Rethinking the Ozone Problem in Urban and Regional Air Pollution (1991) / Chapter Skim
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5 Atmospheric Chemistry of Ozone and Its Precursors
5 Atmospheric Chemistry of Ozone and Its Precursors
Pages 109-162
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From page 109... ...
The tropospheric chemistry of organic compounds of anthropogenic and biogenic origin, respectively, is discussed in detail, and the calculated tropospheric lifetimes of these compounds are presented. The formulation and testing of chemical mechanisms for use in urban and regional airshed computer models are discussed briefly, and the reactivities of organic compounds with respect to ozone formation, as calculated using these models, are discussed.
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From page 110... ...
The formation of OH radicals leads to cycles of reactions that result in the photochemical degradation of organic compounds of anthropogenic and biogenic origin, the enhanced formation of ozone, and the atmospheric formation of acidic compounds (see, for example, Heicklen et al., 1969; Stedman et al., 1970; Finlayson-Pitts and Pitts, 1986; WMO, 1986~. The generation of the OH radical from ozone is shown in the following reactions: O3 + he ~ O2 + Of D)
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From page 111... ...
It should be noted that knot depends on temperature, and hence on altitude, and that the OH radical concentration is temporally and spatially dependent. The lifetime of methane in the troposphere is long enough that a diurnally and annually averaged concentration of global tropospheric OH radical can be used to calculate the lifetime of methane (and of other similarly long-lived trace species)
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From page 112... ...
. Because the reactions of the CH302 radical with NO and NO2 have comparable rate constants for the temperatures and pressures encountered In the troposphere (Atkinson, 1990a)
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From page 113... ...
, and under these conditions the HO2 radical reaction to form methyl hydropero~nde, CH3OOH, is important. The subsequent reactions of CH3OOH under tropospheric conditions are photolysis and reaction with the OH radical (Ravishankara, 1988; Atkinson, 1989, 1990a,b)
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From page 114... ...
HCHO + 1.75 H2O The NOX concentrations in the atmospheric boundary layer over continental areas in the northern hemisphere are generally high enough that the reactions
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From page 115... ...
, and the calculated lower tropospheric lifetime of HCHO due to photolysis and, to a lesser extent, reaction with the OH radical is ~4 hours at the sun's zenith angle of 0° (Rogers, 1990~. The tropospheric removal of CO is by reaction with the OH radical, with a calculated lower tropospheric lifetime of ~2 months.
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From page 116... ...
(Here Jo is the diurnally, seasonally, and latitudinally dependent rate of photolysis of NO2, and k2 is the rate constant for Reaction 5.27.) For an NO2/NO concentration ratio of one, a reasonable mid-day value in the clean lower troposphere, and a temperature of 298 K, the resulting ozone concentration is ~5 x 10~ molecule/cm3 (20 parts per billion (ppb)
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From page 117... ...
other tropospherically important reactions involve oxides of nitrogen (Finlayson-Pitts and Pitts, 1986; WMO, 1986; Atkinson et al., 1989a; DeMore et al., 1990~. The recombination reactions M OH + NO - HONO M HO2+ NO2- HOONO2 (532)
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From page 118... ...
for NOX mixing ratios 20.1 ppb, and under these conditions the removal of OH radicals by Reaction 5.35 balances the formation of HOX sonnies of hydrogen) radicals from the photolysis of ozone and HCHO.
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From page 119... ...
. The homogeneous gas-phase reaction of N2Os with water vapor to form nitric acid N2OS ~ H2O - 2HONO2 (5-40, is slow enough that only an upper limit can be placed on the rate constant (Atkinson et al., 1989a; Hatakeyama and Leu, 1989)
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From page 120... ...
photolyzes to generate the OH radical HONO + ha - OH + NO (5.42) and this photolysis reaction is rapid (~10-3 s-l at a 0° zenith angle of the sun)
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From page 121... ...
Although in principle an extension of the chemistry of the clean, methane-dom~nated troposphere, the chemistry of the polluted troposphere, including urban and rural air masses, is significantly more complicated because of the presence of many VOCs of various classes (alkalies, alkenes, and aromatic hydrocarbons) and the added complexities in the chemistry of these organic species (see, for example, Atkinson, 1990a)
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From page 122... ...
