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FIGURE 14.20 Monthly average 03 concentrations at Mon-calieri, Italy; Montsouris, France; and Zagreb, Croatia in the 1868-1900 period and Arkona (Baltic Sea) in 1983 (adapted from Sandroni et al, 1992).

the reactions of precursors transported from the boundary layer.

In short, the concentrations of tropospheric ozone, which is also a greenhouse gas, have also increased over the past century, an increase attributed to increased oxides of nitrogen emissions associated with fossil fuel combustion (e.g., see Volz and Kley, 1988; and Janach, 1989).

The potential effects of ozone on climate are particularly complex in that the net effect is very sensitive to the vertical distribution profile, with changes at the tropopause having the largest impact (Wang et al., 1980; Lacis et al., 1990). The reason for this is that 03 near the ground is at temperatures close to those of the earth's surface. As a result, emission and absorption are occurring at essentially the same temperature, resulting in no contribution to the greenhouse effect. However, because the temperature falls with altitude up to the tropopause, the Boltzmann distribution (Eq. (A)) shifts to smaller relative populations in the excited states. Thus, as discussed earlier, the net emission from 03 becomes smaller relative to absorption. While the same is true for other greenhouse gases such as C02 and CH4, their sources and sinks are such they are relatively well mixed in the atmosphere and their vertical distributions are not subject to the variability associated with 03. Another important difference is that 03 absorbs strongly in the UV as well, which leads to heating in the stratosphere, in contrast to C02, which cools it. Thus, changes in the concentrations of ozone and its vertical distribution affect not only infrared but also solar UV radiation, with associated effects on climate (see also Section B.3b).

These phenomena are illustrated in Fig. I4.2fa, which shows a model calculation of the change in global surface temperature when fO Dobson units (DU) of 03 (10 DU = total column 03 equivalent to a layer of thickness 0.1 mm at 273 K and a pressure of 1 atm; see Chapter 12.A) are added one at a time to each of 33 vertical layers of the atmosphere (assuming no feedbacks). An increase in the global surface temperature is predicted when the ozone is added in layers up to

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