Encyclopedia of Environmental Science and Engineering, Volume I and II

126 ATMOSPHERIC CHEMISTRY

This concept of changing the VOC mixture is the basis

for the use of reformulated or alternative fuels for the reduc-

tion of ozone production. Oxygenated fuel components,

such as methanol, ethanol, and methyl t-butyl ether (MTBE),

generally have smaller incremental reactivities than those

of the larger alkanes, such as n-octane, which are more

characteristic of the fuels used in automobiles. The use of

these fuels would be expected to reduce the reactivity of the

evaporative fuel losses from the automobiles, but the more

important question is how they will change the reactivity of

the exhaust emissions of VOCs. The data that are currently

available suggests that there should also be a reduction in the

reactivity of the exhaust emissions as well.

Ozone Isopleths

Ozone production depends on the initial amounts of VOC

and NO x in an air mass. Ozone isopleths, such as those

shown in Figure 5, are contour diagrams that provide a con-

venient means of illustrating the way in which the maximum

ozone concentration reached over a fixed irradiation period

depends on the initial concentrations of NO x and the initial

concentration of VOCs. The ozone isopleths shown in Figure

5 represent model results for Atlanta, using the Carbon Bond

4 chemical mechanism (Seinfeld, 1995). The point on the

contour plot represents the initial conditions containing

600 ppbC of anthropogenic controllable VOCs, 38 ppbC

of background uncontrollable VOCs, and 100 ppb of NO x.

These conditions represent morning center-city conditions.

The calculations are run for a 14-hour period, as chemistry

proceeds and the air mass moves to the suburbs, with associ-

ated changes in mixing height and dilution. The air above the

mixing layer is assumed to have 20 ppbC VOC and 40 ppb

of O 3. The peak ozone concentration reached in the calcula-

tion is about 145 ppb, as indicated at the point. The isopleths

arise from systematically repeating these calculations, vary-

ing the initial VOC and initial NO x with all other conditions

the same.

The base case corresponds to the point, and the horizon-

tal line represents a constant initial NO x concentration. At a

fixed initial NO x , as one goes from the point to a lower initial

VOC, the maximum O 3 decreases, while increasing the initial

VOC leads to an increase in the maximum O 3 concentration

until the ridge line is reached. The ridge line represents the

VOC-to-NO x ratio that leads to the maximum ozone produc-

tion at the lowest concentrations of both VOC and NO x. The

region of the isopleth diagram below the ridge line is referred

to as the NOx -limited region; it has a higher VOC:NO x ratio.

The region of the diagram above the ridge line is referred to

as the VOC-limited region; it has a lower VOC:NO x ratio. In

200

160

120

80

40

0 400 800 1200 1600 2000

Initial VOC, ppbC

Initial NO

, ppbx

180

140

FIGURE 5 Ozone isopleth diagram for Atlanta, Georgia. Adjacent ozone isopleth lines are

10 ppb different. The point on the constant NOx line represents the base case. From Seinfeld

(1995). With permission.

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