1700 1750 1800 1850 1900 1950 2000 Year

FIGURE 14-19 Fitted curve for atmospheric N20 concentrations from 1735 to 1991 obtained from ice cores in Antarctica (adapted from Machida et al., 1995).

(Machida et al., 1995). The rate of growth has been variable, averaging 0.8 + 0.2 ppb per year from 1977 to 1988 (Khalil and Rasmussen, 1992), but was only 0.5 ppb per year in 1993 (IPCC, 1996).

As discussed in other chapters of this book and summarized in Chapter f6, the formation of tropospheric ozone from photochemical reactions of volatile organic compounds (VOC) and oxides of nitrogen (NOx) involves many reactions. Concentrations are therefore quite variable geographically, temporally, and altitudinally. Additional complications come from the fact that there are episodic injections of stratospheric 03 into the troposphere as well as a number of sinks for its removal. Because 03 decomposes thermally, particularly on surfaces, it is not preserved in ice cores. All of these factors make the development of a global climatology for 03 in a manner similar to that for N20 and CH4, for example, much more difficult. In addition, the complexity of the chemistry leading to 03 formation from VOC and NOx is such that model-predicted ozone concentrations can vary from model to model (e.g., see Olson et al., 1997).

Shortly after the discovery of ozone by Schonbein in 1839, measurements of this newly discovered atmospheric gas were initiated in a number of locations around the world, including Europe, South America, and North America (e.g., see Bojkov, 1986; Volz and Kley, 1988; McKeen et al., 1989; Anfossi et al., 1991; Sandroni et al., 1992; and Marenco et al., 1994). Although these very early measurements used wet chemical techniques (see Chapter 11), potential interferences can be estimated to make some approximate corrections to the data. Typical annual variations in tropospheric 03 observed at Moncalieri in Italy from 1868 to 1893, Montsouris, France, from 1876 to 1886, and Zagreb, Croatia, in 1900 are compared in Fig. 14.20 to more recent data (1983) at Arkona, on an island believed to be relatively remote, in the Baltic Sea (Sandroni et al., 1992). All of the early measurements peak around 10 ppb, whereas 30-40 ppb is a typical tropospheric 03 concentration found essentially everywhere in the world today. Global increases in ozone have also been documented over more recent times, although the geographic distribution and temporal changes are complex (e.g., see Volz et al., 1989; Janach, 1989; Lefohn et al., 1992; Low et al., 1990; Logan, f 994; Jiang and Yung, 1996; and Stockwell et al., 1997). Berntsen et al. (1997) carried out model calculations that showed that significant radiative effects must have resulted from changes in 03 since preindustrial times in the upper troposphere due to in situ formation from

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