Chemistry of the stratosphere

The stratosphere has been emphasized as a reservoir of smaller size than the troposphere and hence more easily affected by small amounts of trace gases. These can be injected by high flying aircraft or powerful volcanic explosions. However, 

A parcel of air over a rural area of an industrial continent would typically be expected to contain sulphur dioxide (S02) at a concentration of 5 x 10 9atm. This means that a cubic metre of air contains 5 x 10 9m3 of S02. We can convert this to moles quite easily because a mole of gas occupies 

0245 m3 at 15°C and atmospheric pressure. Thus our cubic metre of air contains 5 x 10 9/0.0245 = 2.04 x 10 7mol of S02. In a rain-laden cloud we can expect one cubic metre to contain about 1 g of liquid water, 

If the S02 were all removed into the droplet and oxidized to sulphuric acid 

Thus the proton activity will be 4.08 x lO moll, or the pH 3.4. Evaporation of water from the droplet and removal of further S02 as the droplet falls through air below the cloud can lead to even further reduction in pH. 

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Because of the expanded scale and need to describe additional physical and chemical processes, the development of acid deposition and regional oxidant models has lagged behind that of urban-scale photochemical models. An additional step up in scale and complexity, the development of analytical models of pollutant dynamics in the stratosphere is also behind that of ground-level oxidant models, in part because of the central role of heterogeneous chemistry in the stratospheric ozone depletion problem. In general, atmospheric Hquid-phase chemistry and especially heterogeneous chemistry are less well understood than gas-phase reactions such as those that dorninate the formation of ozone in urban areas. Development of three-dimensional models that treat both the dynamics and chemistry of the stratosphere in detail is an ongoing research problem. 

The chemistry of the stratospheric ozone will be sketched with a very broad brush in order to illustrate some of the characteristics of catalytic reactions. A model for the formation of ozone in the atmosphere was proposed by Chapman and may be represented by the following "oxygen only" mechanism (other aspects of... 

Nitrogen oxides also play a significant role in regulating the chemistry of the stratosphere. In the stratosphere, ozone is formed by the same reaction as in the troposphere, the reaction of O2 with an oxygen atom. However, since the concentration of O atoms in the stratosphere is much higher (O is produced from photolysis of O2 at wavelengths less than 242 nm), the concentration of O3 in the stratosphere is much higher. 

Even with our much-advanced understanding of the chemistry of the stratosphere, it appears that there are still some discrepancies between the calculated amount of ozone in the stratosphere and the amount measured. Toumi and Kerridge have summarized data showing that the range of calculated concentrations is some 10-15% below the range of measured... 

Chlorine nitrate and bromine nitrate are recognized as key species in the chemistry of the stratosphere. In... 

Before we discuss the chemistry of the stratosphere in detail, let us first briefly consider how chemicals emitted at the earth s surface are transported into the stratosphere and, conversely, how stratospheric species are transported into the troposphere. This issue is important as it determines which species end up in the stratosphere as we shall see, it is only those that survive for a sufficiently long time in the troposphere that are transported into the stratosphere. Thus, reactive organics (larger than CH4) from the troposphere do not survive to reach the stratosphere in appreciable quantities. Similarly, transport from the stratosphere to the troposphere is the major mechanism for removal of the products of the reactions of anthropogenic species that occur in the upper atmosphere. In addition, some 03 is injected into the troposphere, which is a natural source upon which anthropogenic production of 03 is imposed (see Chapter 16). 

The finding that the heterogeneous chemistry that occurs on polar stratospheric clouds also occurs in and on liquid solutions in the form of liquid aerosol particles and droplets in the atmosphere provided a key link in understanding the effects of volcanic eruptions on stratospheric ozone in both the polar regions and midlatitudes. As discussed herein, the liquid particles formed from volcanic emissions are typically 60-80 wt% H2S04-H20, and hence the chemistry discussed in the previous section can also occur in these particles (Hofmann and Solomon, 1989). We discuss briefly in this section the contribution of volcanic emissions to the chemistry of the stratosphere and to ozone depletion on a global scale. For a brief review of this area, see McCormick et al. (1995). 

Additional sulfates continue to form after the eruption as gaseous S02 is oxidized to sulfuric acid and sulfates. While we shall focus here on the effects of these sulfate particles on the heterogeneous chemistry of the stratosphere, there may be other important effects on the homogeneous chemistry as well. For example, model calculations by Bekki (1995) indicate that this oxidation of S02 by OH leads to reduced OH levels, which alters its associated chemistry. 

In summary, the chemistry of the stratosphere and the effects of anthropogenic perturbations on it have a rich history, with new chemistry that continues to unfold. For reviews of various aspects of the chemistry and history, see Cicerone (1981, 1987), Rowland (1989, 1992,1993), Molina (1991), Rowland and Molina (1994), Toohey (1995), Brasseur et al. (1995), chapters by Li et al. (1995a), Anderson, and Sander et al. in the book edited by Barker (1995), chapters by Brune, Middle-brook and Tolbert, Wilson, and Brasseur et al. in the book edited by Macalady (1998), and the World Meteorological Organization (WMO) 1995 and 1999 reports Scientific Assessment of Ozone Depletion. ... 

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