Although there is sometimes a tendency to think of the chemistry of the "natural" troposphere as somehow different from that of more polluted areas, it is essen tially the same VOC-NOx chemistry described earlier in this chapter. However, given that there are the significant biogenic sources of a wide variety of organ-ics, the major difference lies in the NOx levels, which are much lower in remote regions.
Because of these lower concentrations of NO, the self-reactions of HOz and R02 radicals can become competitive with NO reactions:
Because reactions (11), (12), and (29) do not oxidize NO to N02, they do not result in the formation of 03 (via N02 photolysis). Indeed, an additional reaction of H02, that with 03,
actually results in the destruction of ozone.
As developed in more detail in Chapter 6, the NO concentration at which other reactions such as (11), (12), and (29) occur at approximately the same rate as the H02/R02 + NO reaction is in the 10-50 ppt range. These concentrations are sufficiently low that they are encountered only in remote atmospheres, where the influence of anthropogenic emissions is minimized.
However, it is noteworthy that on a global level, 03 levels have been increasing significantly over the past century, coinciding with the increase in fossil fuel usage and the associated increase in NOx emissions (Bojkov, 1986; Volz and Kley, 1988; McKeen et al., 1989; An-fossi et al., 1991; Sandroni et al., 1992; Marenco et al., 1994; Anfossi and Sandroni, 1997). Figure 1.6 shows that 03 levels appear to have been about 10-15 ppb a century ago, compared to 30-40 ppb measured around the world today. This is consistent with a shift in the
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