Ozone isopleths

Fig. 2. Examples of ozone isopleths generated using four different chemical mechanisms (a) EPA (b) ESM (c) CBII (d) ELSTAR NMOC = nonmethane...

FIGURE 4-18 Ozone isopleths (vertical cross section) from southern Caltfomta coast air basin survey, August 11, 1971. Reprinted with permission from Gloria et al. ... 

FIGURE 16.14 Typical peak ozone isopleths generated front initial mixtures of VOC and NO, in air. (a) Two-dimensional depiction generated from the EKMA model (Dodge, 1977a) (b) three-dimensional depiction prepared by B. Dickerson. The VOC-liniited region (e.g., at point D) is found in some highly polluted urban centers while the NOA (e.g., at point A) is typical of downwind suburban and rural areas (adapted from Finlayson-Pitts and Pitts, 1993). 

A second reason for the response of 03 to NOx at low VOC/NOx ratios is that at these high NOx concentrations, N02 competes with VOC for the OH radical by forming HN03 (reaction (13)). This terminates the chain oxidation of VOC and removes N02 from the system without forming 03. This chemistry has been confirmed by direct OH radical measurements thus, Eisele et al. (1997) report that OH concentrations increase with the NO concentration up to l-2 ppb but decrease thereafter due to the OH + N02 reaction. This is consistent with model calculations of isopleths of OH concentrations as a function of the VOC/NOx ratio, similar to the ozone isopleths in Fig. 16.14 (e.g., Kley, 1997) in the high NOx-low VOC... 

FIGURE 16.16 Peak ozone isopleths calculated for downtown Los Angeles (DTLA) and Rubidoux, approximately 100 km east and downwind of DTLA under typical meteorological conditions. Spatially uniform reductions of VOC and NOx were employed in an airshed model by Milford el at. (1989). The top shows isopleths in two dimensions as presented by Milford et at. (f989), and the bottom shows these data extrapolated to three dimensions (from Finlayson-Pitts and Pitts, f993). 

The kinetic modeling nomenclature arises from the incorporation of chemical kinetic submodels in EKMA. The empirical term comes from the use of observed 03 peaks in combination with the model-predicted ozone isopleths to develop control strategy options. Thus, the approach historically was to use the model to develop a series of ozone isopleths using conditions specific for that area. The second highest hourly observed 03 concentration and the measured... 

Figure 16.26, for example, shows ozone isopleths predicted for the Los Angeles area using the CIT Eulerian model under two different assumptions for the boundary and initial conditions (Winner et al., 1995). In Fig. 16.26a, the pollutant concentrations measured in the past were used to set the boundary and initial conditions. In Fig. 16.26b, concentrations measured in clean air over the Pacific Ocean upwind of the modeled area were used instead. The axes are given in terms of the percentage of VOC or NOx emissions compared to the baseline level on August 28, 1987, rather than in terms of absolute concentrations as for... 

FIGURE 16.26 Effect of assumed boundary and initial conditions on ozone isopleths in the Los Angeles area for reductions in VOC and NOA from August 28, 1987 levels (a) using historical values of measured concentrations (b) clean air values for boundary and initial conditions (adapted from Winner et al., 1995). 

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... 

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]...

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... 

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. 

Figure 17.4.1. Ozone isopleth diagram showing the dependencies of ozone on varying levels of initial VOCs and NO,. Concentrations are given in ppb. [Adapted from M.S. Bergin et at, Enc. ofEnv. Analysis and Remediation, 29, 3029,(1998)]...

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... 

Fig. 7.11 Ozone isopleths for the maximum concentration of O3 obtained by (a) a smog chambtr experiment after long-time irradiation to reach ultimate maximum (Sakamaki et al. 1982), and (b) EKMA model with limited-time irradiation to simulate daily ozone (Dodge 1977)...

Sakamaki, F., Okuda, M., Akimoto, H., Yamazaki, H. Computer modeling study of photochemical ozone formatirai in the propene-nitrogen oxides-dry air system. Generalized maximum ozone isopleth. Environ. Sd. Technol. 16,45-52 (1982)... 

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