TABLE 5-1 Calculated Tropospheric Lifetimes of Selected VOCs Due to Photolysis and Reaction with OH and NO' Radicals and Ozone Lifetime due to reaction with VOC OH NOR Methane ~ 12 yearsb > 120 years Ethane 60 days >12 years Propane L3 days >2.5 years e-Butane 6.1 days ~2.5 years e-Octane 1.8 days 260 days Ethene 1.8 days 225 days >4,500 years >4,500 years >4,500 years >4,500 years >4,500 years 9.7 days
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From page 123... ...
bFrom Vaghjiani and Ravishankara (1991~. CExpected to be of negligible importance.
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From page 124... ...
124 RETHINKING THE OZONE PROBLEM All of the tropospheric processes represented in Table 5-1 lead to the formation of organic Perot radicals (Rod. For example, for the reactions of OH and NO3 radicals with alkalies (Reactions 5.45 and 5.46)
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From page 125... ...
The difference is that for the RO2 radicals with more than two carbon atoms, the reaction with NO also can lead to the formation of organic nitrates, PRO + NO2 RO2- + NO ~ MONO (5.49) with this organic nitrate formation increasing with increasing pressure, decreasing temperature, and (for the n-alkane series)
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From page 126... ...
, and the kinetics and initial reaction mechanisms of these OH radical reactions are well understood (Atkinson, 1989~. The major pathway of the OH radical reaction involves initial OH radical addition to the aromatic ring to yield a hydroxycyclohexadienyl-type radical (Atkinson, 1989~.
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From page 127... ...
. Furthermore, the HCO radical formed from the photolysis and OH radical reaction of HCHO reacts with oxygen to form the HO2 radical and CO, HCO + O2 - HO2 + CO whereas the acyl (RCO)
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From page 128... ...
is 2.2, independent of temperature over the range ~280-320 K For the acetylperoxy radical, the ratio of the rate constant for Reaction 5.56a divided by the sum of rate constants for Reactions 5.56a and 5.56b is 0.67, independent of temperature (Moortgat et al., 1989)
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From page 129... ...
(5.62) where R', an alkyl radical with fewer carbon atoms than the parent RH alkane, then undergoes an analogous series of reactions that lead to the formation of carbonyl compounds (which react further in the atmosphere by photolysis and reaction with the OH radical)
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From page 130... ...
lead to conversion of NO to NO2, to the generation or regeneration of OH radicals, and to the formation of ozone. ATMOSPHERIC CHEMISTRY OF ANTHROPOGENIC VOCS The general features of the atmospheric chemistry of alkalies, alkenes, and aromatic hydrocarbons emitted from anthropogenic sources are understood, although there are still some significant uncertainties (Atkinson, 1990a)
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From page 131... ...
In competition with the formation of NO2 and the corresponding alkoxy radical, becomes increasingly important, and the alkyl nitrate formation yields at 298 K and 760 Torr total pressure increase from ~4% from propane to ~33% from e-octane (Carter and Atkinson, 1989a)
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From page 132... ...
The rate constants for the initial reactions of these species are reasonably well defined and the initial steps of the reaction mechanisms are known (Atkinson, 1990a)
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From page 133... ...
Aromatic VOCs The greatest uncertainties in the atmospheric chemistry of anthropogenic VOCs concern the aromatic compounds. The aromatic hydrocarbons react only with the OH radical under tropospheric conditions, by two pathways, one involving H-atom abstraction from the substituent groups (or, for benzene, from the aromatic ring C-H bonds)
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From page 134... ...
This finding, that the hydroxycyclohexadienyl-type radicals react rapidly with NO2 CH3 ~ HONO + CH3 + NO2 CH3 /\ (5.74) THEO + ~ NO2 Other products and only very slowly with O2, differs from the reaction sequences in the current chemical mechanisms of Gery et al.
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From page 135... ...
Unfortunately, there are several areas of uncertainty concerning the atmospheric chemistry of all of the carbonyl compounds other than formaldehyde and acetaldehyde. In particular, there is a need for data concerning the absorption cross-sections and photolysis products and the photodissociation quantum yields (as a function of wavelength)
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From page 136... ...
