

¿4, cm3

z, km

T, K

molecule

•s"1

jo2, s

1

jo}< s 1

Tss u x03' n

20

217

6 x 10-

16

1 x 10-

11

0.7 x 10-

-3

-1400

25

222

7.5 x 10"

16

2 x 10"

-11

0.7 x 10-

-3

~600

30

227

9.2 x 10-

16

6 x 10"

-11

1.2 x 10"

-3

~ 160

35

237

1.3 x 10"

-15

2 x 10-

-10

1.6 x 10-

-3

-40

40

251

2.2 x 10"

-15

5 x 10-

-10

1.9 x 10"

-3

-12

45

265

3.4 x 10-

-15

8 x 10-

-10

6 x 10"

-3

~ 3

Again, we caution that x^ is not the overall lifetime of an 03 molecule; rather, this quantity is the characteristic time required for the Chapman mechanism to achieve a steady-state balance after some perturbation. When Xq is short relative to other

TABLE 5.2 Chemical Families in Stratospheric Chemistry

Symbol

Name

Components

0,

Odd oxygen

0 + 03

NO,

Nitrogen oxides

NO + N02

NOy

Oxidized nitrogen

NO + N02 + HNO3 + 2N205 + CIONO2 + NO3+ HOONO2 + Br0N02

HO,

Hydrogen radicals

OH + H02

Cly

Inorganic chlorine

Sum of all chlorine-containing species that lack a carbon atom (CI + 2C12 + CIO + OCIO + 2C1202 + HOC1 + CIONO2 + HC1 + BrCl)

CIO,

Reactive chlorine

CI + CIO

CCly

Organic chlorine

CF2C12 + CFCI3 + CCI4 + CH3CCI3 + CFC12CF2C1 (CFC - 113) + CF2HC1(CFC - 22)

Bîy

Inorganic bromine

Sum of all bromine-containing species that lack a carbon atom (Br + BrO + HOBr + Br0N02)

stratospheric processes, it can be assumed that the local 03 concentration obeys the steady state, (5.12)—(5.14). When Tq3 is relatively long, the Chapman mechanism does not actually attain a steady state. We see that above ~ 30 km, [03] can be assumed to be in a local steady state. At these altitudes the 03 concentration over a year is governed by the seasonal cycle of production and loss terms.

Finally, the Ox chemical family is the first of a number of chemical families that are important in stratospheric chemistry. Table 5.2 summarizes chemical families important in stratospheric chemistry. As we proceed through this chapter, each of these families will emerge.

Figure 5.3 shows the ozone distribution in the stratosphere. Note that the regions of highest ozone concentration do not coincide with the location of the highest rate of

Spring Autumn

Latitude

FIGURE 5.3 Zonally averaged ozone concentration (in units of 1012 molecules cm-3) as a function of altitude, March 22 (Johnston 1975). Ozone concentration at the equator peaks at an altitude of about 25 km. Over the poles the location of the maximum is between 15 and 20 km. At altitudes above the ozone bulge, 03 formation is oxygen-limited; below the bulge, 03 formation is photon-limited.

formation of 03. Rates of 03 production are highest at the equator and at about 40 km, altitude, whereas ozone concentrations peak at northern latitudes. Even at the equator, maximum ozone concentrations occur at about 25 km as opposed to 40 km, where the production rate is greatest. At the poles, maximum 03 production occurs at altitudes even higher than those over the equator, whereas the maximum 03 concentrations are at lower altitudes. There is even a north-south asymmetry. Figure 5.4 shows the historical total column ozone as a function of latitude and time of year, measured in Dobson units, prior to anthropogenic ozone depletion. Highest ozone column abundances are found at high latitudes in winter, and the lowest values are in the tropics.

FIGURE 5.4 Zonally averaged total ozone column density (in Dobson units) as a function of latitude and time of year (Diitsch 1974). This distribution is representative of that prior to anthropogenic perturbation of stratospheric ozone.

The steady-state odd oxygen model of (5.10)—(5.12) predicts that the 03 concentration should be proportional to the square root of the 02 photolysis rate. We see that, in fact, ozone concentration and 02 photolysis rate do not peak together. The explanation for the lack of alignment of these two lies in the role of horizontal and vertical transport in redistributing stratospheric airmasses. The ozone bulge in the northern polar regions is a result, for example, of northward and downward air movement in the NH (Northern Hemisphere) stratosphere that transports ozone from high-altitude equatorial regions where ozone production is the largest.

That stratospheric 03 concentrations are maximum in areas far removed from those where 03 is being produced suggests that the lifetime of 03 in the stratosphere is longer than the time needed for the transport to occur. The stratospheric transport timescale from the equator to the poles is of order 3-4 months.

Until about 1964, the Chapman mechanism was thought to be the principal set of reactions governing ozone formation and destruction in the stratosphere. First, improved measurement of the rate constant of reaction 4 (above) indicated that the reaction is considerably slower than previously thought, leading to larger abundances of 03 as predicted by (5.10)—(5.12). Then, measurements indicated that the actual amount of ozone in the stratosphere is a factor of ~ 2 less than what is predicted by the Chapman mechanism with the more accurate rate constant of reaction 4 (Figure 5.5). It was concluded that significant additional ozone destruction pathways must be present beyond reaction 4.

FIGURE 5.5 Comparison of stratospheric ozone concentrations as a function of altitude as predicted by the Chapman mechanism and as observed over Panama (9°N) on November 13, f970.

12 i Ozone Concentration, 10 molecules cm

FIGURE 5.5 Comparison of stratospheric ozone concentrations as a function of altitude as predicted by the Chapman mechanism and as observed over Panama (9°N) on November 13, f970.

For an atmospheric species to contribute to ozone destruction, it would either have to be in great excess or, if a trace species, regenerated in a catalytic cycle. Following the initial studies of the effect of NO, emissions from supersonic aircraft and chlorofluo-rocarbons, an intricate and highly interwoven chemistry of catalytic cycles involving nitrogen oxides, hydrogen radicals, chlorine, and bromine emerged in one stunning discovery after another. The result is a tapestry of different catalytic cycles dominating at different altitudes in the stratosphere. The remainder of this chapter develops each of these systems, culminating in a sythesis of the stratospheric ozone depletion cycles not accounted for in the Chapman chemistry.

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