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Isopleth diagrams are a convenient means of representing the complex relationship between initial concentrations of volatile organic compounds (VOCs) and oxides of nitrogen (NOx) and peak concentrations of ozone subsequently formed via chemical reactions in the troposphere. Ozone isopleth diagrams generated for urban areas using laboratory experiments or models (either EKMA [empirical kinetic modeling approach] or three-dimensional, grid-based models) show that the extent of reduction of peak ozone concentrations resulting from reductions of precursor emissions depends on the initial VOC/NOx ratio. At higher VOC/NOx ratios (greater than ˜8-10), ozone concentrations are relatively insensitive to VOC concentrations, and NOx control is more effective in lowering ozone. Measurements of 6:00 a.m. to 9:00 a.m. average VOC and NOx show that most urban areas appear to fall into this category, with measured image (see chapter 8). At VOC/NOx ratios less than ˜8-10, found in some highly polluted urban areas, such as Los Angeles, lowering VOC reduces ozone, whereas NOx control might actually increase ozone at some locations.

Isopleth diagrams generated in the traditional manner, using EKMA, have several shortcomings, especially their failure to treat the effects of VOC and NOx controls throughout an airshed. Because the VOC/NOx ratio generally increases as an air mass moves downwind from major NOx sources, control strategies derived from the isopleths for upwind locations often are inappropriate for downwind areas within the same air basin.

This problem has recently been overcome by applying three-dimensional urban airshed models to generate ozone isopleth diagrams for some air basins where the requisite detailed model input is available. However, because of the limitations associated with developing information for urban airshed models (see Chapter 10), generating such isopleth diagrams for all regions in the United States is not now feasible. The relationship between VOC and NOx control and ozone concentrations, as determined by three-dimensional models, is discussed further in Chapter 11.

Changes in VOCs and NOx will, because of their complex chemical interactions, also lead to changes in a variety of other pollutants associated with ozone, such as nitric acid, peroxyacetyl nitrate, nitrogen dioxide, and aerosol particles. Some of these pollutants have known harmful effects on human health and welfare. Hence, it is important to recognize that control strategies implemented for ozone will simultaneously affect other species.

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