This reaction has a rate constant at 298 K of 3.3 x 10-~2 cm3/molecule-s (Atkinson, 1989~. The OH radical reaction can proceed by three channels (the percentages are for room temperature)
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From page 137... ...
ATMOSPHERIC CHEMISTRY 137 Under tropospheric conditions, the major reactions of these initially formed radicals are: for CH2CH2OH for CH3CHOH for CH3CH2O .
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From page 138... ...
+ O2 ~ CH3CHO + HO2 (5.85) At room temperature and 760 Torr total pressure of air in the presence of NO, the overall OH radical reaction is OH + CH3CH2OH + 0.05 NO+~.05 O2' H2O + 0.95 CH3CHO + 0.078 HCHO + (5.8O 0.011 HOCH2CHO + 0.05 NO2 Methyl t-butyl ether The only significant reaction under tropospheric conditions is with the OH radical, with a rate constant at room temperature of 2.8 x 10-~2 cm3/molecule-s (Atkinson, 1989~.
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From page 139... ...
Rate constants have been determined at room temperature for the gas-phase reactions of isoprene, a series of monoterpenes, and related compounds with OH and NO3 radicals and ozone; these data are given in Table 5-2. TABLE 5-2 Room-Temperature Rate Constants for the Gas-Phase Reactions of a Series of Organic Compounds of Biogenic Origin with OH and NO3 Radicals and Ozone Rate constant, cm3/molecule-s, for reaction with VOC Structure OHa Numb O1c Isoprene >a Camphene [A 1.0 X 10-~° 5.9 X 10-~3 1.4 X 10-~7 5.3 X 10-~l 6.5 X 10-~3 9.0 X 10-~9
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From page 140... ...
140 RETHINKING THE OZONE PROBLEM Rate constant, cm3/molecule-s, for reaction with VOC Structure OHa No3b O3C 2-Carene ,~ 8.0 x 10-11 1.9 x 10-11 2.4 x 10-16 3-Carene ~J 8.8 x 10-11 1.0 x 10-11 3.8 x 10-17 d-Limonene Myrcene I 11 2.2 x 10-1° Ocimene ~11 2.5 x 10-1° c'-Phellandrene ~ c`-Pinene 76 1.7 x 10-1° 1.3 x 10-11 2.1 x 10-16 1.1 x 10-11 49 X 1o-l6 2.2 x 10-11 56 X 1o-l6 3.1 x 10-1° 8.5 x 10-11 1.9 x 10-15 5.4 x 10-11 5.8 x 10-12 8.7 X 10-17 ,6-Pinene 7 ~7.9 x 10-11 2.4 X 10-12 1.5 X 10-17
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From page 141... ...
ATMOSPHERIC CHEMISTRY 141 Rate constant, cm3/molecule-s, for reaction With VOC Structure OHa NO3b O3C Sabinene a-Terpinene '-Terpinene h Terpinolene ~0 1,8-Cineole ~ p-Cymene /\ 1.2 x 10-1° 1.0 x 10-11 8.8 x 10-17 3.6 x 10-1° 1.8 x 10-1° 8.7 x 10-15 1.8 x 10-1° 2.9 x 10-11 1.4 x 10-16 2.3 x 10-1° 9.6 x 10-11 1.4 x 10-15 1.1 X 10-11 1.7 X 10-16 <1.5 X lo-19 1.5 x 10-11 9.9 x 10-16 <5 x 10-2° aFrom Atkinson, 1989; Atkinson et al., 1990a, and Corchnoy and Atkinson, 1990. bFrom Atkinson et al., 1988; Atkinson et al., 1990a; and Corchnoy and Atkinson, 1990.
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From page 142... ...
