varied. However, for application to the atmosphere, such data need to be corrected, for example, for chamber-wall effects on the chemical reactions, the relatively high concentrations used, and the level of dilution. Although the ozone isopleth diagram was put forward by Haagen-Smit as an empirical representation of the VOC-NOx-O3 relationship, the chemistry that gives rise to the characteristic isopleth shape is now well understood. Isopleths are now generated by models that use photochemical reaction mechanisms, and they are tested against smog chamber data.
EKMA, which is largely being supplanted for use in ozone NAAQS attainment demonstration by grid-based models, simulates urban ozone formation in a hypothetical box of air that is transported from the region of most intense source emissions (a center city, for example) to the downwind point of maximum ozone accumulation. Emissions of VOCs and NOx are assumed to be well mixed in the box, which varies in height, to account for dilution caused by changes in the height of the mixed layer of air; ozone formation is simulated using a photochemical mechanism. By simulating an air mass as a box of air over its trajectory for a large number of predetermined combinations of initial VOC and NOx concentrations, EKMA generates ozone isopleths that are, to varying degrees, specific to particular cities. Once the maximum measured ozone concentration in a city has been identified, the VOC and NOx reductions needed to achieve the National Ambient Air Quality Standard (NAAQS) are determined in EKMA from the distances along the VOC and NOx axes to the isopleth that represents the 120 ppb (parts per billion) peak ozone concentration mandated by the NAAQS.
The location of a particular point on the ozone isopleth is defined by the ratio of the VOC and NOx coordinates of the point, referred to as the VOC/ NOx ratio. Figure 6-1 shows that the shape of the ozone isopleths depends on the VOC/NOx ratio. (The lines in Figure 6-1 correspond to VOC/NOx ratios of 15, 8, and 4 ppb carbon (C)/ppb.) The 0.32 parts per million (ppm) (320 ppb) ozone isopleth, for example, spans a wide range of VOC/NOx ratios. As a result, the degrees of VOC and NOx reductions required to move from the 320 ppb isopleth to the 120 ppb isopleth vary considerably depending on the VOC/NOx ratio at the starting point on the 320 ppb isopleth.
The VOC/NOx ratio is important in the behavior of the VOC-NOx-O3 system. Moreover, it has a major effect on how reductions in VOC and NOx affect ozone concentrations. This chapter examines how ozone isopleths depend on the VOC/NOx ratio. Chapter 8 discusses the data on ambient VOC/NOx ratios in urban, suburban, and rural areas of the United States, and Chapter 11 addresses the implications of these ratios for the effectiveness of VOC and NOx control in reducing ozone.