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ARCTIC (N.H.) Polar Vortex Minimum Temperature, 20 km

POtAR STRATOSPHERIC ClOUD

0 2 4 6 8 10 Ozone Number Density, 1012 molecules cm-3 FIGURE 5.23 Vertical 03 profiles determined by balloonborne sensors over Spitzbergen, Norway (79°N), on March 18, 1992 and March 20, 1995 (Reprinted with permission from Chemical & Engineering News, June 12, 1995, p. 21. Copyright 1995 American Chemical Society.)

0 2 4 6 8 10 Ozone Number Density, 1012 molecules cm-3 FIGURE 5.23 Vertical 03 profiles determined by balloonborne sensors over Spitzbergen, Norway (79°N), on March 18, 1992 and March 20, 1995 (Reprinted with permission from Chemical & Engineering News, June 12, 1995, p. 21. Copyright 1995 American Chemical Society.)

5.8 HETEROGENEOUS (NONPOLAR) STRATOSPHERIC CHEMISTRY

Evidence now exists for significant loss of 03 in all seasons in both hemispheres over the decades of the 1980s and 1990s. It is difficult to account for the global average (69°S-69°N) total ozone decrease of 3.5% over the 11 year period, 1979-1989, based on gas-phase chemistry alone; the reduction expected on that basis is only about 1 % over that period. The process of unraveling the chemistry responsible for the Antarctic ozone hole led to the realization of the pivotal importance of heterogeneous reactions in the polar stratosphere. When midlatitude ozone losses could not be explained on the basis of gasphase chemistry alone, it was recognized that midlatitude ozone destruction is also tied to surface reactions. In this case the reactions occur on the sulfuric acid aerosols that are ubiquitous in the stratosphere (Brasseur et al. 1990; Rodriguez et al. 1991).

5.8.1 The Stratospheric Aerosol Layer

The stratosphere contains a natural aerosol layer at altitudes of 12-30 km. This aerosol is composed of small sulfuric acid droplets with size on the order of 0.2 pm diameter and at number concentrations of 1-10cm-3. In the midlatitude lower stratosphere (about 16 km) the temperature is about 220 K, and the particles in equilibrium with 5 ppm H20 have compositions of 70-75 wt% H2S04. As temperature decreases, these particles absorb water to maintain equilibrium; at 195 K, they are about 40 wt% H2S04.

The role of carbonyl sulfide (OCS) as a source of the natural stratospheric aerosol layer was first pointed out by Crutzen (1976). Because OCS is relatively chemically inert in the troposphere, much of it is transported to the stratosphere where it is eventually photo-dissociated and attacked by O atoms and OH radicals. The gaseous sulfur product of the chemical breakdown of OCS is SO2, which is subsequently converted to H2SO4 aerosol. As noted in Section 2.2.2, an approximate, global average tropospheric OCS mixing ratio is 500 ppt. In the stratosphere the OCS mixing ratio decreases rapidly with altitude, from near 500 ppt at the tropopause to less than 10 ppt at 30 km. Chin and Davis (1995) evaluated existing data on stratospheric OCS concentrations and used the measured vertical profile of OCS to calculate an average OCS stratospheric mixing ratio of 380 ppt.

5.8.2 Heterogeneous Hydrolysis of N2Os

In NO* cycle 2, which is important in the lower stratosphere, the nitrate radical N03 is formed from the N02 + 03 reaction

During daytime, N03 rapidly photolyzes, but at night, N03 reacts with N02 to produce dinitrogen pentoxide, N205:

N205 can decompose back to N03 and N02 either photolytically or thermally. N205 itself is photolyzed in the 200-400 nm region; since this wavelength region overlaps that of the strongest 03 absorption, the photolysis lifetime of N205 depends on the overhead column of ozone. The photolytic lifetime of N205 is typically on the order of hours at 40 km and many days near 30 km.

The key reaction that N205 undergoes is with a water molecule to form two molecules of nitric acid. Whereas the gas-phase reaction between N205 and a H20 vapor molecule is too slow to be of any importance, the reaction proceeds effectively on the surface of aerosol particles that contain water:

This reaction has been demonstrated in the laboratory to proceed rapidly on the surface of concentrated H2S04 droplets (Mozurkewich and Calvert 1988; Van Doren et al. 1991).

When an N2O5 molecule strikes the surface of an aqueous particle, not every collision leads to reaction. In Section 3.7, a reaction efficiency or uptake coefficient y was introduced to account for the probability of reaction. Values of y for this reaction ranging from 0.06 to 0.1 have been reported. The first-order rate efficient for this reaction can be expressed as in (3.38)

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