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It is useful to focus on four regions of the atmospheric boundary layer, each of which has a distinct mix of anthropogenic and natural VOC and NOx emissions:

• The urban-suburban atmosphere, which is the area most strongly affected by anthropogenic emissions

• The rural atmosphere, which is somewhat less affected by anthropogenic emissions and more affected by natural emissions than is the urban-suburban atmosphere

• The atmosphere over remote tropical forests, which is essentially free of anthropogenic VOC and NOx emissions and strongly affected by natural emissions

• The remote marine atmosphere, which is not only free of anthropogenic emissions, but also has relatively small biogenic sources of VOCs and NOx

Because we are most interested in the conditions that foster episodes of high concentrations of ozone, our discussion concentrates on observations made during the daylight hours of the summer months. In the sections below, we first briefly examine the typical concentrations of ozone in these four regions and then turn to the more complex issues of NOx and VOC concentrations and their variability.

Observations of Ozone

Compared with those for NOx and VOCs, the data base of ozone observations is fairly extensive, especially for urban and suburban areas. At most rural surface sites, ozone concentrations have been found to vary over a diurnal cycle with a minimum in the early morning hours before dawn and a maximum in the late afternoon (Figure 8-1). This pattern is believed to result from daytime photochemical production or downward transport of ozone-rich air from above, combined with ozone loss by dry deposition and reaction with nitric oxide (NO) at night, when photochemical production ceases and vertical transport is inhibited by an inversion of the normal temperature profile. In locations near large sources of NO, the nighttime minimum in ozone can be quite pronounced because of the rapid reaction between ozone and NO. In fact, in many urban areas the NO source is strong enough to cause the complete nighttime disappearance of ozone. A somewhat different pattern has been observed at high-altitude sites (i.e., sites located 1 km or more above the local terrain). At these sites, ozone often exhibits a shallow maximum rather than a minimum at night (Figure 8-1b). This diurnal pattern is thought to

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