nil 1,000

OH reactivity adjusted VOC (ppbC)

FIGURE 16.38 Observed NOA and OH-reactivity-adjusted VOC (expressed as propene) in various regions of the troposphere. Isopleths shown are midday rates of 03 production (ppb h-1) calculated using a box model (adapted from Chameides et al., 1992).

in Fig. 16.38 indicate a strong relationship between 03 and NOx, but little with the reactivity-adjusted VOC concentrations in continental areas. For example, while the reactivity-adjusted VOC increases by almost two orders of magnitude from the remote marine region to the tropical forest sites due to biogenic organic emissions, 03 (and NOx) remain approximately constant. These trends are similar to the isopleths predicted using a simple photochemical box model and suggest that in remote and rural regions, 03 is most sensitive to NOx control, whereas in polluted urban-suburban areas, it can be sensitive to either NOx or VOC control depending on the particular conditions, i.e., which side of the ridge line one is on in the top right corner of Fig. 16.14.

Given our knowledge of the detailed chemistry leading to 03 formation from VOC and NOx (see Chapters 6 and 7), one might expect that any NOx that is oxidized will form either 03 or other oxidized nitrogen compounds such as PAN, HN03, and N205. Trainer and co-workers (1993) showed that there was indeed a linear relationship between the amount of reacted oxides of nitrogen, expressed as NOz = (NO>( — NOx) (see Chapter 7 for definitions of NOz, NO , and NOx), and 03, with the slope giving 8.5 ppb of 03 per ppb of NOx oxidized. This slope, i.e., the number of molecules of 03 generated per molecule of NOx oxidized, is known as the ozone production efficiency.

Figure 16.39 shows a similar relationship measured at a site downwind of metropolitan Toronto, Canada (Roussel et al., 1996). The slope of this line gives 13.8 ppb of 03 formed per ppb of NOx oxidized. The

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