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Assuming steady-state conditions, Equation (4.20) can be rewritten as

[ClONO,] ¿oo+so^NO,] ¿ci-roJO.,] AIICWih[OH] ----=-x---------------------------- (4.21)

In the lower and mid-stratosphere, A( io+no[NO| > Arl(w,[0]. And substituting the steady-state relation (ANO+(><[0,| + ArN0+(.10[C10|)/./N0, for [NO,]/[NO], we obtain the expression

[ClONOJ ^ AVichno„ x {k^k^O^ + knM^M.ACio\m) [HCI1 -^ciono-j ^cio+noa'o, x W»!^ (4.22)

Acn^nlCHJ

In the lower stratosphere, ¿no+o^o.oJO.,]2 is several orders of magnitude larger than An+()3/:N0+n0[CI0j[0:,], decreasing to about live times larger in the mid-stratosphere. Thus, the ratio in Equation (4.22) is approximately quadratically dependent on [0,|, linearly dependent on [OH] and [CH.,j, and less sensitive to changes in [CIO]. Note that we have neglected chlorine-activating heterogeneous reactions, which are generally unimportant.

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Figure 4.12 Diurnal cycle of the components of Cl„ derived from a model incorporating the reaction set of JPL 94 [911. The model run is at 45°N on March 21, 1996 and using background aerosol abundances. "Upper stratosphere" is the 3.16 hPa level of the model, "mid-stratosphere" is the 14.7 hPa level, and "lower stratosphere" is the 57.0 hPa level.

Note that the lifetime of HCl is sufficiently long that we do not expect Equation (4.22) to be satisfied at any particular instant in time but only when the abundances and other parameters are averaged over the lifetime of HCl.

In the upper stratosphere, during the day [CI,| [ClONOJ, and the ratio [C1V]/[HC1| is the important partitioning parameter. Following the logic used for [C10N02]/[HC1[, we rewrite the ratio [C1,]/[HC1] in terms of the ratios of constituents that are directly related by chemical reactions:

Using a photochemical steady-state relation between CI and HCl and Equation (4.9), the photochemical steady-state relation between CI and CIO, we get

Owing to the reasonably short lifetime of HOCI, over most of the day HOC! is in the photochemical steady state with CI,. The relative abundance of HOCI and CI, can be written [92] as

where we utilize the approximation [CI,] ~ [CIO]. The ratio is near unity in the lower stratosphere, rising to 10-20 at 1 hPa. Both production and loss cease at sunset, leaving the concentration of HOCI to hold steady throughout the night. Despite its relatively short lifetime throughout the stratosphere, HOCI shows little diurnal cycle.

Finally, it should be noted that the behavior of the abundance of a species during the day is constrained by its lifetime. If the species has a lifetime longer than a day—HCl, for example—then its abundance can exhibit only small changes during a day. Species with lifetimes shorter than a day can exhibit significant diurnal variations (Figure 4.12). The abundance of CIO, for example, rises rapidly in the morning, remains elevated throughout the day, and then decreases significantly after sunset. It should be noted, however, that short-lived species are not guaranteed to exhibit large diurnal variations. HOCI, which has a lifetime comparable to or shorter than CI,, for example, shows little diurnal change. In this case, the production rate and loss frequency of HOCI vary diurnally so that the ratio P/L remains approximately constant throughout the day, leading to an approximately constant abundance of HOCI.

Another aside: how do we know that CFCs are responsible for stratospheric chlorine?

In the last aside, we explained CFCs are expected to reach the stratosphere despite being heavier than air. In this brief aside, we present evidence that CFCs actually make it and are responsible for most of the chlorine found there:

(1) CFCs have been detected in the stratosphere [93]. And since they have no sources other than the surface, this proves they must be transported there from the troposphere.

(2) HF has been detected in the stratosphere [84j. The only source of stratospheric HF is the breakdown of CFCs, so its presence in the stratosphere is positive proof that CFCs are being destroyed in the stratosphere, and their chlorine and fluorine are being released.

(3) The abundance of total chlorine in the stratosphere (-3.6 ppbv in the mid-1990s) corresponds well to the tropospheric abundance of CFCs and other long-lived halogenated organic molecules.

Combining these lines of reasoning, it is indisputable that CFCs are the main source of stratospheric chlorine.

4.2.3 Loss of Cly

There are no chemical loss processes for the CI, family in the stratosphere. Instead, the concentration of Clv in an air parcel increases until all of the CO,, has been destroyed.

4.3.1 Source

Much like CFCs, nitrous oxide (N,0) is a stable molecule with a tropospheric source, which, because of its stability, is transported into the stratosphere. Once in the stratosphere, destruction of N,0 creates NO,..

N20 is thought to be primarily of natural origin, although significant anthropogenic sources do exist (see WMO [I3|, Section 2.5) [941. In the mid-1990s, the tropospheric concentration of N20 was 310 ppbv (see WMO [ 1.3], Section 2.5), and had been growing at 0.5-1.2 ppbv/year [94[, about 0.16-0.39% per year. The major stratospheric loss process for N ,0 is photolysis:

which accounts for -90% of the loss (see WMO [13], Section 2.5). Because of its strong triple bond, N2 is essentially inert in the lower and middle atmosphere; as a result, N, is not included in NOv. Therefore, reaction (4.26) does not produce NOv. Other loss processes include [95]

Reaction (4.27) does not produce NOv. Reaction (4.28), while a minor loss process for N,0, is the major source of NO, in the stratosphere. Reaction (4.29) is also a source of NOv, but it is much less important than reaction (4.28).

NO, is also formed in the stratosphere from dissociation of N2 caused by collisions with solar protons, galactic cosmic rays, and associated electrons [96]. In addition, NOv produced in the mesosphere and thermosphere from photolysis of N2 and precipitating electrons dissociating N2 can be transported into the stratosphere [97]. These NO, sources, however, are minor compared to the oxidation of N ,0.

43.2 Partitioning

Odd nitrogen, NO,., is defined to be

[NO,,| = |N] + [NO] + [NO,] + [NO,] + 2 x [Nft] (4 30)

Thus, NO, represents the abundance of N atoms that are not bound up in either N; or N20. This is analogous to the definition for Cl„ which contained all of the CI atoms that were not in a CFC or other organic molecule.

There are three other points of note. First, the concentration of N205 in Equation (4.30) is multiplied by 2 to account for the fact that each N205 molecule contains two N atoms. Second, CIONO,. which is a member of the CI, family, is also a member of the NO, family. This shows that the various chemical families are coupled—i.e. changes in the partitioning of one family can affect the partitioning of another. Third, in the stratosphere, N and H02N02 make up a small fraction of NO, species, and play a limited role in stratospheric chemistry.

Figure 4.13 shows typical lifetimes for the NO,, species. Following the pattern set in our discussion of the CI, family, we group the short-lived species NO, N02, and NO, together and define their sum to be NO,:

NOa includes the members of the NO, family that participate in catalytic cycles that destroy ozone—in other words the "active" components of NOr

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