Ozone-Forming Potential of Reformulated Gasoline (1999) / Chapter Skim
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3 The Concept of Ozone-Forming Potential and Its Quantification
3 The Concept of Ozone-Forming Potential and Its Quantification
Pages 33-72
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From page 33... ...
The principle behind ozone-forming potential or reactivity is the notion that, in addition to the amount of a specific VOC species emitted into a given airshed, the difference in the chemistry of each of the VOCs needs to be considered when assessing the impact of those species on ozone formation. The utility of the concept of ozone-forming potential can be illustrated through a comparison of the impacts on ozone concentrations in an urban airshed of two ubiquitous VOC species: ethane and propene.
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From page 34... ...
Little of the ethane emitted in an urban area reacts within that area before it is transported away. Its contribution to ozone formation within the urban area is therefore very small.
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From page 35... ...
Environmental Protection Agency at http://www.epa.gov/docs/OCEPAterms.7 unreactive is a misnomer, because even compounds such as ethane and methane do react and contribute to tropospheric ozone formation, terms are listed in Table 3-1. Unless noted otherwise, VOCs is the term used in this report to represent the general class of gaseous organic compounds.
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From page 36... ...
OPERATIONAL DEFI N ITION OF OZON E-FORM ING POTENTIAL USING REACTIVITY The photochemical degradation of most VOC species is initiated by reaction with the OH radical (i.e., Reaction 2. ~ in Chapter 2)
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From page 37... ...
b Rate constants at 298 K are taken from Atkinson (1994,1997) and Le Calve et al.
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From page 38... ...
19921. There are, however, significant limitations to using the OH-reactivity scale to characterize the roles of VOCs: The method does not account for the potentially different yields of peroxy radicals formed from different VOCs, the different reactive pathways these peroxy radicals can take once they are produced, and the varying tendency of VOCs to enhance or inhibit radical levels, and thus influence the contribution of other VOC species to ozone formation.
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From page 39... ...
is the change in some ozone metric used to assess the impact of VOCs on air quality (e.g., the I-hr peak or S-hr averaged ozone concentration in an airshed) or the total human exposure to ozone above some threshold concentration)
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From page 40... ...
Smog chambers do not realistically represent the physics of pollutant transport and the impact of fresh emissions. Moreover, most do not operate over the full range of NOx
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From page 41... ...
Because models can be run for conditions that more accurately reflect actual atmospheric conditions, they can, in principle, provide a more appropriate measure of a species' reactivity than that obtained from a smog chamber. However, virtually all photochemical mechanisms used in current air-quality models are based on data from smog chambers.
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From page 42... ...
The principal chemical mechanisms used in current air-quaTity models, along with representative airshed modeling applications and their key attributes, are listed in Table 3-3. With the exception of the Harwell Master Chemical Mechanism, all the chemical mechanisms in use today include various kinds of parameterizations, approximations, and condensations to simplify the very complex chemical processes that actually occur when VOCs are oxidized in the atmosphere.
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From page 43... ...
Irradiations of single VOCs in the presence of NOx are used to test the mechanism's ability to simulate the oxidation of and ozone production from an individual VOC; NOx-air irradiations of more complex VOC mixtures test the performance of the mode} as a whole; and experiments in which the effect of adding single VOCs to irradiations of NOx and complex mixtures test mode} predictions of the VOC's incremental reactivity. Evaluation of chemical mechanisms with smog chamber data is complicated by uncertainties in chamber effects, and separate characterization experiments are needed to evaluate those effects.
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From page 44... ...
Although these results have limited applicability for the reasons discussed above, they can be quite valuable for evaluating and verifying reactivities calculated using air-quality models. Studies based on smog chambers include those of Carter at the University of California at Riverside, Kelly at the General Motors Research Laboratories, and ~Jeffries at the University of North Carolina.
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From page 45... ...
The purpose of the ~Jeffries et al. studies was to evaluate chemical mechanisms, and to compare, directly, ozone formation from various chemically realistic mixtures.
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From page 46... ...
Three-dimensional, urban-scale photochemical model. Specified by the EPA for regulatory applications.
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From page 47... ...
However, these models require large quantities of detailed input data and have relatively high computational demands. In addition to uncertainties in chemical mechanisms (a feature common to both box and grid models)
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From page 48... ...
developed IS separate reactivity scales for quantifying VOC reactivity under different conditions, in this case using the SAPRC-90 chemical mechanism in a single-cell trajectory model. Those reactivity scales were derived using nine different approaches for dealing with the dependence of reactivity on environmental conditions and on two methods for quantifying ozone impacts.
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From page 49... ...
Simulations were performed for the SCAQS episode. Calculation of the contributions of 18 compounds to ozone concentrations in the Lower Fraser Valley.
