Rethinking the Ozone Problem in Urban and Regional Air Pollution (1991) / Chapter Skim
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6 VOCs and NOx: Relationship to Ozone and Associated Pollutants
6 VOCs and NOx: Relationship to Ozone and Associated Pollutants
Pages 163-186
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From page 163... ...
Because ozone isopleth diagrams are a concise way to depict the effect of reducing initial VOC and NOx concentrations on the peak ozone concentrations, they have been used quantitatively to develop control strategies for ozone reduction by the U.S. Environmental Protection Agency's EKMA (empirical kinetic modeling approach)
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From page 164... ...
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.
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From page 165... ...
It is useful to consider two areas on the graph: those to the right of the ridge line and those to the left. The variation of peak ozone concentration with the VOC/NOx ratio can be explained on the basis of the atmospheric chemistry discussed in Chapter 5 and summarized as follows:
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From page 167... ...
At VOC/NOx ratios to the left of the ridge line (characteristic of some highly polluted urban areas) lowering VOC at constant NOX results in lower peak ozone concentrations; this is also true if VOCs and NOX are decreased proportionately and at the same time.
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From page 168... ...
Reducing the VOCs reduces aldehyde production, resulting in smaller RO2 and HO2 radical concentrations, which lowers the rate of NO-toNO2 conversion by Reaction 6.9. The increase in peak ozone concentration at relatively low VOC/NOX ratios that occurs when NOX is reduced has been a major issue in the development of ozone control strategies.
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From page 169... ...
All reactions of a certain class of VOCs may be represented by those of a single species, or VOCs may be segmented according to the kinds of carbon bonds in the molecules. Because different mechanisms use somewhat different approximations in lumping the VOC chemistry, the ozone concentrations predicted for a given set of initial conditions by different chemical mechanisms will not agree exactly, and the resulting isopleths can differ.
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From page 170... ...
Sensitivity to VOC Composition Because different VOCs show widely varying reactivities in terms of ozone formation (see Chapter 5) , the peak ozone generated in a given VOC-NOX mixture and hence the shapes of the ozone isopleths, particularly when the VOC/NOX ratio is low, are sensitive to the initial VOC composition.
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From page 171... ...
the change is from 160 to 120 ppb ozone. These reductions in the peak ozone concentration are expected because of the photochemical reactivity of aldehydes and their efficiency at generating free radicals.
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From page 172... ...
isopleths generated using the Lurman, Carter, and Coyner (LCC) mechanism and VOC compositions (including methane)
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From page 173... ...
Another limitation in the application of these isopleth diagrams lies in the difficulty of selecting the appropriate VOC/NOX ratio in defining the base-year point on the peak ozone isopleth. VOC/NOX ratios are discussed in detail in Chapters 8 and 11.
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From page 174... ...
It must be concluded that the concept of a single VOC/NOX ratio for an
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From page 175... ...
Figure 6-4 shows the effect of VOC and NOX controls on peak ozone formation in the Los Angeles air basin as a whole, irrespective of the location of the peak. The isopleths in Figure 6-4 show that when the air basin as a whole is considered, as much as 80% control of VOCs alone will not result in attainment of the NAAQS; if biogenic VOC emissions were included, VOC control would be even less effective.
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From page 176... ...
regardless of lcoation in the Los Angeles air basin. A decrease in percent control along an axis corresponds to a higher concentration of a precursor in the atmosphere.
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From page 177... ...
Nitrates from nitric acid, HNO3, are a significant component of acid aerosols, which are under consideration for inclusion on the list of criteria air pollutants (Lipfert et al., 1989~. These particles are typically in the respirable 0.1-1.0 micrometer (pm)
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From page 178... ...
The increase in the al for nitrate with year could result from the collection of gaseous nitric acid as a filter artifact in earlier studies (Appel et al., 1985~. The most recent studies, in which this artifact should be minimized, show that, on a mass basis, nitrate is of comparable importance to sulfate with respect to light scattering.
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From page 179... ...
a) ~ 0 ._ ~ ' 1 con ~ ~ O _' 2 MONO 0600 o3 HCHO 1000 Time ~ 400 FIGURE 6-5 Predicted sources of HO radicals as a function of time of day for a typical polluted urban atmosphere.
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From page 182... ...
Peroxyacetyl Nitrate PAN is formed from the reaction of acetylperoxy radicals with NO2: O O 11 11 CH3COO + NO2 ~ C~COONO2 (6~22) PAN thermally decomposes over the range of temperatures found in the atmosphere, reforming acetylperoxy radicals and NO2.
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From page 183... ...
As discussed above, hydrogen abstraction reactions of NO3 form nitric acid, as does the hydrolysis of N2Os, which is a major source of nitric acid in the atmosphere (Russell et al., 1985~. NO3 works in the nighttime oxidation of naturally produced organic compounds, such as isoprene and the pinenes, as well as dimethylsulfide and methyl mercaptan (Finlayson-Pitts and Pitts, 1986~.
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From page 184... ...
Finally, several inorganic halogen-containing species, including ClNO, BrNO, ClNO2, and BrNO2, can be formed in coastal marine areas by the reactions of NO2, N2Os, and possibly NO3 with the components of sea salt (Finlayson-Pitts, 1983; Finlayson-Pitts et al., 1989a,b, 1990; Livingston and Finlayson-Pitts, 1991~. These species are highly labile photochemically, and could play a role in ozone formation, in the case of the chlorine compounds, or ozone destruction, in the case of the bromine derivatives.
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From page 185... ...
Secondary Organic Particles and Acids Recent atmospheric measurements suggest that organic acids constitute a significant fraction of total acidity, even in areas such as Los Angeles, which has large NOX emissions (Keene et al., 1983; Grosjean, 1989, 1990; Grosjean et al., 1990a; Grosjean and Parmar, 1990~. The sources are not well established, but both direct emissions from mobile sources and atmospheric oxidation of VOCs (reactions of Criegee biradicals from ozone-alkene reactions, for example)
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From page 186... ...
Because the VOC/NOX ratio generally increases as an air mass moves downwind from major NOX sources, control strategies derived from the isopleths for upwind locations often are inappropriate for downwind areas within the same air basin. This problem has recently been overcome by applying three-dimensional urban airshed models to generate ozone isopleth diagrams for some air basins where the requisite detailed model input is available.
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