Ozone-Forming Potential of Reformulated Gasoline (1999) / Chapter Skim
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2 Ozone Photochemistry
2 Ozone Photochemistry
Pages 23-32
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From page 23... ...
It turns out that the rate of ozone formation is a complex and variable function of the concentrations of VOC and NOx as well as meteorological conditions. As a result, establishing the relative benefits of VOC and NOx emissions controls can be a difficult and challenging task.
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From page 24... ...
Ozone production occurs as a result of a series of reactions initiated by the oxidation of VOCs or CO by the hydroxy} radical (OH)
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From page 25... ...
. Therefore, ozone concentrations are most effectively reduced by lowering NOx emissions, and subsequent concentrations of NOx, instead of lowering emissions of VOCs.
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From page 26... ...
) calculated as a function of initial VOC and NOx concentrations and the regions of the diagram that are characterized by VOC limitation or NOx limitation.
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From page 27... ...
For each mixing rate, simulations for three initial VOC and NOX concentrations are presented: "Base" with initial VOC = 0.15 ppm and NOX= 1.5 ppm; "VOC/2" with initial VOC = 0.75 and NOX= 0.15 ppm; and "NOJ2" with initial NOX= 0.075 and VOC = 1.5 ppm. Note the characteristic tendency for the system to evolve from VOC limitation to NOX limitation with time and for the point of transition to be delayed as mixing decreases.
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From page 28... ...
Because NOx is processed and removed rapidly, the NOx disbenefit tends to be fairly short-lived. Moreover, as NOx concentrations continue to fall, the air parcel begins to move from VOC limitation to the transitional region and often reaches NOx limitation within many areas of the country.
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From page 29... ...
The distribution of NOx emissions can also affect where and if air parcels within a given airshed make the transition from VOC limitation. Like stagnation, the presence of dispersed NOx sources in a large metropolitan area or megalopolis can lead to high NOx concentrations throughout an area, fostering continuous VOC limitation.
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From page 30... ...
, with the major reaction pathway (~90% of the overall OH radical reaction) proceeding by OH + CH3CH2OH- H2O + CH3CHOH (2-1 O)
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From page 31... ...
in isolated large urban cores and similar source regions, ozone concentrations during severe air-pollution episodes are most effectively reduced by reductions in VOC emissions and might even increase as a result of NOx-emission controls; (2) ozone concentrations in rural areas and over large regional expanses are most effectively reduced by reductions in NOx emissions from the pollution sources that affect that area or region (e.g., upwind urban sources and important local sources)
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From page 32... ...
The concept of ozone-forming potential, discussed in the next chapter, attempts to account for the differing chemical characteristics of VOCs as they relate to ozone photochemical production.
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