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From page 50... ...
. The MOIR scale is the incremental reactivity computed for conditions that maximize the ozone concentration (see Figure 3-~)
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From page 51... ...
CARD (1990) proposed using the MIR scale for regulator applications, because the MIR scale reflects reactivities under environmental conditions that are most sensitive to the effects of VOC controls.
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From page 52... ...
The top graph illustrates peak ozone concentrations (as isopleths) as a function of both VOC and NO.
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From page 53... ...
The MIR and MOIR scales usually give similar relative reactivities for most compounds, and are consistent in their predictions of which compounds are highly reactive and which are not. However, for reasons indicated above, the MOIR scale gives lower relative reactivities for aromatics, and also predicts lower relative reactivities for radical 4RAF (reactivity adjustment factor)
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From page 55... ...
Effects of differences and uncertainties in chemical mechanisms on calculated incremental-reactivity scales are discussed in more detail later in this chapter. Eulerian-Mode!
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From page 56... ...
The MOIR scale did not compare as well as the MIR scale with results derived from airshed mode} for either the peak ozone concentration or ozone exposure concentrations greater than the air-quality standard. Subsequent to the study of McNair et al.
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From page 57... ...
The impact of uncertainties in chemical mechanisms on the reliability of reactivities derived from models should be discussed at two levels. First, how uncertainties affect the reactivity of individual VOCs is addressed in this section.
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From page 58... ...
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From page 59... ...
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From page 60... ...
However, because uncertainties in the rate constants and parameterizations used in the chemical mechanisms apply to the calculations for all VOC reactiv~ties, the effects of these uncertainties on the reactivities of individual VOCs are strongly correlated between VOCs. For example, an increase in the photolysis rate for NO2 increases the reactivity of most species by about the same proportion.
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From page 61... ...
TABLE 3-7 Uncertainty in the Mean Absolute and Relative MlRs from 39 Separate Trajectory Simulations Representing Different Environmental Conditions 95% Confidence Interval {% of Mean Value) Absolute Reactivity 28 Compound Formaldehyde Methanol Ethane Toluene Pentene Relative Reactivity 16 39 56 38 39 23 38 21 21 Source: Derived from Russell et al.
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From page 62... ...
Reactivities from box models, on the other hand, focus on a single set of environment conditions corresponding to a specific air mass following a specific trajectory. In Figure 3-4 the relative reactivities for a variety of VOCs calculated on the basis of peak ozone concentrations, population-weighted ozone exposures, and spatially weighted ozone exposures from a three-dimensional-model simulation are compared along with box-model-derived results.
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From page 64... ...
Simulations were performed for the SCAQS episode. Rate constant and exhaust composition uncertainty calculations for the RAFs from reformulated gasolines and methanol.
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From page 65... ...
, this issue has been explored in most detail for motor-vehicle exhaust emissions. When CARB implemented regulations for the LEV/CF6 program, it introduced the concept of reactivity adjustment factors (RAFs)
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From page 66... ...
The relative reactivities of the other species did not change appreciably. This result, albeit limited, appears to suggest that reactivity scales derived for peaks of 1-fur averaged ozone concentration should largely apply to peaks of Shr averaged ozone concentrations in urban areas.
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From page 67... ...
In the first place, recall that Eulierian-modelderived reactivities based on the model's predicted peak ozone concentration did not compare well with the trajectory-model-derived MIRs. Second, reactiv~ties derived from trajectory models are typically teased on very limited simulation times, and thus the use of those models to derive a peak S-hr averaged ozone-reactivity scale is questionable.
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From page 68... ...
Also, as was found in Los Angeles, the impact on the peak ozone concentration is not likely to be the same as the impact on ozone exposure surrogates. Another important issue relates to the role of NOx.
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From page 69... ...
Thus, not only does ozone formation respond differently to different VOC species, but it will often respond differently to the same compound in different locations or during different episodes at the same location. A variety of metrics or scales have been proposed to quantify the ozone-forming potential of an individual VOC or a mixture of VOCs arising from a specific source or type of emission.
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From page 70... ...
Moreover, reactivity is expressed in a variety of ways. The specific reactivity, derived from box modeling, is the reactivity normalized to the change of mass of VOC emissions and has units of grams of ozone formed per grams change of VOC emitted or grams of ozone per grams change of VOC.
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From page 71... ...
, in general, the relative reactivity of VOCs derived from different models and models using different chemical mechanisms tend to be reasonably consistent. For this reason, it is believed that the uncertainties (or potential errors)
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From page 72... ...
uncertainty in the relative reactivities in most of ubiquitous VOCs (that have been studied extensively) is about 20-40%.
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