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VOC (ppbC)

FIGURE 16.15 Isopleths of the calculated daytime average rates of net ozone production (ppb h_1) as a function of initial VOC and NOA.. The model-predicted changes in VOC and NOA. concentrations over an 8-h time period as the air mass is transported and reacts are shown by the arrows (adapted from Milford et al., 1994).

ford et al., 1994). As the air parcels age, they move toward higher VOC/NOx ratios, i.e., from the VOC-limited regime to the NOx-limited regime. The reasons for this are that oxides of nitrogen are removed more rapidly from the atmosphere and, at the same time, biogenic emissions contribute to the VOC concentrations. (Biogenic emissions of NOx are small compared to anthropogenic emissions in urban areas; for example, Aneja et al. (1997) estimate that less than 1% of the total NOx in Raleigh, North Carolina, is due to natural emissions.) As a result, the relative effectiveness of VOC versus NOx controls is expected to change as the air parcel moves downwind and ages.

Because of this increase in the VOC/NOx ratio as the air mass moves downwind, isopleths appropriate for one portion of an air basin, e.g., the upwind portion, will not necessarily be applicable to other regions, such as the downwind areas. The development of sophisticated grid-based models and their application to entire air basins have allowed modelers to include continuing emissions, transport, and transformation processes (see Section A.3).

For example, Fig. 16.16 shows the results of modeling by Milford et al. (1989) to examine the effects of control of VOC and NOx on ozone under conditions typical of the base case, August 30-31, 1982, at several locations in southern California. The results for two of the sites modeled, downtown Los Angeles (DTLA) and Rubidoux, about 100 km east and downwind, are presented in the form of two-dimensional isopleths analogous to those in Fig. 16.f4a as well as three dimensions analogous to Fig. 16.f4b. In this case, points A and B represent the situation at these two locations in 1982, and the isopleths give the predicted changes in ozone resulting from various percentage reductions in basin-wide emissions of NOx and VOC from the base case.

in the Los Angeles area during the day, the winds are predominantly from the west, carrying the pollutants inland. At night the direction is reversed and "aged" air masses are carried out over the Pacific Ocean, only to return the following morning carrying enhanced levels of OH sources such as nitrous acid (HONO) and formaldehyde. As a consequence, under multiday stagnant meteorological conditions, VOC oxidation and the NO to N02 conversion are enhanced and increasingly high peak 03 levels are reached on successive episode days.

Figure 16.16 shows that in DTLA, which is most heavily impacted by direct mobile source emissions and has relatively low VOC/NOx ratios, VOC control is predicted to be most effective. Thus, starting at the current emissions marked as point A, controlling VOC is clearly most effective in rolling down the ozone hill. Reducing NOx in the absence of VOC control from

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