Hoci

Figure 4.5 Schcmatic of the conversion pathways between species of the CI, family.

The reaction between CIO and OH generally forms CI and HO, as products. However, a small fraction (~7%) of the reactions between CIO and OH also form HC1 [851:

While this reaction occurs throughout the stratosphere, it is primarily important in the upper stratosphere [64,86,87J.

HOC1 is formed from Clr primarily in the reaction

In rare circumstances, formation of HOC1 in heterogeneous reactions can be important. This will be discussed further in Chapters 6 and 7.

Formation ofClyfrom reservoirs ClOJNO, is converted to Cf through the photolysis reaction

The majority of ClONO. molecules photolyze to produce CI and NO,, with the remainder producing CIO and NO, (see DeMore et al. [5], Table 40). Oxidation of ClONO, by the stratospheric radicals such as OH, O, and CI becomes important above about 35 km, and is the dominant loss process of ClONO, near the stratopause.

HC1 is converted to CI, in the reaction

The pathway for conversion of HOC1 to CI, is photolysis,

with a minor contribution (less than 1%) from oxidation,

In addition to the gas phase reactions discussed above, there are several heterogeneous reactions occurring on the surfaces of sulfate aerosols that involve members of Clv. The st icking coefficients of these reactions are strong functions of temperature, and in general these reactions are important only for an enhanced sulfate aerosol surface area density (SAD), such as that found immediately after a volcanic eruption, or at temperatures below -200 K. These conditions are rarely found in the stratosphere, and therefore these heterogeneous reactions can be generally ignored. There are, however, certain circumstances where these reactions can have an impact [88]. We will discuss the role of these heterogeneous reactions in Chapters 6 and 7.

Figure 4.6 shows vertical profiles of the various members of Clr The figure shows CC1, decreasing rapidly with altitude, consistent with its destruction through photolysis. At the same time, Clv increases with altitude. It should be noted that the total chlorine content (the sum of CO, and Cly) is approximately constant throughout the stratosphere. This is true because the rate of change of CFCs in the troposphere is small compared to the rate at which air is transported through the stratosphere (a few years, see Chapter 5).

Mixing Ratio (ppbv)

Figure 4.6 Measurements of the major components of stratospheric chlorine versus pressure. Data were measured in November 1994 and between 20°N and 49°N [89j.

Mixing Ratio (ppbv)

Figure 4.6 Measurements of the major components of stratospheric chlorine versus pressure. Data were measured in November 1994 and between 20°N and 49°N [89j.

Over the entire stratosphere, HC1 is the dominant form of Cly. In the lower and mid-stratosphere, C10N02 makes up most of the remainder of Clv. In the upper stratosphere, CIO is the second most abundant form of CI,. HOCl makes up at most a few percent of Clv, while the abundance of CI atoms makes up a negligible fraction of Cls.

Figures 4.7 through 4.10 show meridional cross-sections of Clv, CIO, C10N0:, and HC1. The abundance of CI. increases with both altitude and latitude, consistent with the mean overturning circulation (discussed in Chapter 5) combined with the rapid destruction of CC1V in the mid- and upper-stratosphere.

The HQ distribution (Figure 4.8) has a shape similar to the distribution of Clv, with the abundance of HQ increasing with both altitude and latitude. The abundance of ClONOj (Figure 4.9) increases with altitude in the lower stratosphere, reaches a maximum in the mid-stratosphere, and decreases at higher altitudes. Further, the abundance of lower-stratospheric ClONO, as a fraction of Clv is greater in the winter hemisphere than in the summer hemisphere [90]. And as one might expect, lower-stratospheric HC1 shows the opposite seasonal cycle (Figure 4.8).

Latitude

Figure 4.7 Contours of CI, abundance (ppbv) for December. Values are from the Ooddard two-dimensional climatological circulation model [73]. Based on a model run to the steady state using CFC emission levels for 1990.

Latitude

Figure 4.7 Contours of CI, abundance (ppbv) for December. Values are from the Ooddard two-dimensional climatological circulation model [73]. Based on a model run to the steady state using CFC emission levels for 1990.

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