Ozone isopleth plot

The dependence of 03 production on the initial amounts of VOC and NOx is frequently represented by means of an ozone isopleth diagram. Such a diagram is a contour plot of maximum 03 concentrations achieved as a function of initial VOC and NOx concentrations. The diagram is generated by contour plotting the predicted ozone maxima obtained from a large number of simulations with an atmospheric VOC/NOx chemical mechanism with varying initial concentrations of VOC and NOx while all other variables are constant. 

FIGURE 6.10 Ozone isopleth plot based on simulations of chemistry along air trajectories in Atlanta (Jeffries and Crouse 1990). Each isopleth is lOppb higher in 03 as one moves upward and to the right. 

The ozone ridge line identifies the maximum 03 concentration that can be achieved at a given VOC level, allowing the NO level to vary. If the NO level is either increased or decreased relative to that at the ridge line, the maximum amount of 03 produced at that VOC level will decrease. The region of the ozone isopleth plot below the ridge line is what 

Relation of 03 to NO, From the definition of the ozone production efficiency, the signature of an NO molecule lost is the appearance of a number of 03 molecules, the specific number depending on atmospheric conditions and the HC and NO levels. Thus, the 03 concentration attained in an airmass should be correlated with the quantity [NO,]-[NO ], which is the total concentration of products of NO oxidation (HNO3, PAN, etc.). That this correlation should exist in airmasses was first pointed out by Trainer (1991, 1993), and it has been subsequently pursued in numerous studies [e.g., Kleinman (1994, 1997), CarpenteT et al. (2000)]. To obtain a good correlation between [03] and [NO,]-[NO ], O3 production must have occurred within a day or so in the airmass, before significant removal of NOv can take place, for example by wet and dry deposition of HN03. 

Kleinman et al. (1994) suggested the linear least-squares correlation 

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The EPA s OZIPR model (Ozone Isopleth Plotting Program, Research Version) was designed to be used primarily for calculating emissions reductions needed to reach the NAAQS for 03. Both DOS-based and Windows versions are available and can be downloaded from the Web site http //www.academicpress.com/ pecs/download... 

Figure 6.12 shows isopleths of the maximum 03 mixing ratio achieved over a 10-h period generated from the mechanism in Table 6.3. We see that the simple mechanism in Table 6.3 is able to produce the characteristic features of the ozone isopleth plot in Figure 6.10, based on a much more complex mechanism. 

A simple explanation of why the trends are more pronounced for the stations with the highest initial episodic peak levels would be that these stations show the highest ozone responses to VOC and NOx emission reductions. Whilst this is certainly feasible, it is not a common finding in photochemical ozone modelling. Ozone responses to changing VOC and NOx emissions in photochemical models are usually illustrated as isopleth plots. Ozone isopleths are commonly seen as a set of parallel curves, see Fig. 5 for one such isopleths diagram constructed from a European photochemical trajectory model [23]. It is not at all common for the isopleths to become closer together as would be implied by the ozone responses to... 

Fig. 5 Ozone isopleth diagram for 18th July 2006 at the EMEP GB0036R Harwell station plotted from the results of VOC and NOx emission sensitivity experiments performed with a Photochemical Trajectory Model [23]...

Isopleth plots such as Figure 3 are commonly used to show ozone concentrations as a function of NOjc and VOC emission rates. Representations of ozone concentrations versus emission rates are always critically dependent on assumptions about rates of vertical mixing and the time allowed for... 

The two-dimensional plot showing the dependence of maximum ozone concentration on the initial concentrations of NOx and VOC is called ozone isopleths . Usually, VOC concentration is taken as abscissa and NOx concentration as ordinate as in Fig. 7.1 la, b. Figure 7.10a is the plot of the data of Fig. 7.9 obtained in the... 

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