Gas-phase chemistry in remote areas is, in most cases, analogous to that in more polluted regions. The major difference is in lower NOx emissions and hence concentrations. In addition, in continental regions, there are substantial emissions of biogenic organics, many of which are highly reactive toward OH, 03, N03, and CI atoms and in oceanic regions, dimethyl sulfide (DMS), which reacts with OH, N03, and CI atoms.
As discussed briefly in Chapter 1 and in more detail in Chapter 14, it is unlikely that there are any regions at the earth's surface that have not been significantly impacted by anthropogenic emissions. Even over the central Atlantic and Pacific Oceans, for example, significant contributions to the chemistry from anthropogenic emissions are often observed (e.g., Parrish et al., 1993b; Dickerson et al., 1995). One means of testing for anthropogenic emissions is the use of a correlation between CO and 03. CO is generated primarily from combustion processes in vehicles and industrial processes (e.g., Fig. 2.7 and Table 2.1), and its chemistry is the relatively slow reaction with OH (lifetime of ~ 90 days at an OH concentration of 5 X fO5 cm"3). As a result, as an air mass containing VOC and NOx ages and undergoes the photochemical reactions discussed earlier, 03 is formed. Such air masses therefore often have both increased 03 and CO, with A/A[C0] ~
0.3-0.4 being typical (e.g., Dickerson et al., 1989; Chin et al., 1994; Wang et al., 1996; Harris et al., 1998; Parrish et al., 1993b, 1998; Kajii et al., 1998). This relationship is impacted, of course, by other factors as well such as the production of CO in the oxidation of VOC and by the deposition of 03 during transport, for which corrections can be estimated (e.g., Chin et al., 1994).
In this section, we discuss briefly the distinguishing chemistry associated with remote regions of the troposphere, including continental and marine areas, focusing primarily on regions of minimal anthropogenic influence. There is increasing evidence of interesting chemistry in the upper troposphere, which is also discussed. Finally, we briefly treat some unusual chemistry occurring in the Arctic, which is characterized by long periods of darkness and low temperatures during winter followed by long periods of sunlight.
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