As noted above, the NO3 radical reactions act as a removal process for either the biogenic VOCs or NOX, depending on the relative magnitudes of the biogenic emission fluxes and the formation rate of the NO3 radical from the reaction of ozone with NO2 (Winer et al., 1984~. TABLE 5-3 Calculated Tropospheric Lifetimes of VOCs _Lifetime due to reaction with VOC OHa Cob NO3C Isoprene 1.8 hr 1.2 days 1.7 days Camphene 3.5 hr 18 days 1.5 days 2-Carene 2.3 hr 1.7 hr 36 min ^3-Carene 2.1 hr 10 hr 1.1 hr d-Limonene 1.1 hr 1.9 hr 53 min Myrecene 52 min 49 min 1.1 hr Ocimene 44 min 43 min 31 min cY-Phellandrene 35 min 13 min 8 min a-Pinene 3.4 hr 4.6 hr 2.0 hr ,B-Pinene 2.3 hr 1.1 days 4.9 hr Sabinene 1.6 hr 4.5 hr 1.1 hr
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From page 143... ...
Few definitive data are available concerning the products formed from the atmospheric reactions of isoprene and the monoterpenes. The most studied of the biogenic compounds have been isoprene and its major degradation products methacrolein and methyl vinyl ketone (Arnts and Gay, 1979; Kamens et al., 1982; Niki et al., 1983; Gu et al., 1985; Tuazon and Atkinson, 1989, 1990a,b; Paulson et al., 1992a,b)
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From page 144... ...
The NO3 radical reactions with the monoterpenes led to the formation of aerosols, although for cY- and,6-pinene, spectral features indicated the presence of >C=0 and -ONO2 groups. It should be noted that the initial isoprene and monoterpene concentrations in these experiments were ~ 5 x 10~4 molecule/cm3 (20,000 ppb)
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From page 145... ...
(1992b) concluded that the products formed from the O3 reaction with isoprene are methacrolein, methyl vinyl ketone, and propene, with yields of 68%, 25%, and 7% respectively.
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From page 146... ...
~, and this accounts for the HCHO; methyl vinyl ketone and methacrolein observed. The reported product distributions account for only ~ 60% of the overall products formed.
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From page 147... ...
, and 3-methylfuran (~5%~; the HCHO yield is equal to the sum of the methacrolein and methyl vinyl ketone yields. For the OH radical-initiated reaction of methyl vinyl ketone, the reaction pathways are essentially totally accounted for; the OH radical addition to
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From page 149... ...
The major features of the atmospheric degradations of anthropogenic and biogenic VOCs are shown in Figure 5-3. DEVELOPMENT AND TESTING OF CHEMICAL MECHANISMS Computer models that incorporate emissions of VOCs and NOx, meteorology, and the chemistry of VOC/NOx mixtures simulate the complex physical and chemical processes of the atmosphere and predict the effects of changes of emissions of anthropogenic VOCs, biogenic VOCs, or NOx on photochemical air pollution.
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From page 150... ...
. Apart from the initial reactions of VOCs with OH and NO3 radicals and ozone, the vast majority of the tropospheric degradation reactions of VOCs are not well understood with regard to rate constant or products, so there are large areas of uncertainty, and reaction sequences are formulated by analogy.
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From page 151... ...
In some areas of the chemical mechanism, such as the degradation reaction schemes of the aromatic VOCs, the chemical mechanism has been derived by providing the best fit of an assumed mechanism to the experimental data. Two chemical mechanisms have been developed recently for use in urban airshed simulation models, and predictions of these two mechanisms (Carter et al., 1986a; Gery et al., 1988a, 1989)
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From page 152... ...
. This discrepancy arises because of different assumptions for the temperature dependence of the ratio of the rate constants for Reactions 5.96 and 5.97, where the acetyl peroxy radical reacts with NO and NO2, respectively.
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From page 153... ...
, but this scale does not take into account the reactions subsequent to the initial OH radical reaction and it ignores other tropospheric loss processes, such as photolysis and reaction w~th NO3 radicals and ozone. Thus, for example, the formation of photoreactive products, such as formaldehyde, leads to increased overall reactivity u~th respect to ozone formation, whereas the generation of products such as organic nitrates, which act as sinks for NOX and radical species, leads to a decreased ozone-forming potential (Carter and Atkinson, 1987, 1989b)
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From page 154... ...
~. Table 5-4 shows, as an example, the incremental reactivities calculated when selected VOCs are added to an eight-component urban VOC mixture at various VOC/NOx ratios (Carter and Atkinson, 1989b)
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From page 155... ...
ATMOSPHERIC CHEMISTRY 155 TABLE 5-4 Calculated Incremental Reactivities of CO and Selected VOCs as a Function of the VOC/NOX Ratio for an Eight-Component VOC Mix and Low-Dilution Conditions VOC/NOX, ppbC/ppb Compound 4 8 16 40 Carbon monoxide 0.011 0.022 0.012 0.005 Ethane 0.024 0.041 0.018 0.007 e-Butane 0.10 0.16 0.069 0.019 e-Octane 0.068 0.12 0.027 -0.031 Ethene 0.85 0.90 0.33 0.14 Propene 1.28 1.03 0.39 0.14 trans-2-Butene 1.42 0.97 0.31 0.054 Benzene 0.038 0.033 -0.002 -0.002 Toluene 0.26 0.16 -0.036 -0.051 m-Xylene 0.98 0.63 0.091 -0.025 Formaldehyde 2.42 1.20 0.32 0.051 Acetaldehyde 1.34 0.83 0.29 0.098 Benzaidehyde -0.11 -0.27 -0.40 -0.40 Methanol 0.12 0.17 0.066 0.029 Ethanol 0.18 0.22 0.065 0.006 Urban mixa 0.41 0.32 0.088 0.011 Eight-component VOC mix used to simulate VOC emissions in an urban area In the calculations. Surrogate composition, in units of ppb compound per ppbC surrogate, was ethene, 0.025; propene, 0.0167; r1-butane, 0.0375; n-pentane, 0.0400; isooctane, 0.0188; toluene, 0.0179; m-xylene, 0.0156; formaldehyde, 0.0375; and inert constituents, 0.113.
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From page 156... ...
of the organic compound. Different aspects of the reaction mechanisms, for example, the VOC/NOX concentration ratio, become important under different conditions.
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From page 157... ...
This aspect of the reaction mechanisms becomes important at high VOC/NOx ratios, where the availability of NOx becomes limiting, and the formation of organic nitro compounds competes with the formation of N() ^ (with suh~equent photolysis to generate ozone)
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From page 158... ...
The approach in Chapter 8 to assess the importance of isoprene and other biogenic organic emissions in the formation of ozone in urban-suburban, rural, and remote air masses uses the kinetic reactivities of the VOCs measured in ambient air, and hence is based on the instantaneous rate of formation of RO2 radicals (Reaction 5.99~. While the differences in the mechanistic reactivities of the various VOCs are neglected, this simpification is not expected to significantly alter the conclusions drawn in Chapter 8, especially since isoprene has a high positive mechanistic reactivity compared with the reactivities of the alkalies and aromatic VOCs (Table 5-5)
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From page 159... ...
ATMOSPHERIC CHEMISTRY 159 TABLE 5-5 Calculated Incremental Reactivities and Kinetic and Mechanistic Reactivities for CO and Selected VOCs for Maximum Ozone Formation Conditions, Based on Scenarios for 12 Urban Areas in the U.S. Compound Incremental Kinetic reac- Mechanistic reactivity, mole tivity, fraction reactivity, mole O~/mole C reacted O~/mole C Carbon monoxide 0.019 0.043 0.45 Methane 0.0025 0.0016 1.6 Ethane 0.030 0.049 0.61 Propane 0.069 0.21 0.34 e-Butane 0.124 0.37 0.34 e-Octane 0.081 0.75 0.107 Ethene 0.77 0.81 0.95 Propene 0.82 0.97 0.85 t?
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From page 160... ...
The atmospheric chemistry of anthropogenic VOCs, including alkanes, alkenes, and aromatic hydrocarbons, is generally understood. The kinetics of the initial reactions of the majority of anthropogenic VOCs, and the photolysis rates of these VOCs, have been determined experimentally or can be reliably calculated.
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From page 161... ...
The formation of carbonyl compounds during the atmospheric degradation reactions of VOCs and the atmospheric chemistry of these carbonyl compounds require further study. In particular, there is a need for absorption cross-sections, photodissociation quantum yields, and photodissociation product data (as a function of wavelength)
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From page 162... ...
162 RETHINKING THE OZONE PROBLEM for assessing the effects on ozone formation of conversion from gasoline to alternative fuels and for quantifying the effects of biogenic VOCs on urban, suburban, and rural ozone.
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