DEVELOPMENT OF REVISED SAPRC AROMATICS MECHANISMS

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INTRODUCTION Airshed models are essential for the development of effective control strategies for reducing photochemical air pollution because they provide the only available scientific basis for making quantitative estimates of changes in air quality resulting from changes in emissions. The chemical mechanism is the portion of the model that represents the processes by which emitted primary pollutants, such as volatile organic compounds (VOCs) and oxides of nitrogen (NO x ), interact in the gas phase to form secondary pollutants such as ozone (O 3 ) and other oxidants. This is an important component of airshed models because if the mechanism is incorrect or incomplete in significant respects, then the model's predictions of secondary pollutant formation may also be incorrect, and its use might result in implementation of inappropriate or even counter-productive air pollution control strategies. One airshed model application where the accuracy of the chemical mechanism is important is the calculation of reactivity scales that measure relative impacts of different types of VOCs on ozone formation. VOCs differ significantly in their impacts on O 3 formation, and regulations that take this into account are potentially much more cost-effective than those that regulate all VOCs equally. In view of this, several VOC regulations implemented (or being considered) in California take reactivity into account. The California regulations use the Maximum Incremental Reactivity (MIR) scale that was calculated using the SAPRC-99 chemical mechanism (Carter, 2000a), but these have been updated to values calculated using the more recently developed SAPRC-07 mechanism (Carter, 2010a,b). The SAPRC-07 mechanism is the latest in the SAPRC series of gas-phase chemical mechanisms (Carter, 1990, 2000a, 2010a,b) that are designed for various airshed model applications. The detailed version of the mechanism, which has separate reactions for over 700 different types of VOCs and represents approximately 300 others using the “lumped molecule” approach (Dodge, 2000), is used for calculating the MIR and other ozone reactivity scales, and serves as the basis for deriving more condensed mechanisms for airshed model applications where such chemical detail is not required. For such applications, a lumped mechanism was developed where the many types of emitted VOCs are represented using a more limited number of lumped model species whose mechanisms are derived based on those of the mixture of compounds they represent in a standard ambient mixture taken as representative of anthropogenic VOC emissions (Carter, 2000b, 2010a). Even more condensed versions of lumped SAPRC-07 have been developed using various lumping approximations (Carter, 2009, 2010c,d), but a discussion of this is beyond the scope of this report. But the chemical basis of all these versions of SAPRC-07 is that of the detailed version. The chemical basis of detailed SAPRC-07 is based on results of various laboratory studies, kinetic and mechanistic data evaluations (e.g., Atkinson, 1989; Atkinson and Arey, 2003; Calvert et al. 2000, 2002; IUPAC, 2006; NASA, 2006), theoretical or “best judgment” estimates, extrapolations, and interpolations, and results of model simulations of environmental chamber data. Simulations of chamber data are important because mechanisms for many emitted VOCs are complex and have uncertainties, and available data, theories, and estimates are not sufficient to fully constrain the mechanism. For this reason, the predictive capabilities of the mechanisms need to be evaluated by determining if the mechanism can simulate the results of appropriate environmental chamber experiments, and in some cases uncertain portions of the mechanism may need to be adjusted for the mechanisms to give satisfactory simulations of these data. If a mechanism cannot adequately simulate results of well-characterized chamber experiments, it certainly cannot be relied upon to give accurate predictions in airshed model applications. The need to evaluate and adjust mechanisms based on simulations of chamber data is particularly important when deriving mechanisms for aromatic compounds, because of the complexities and 4
significant uncertainties in their mechanisms, and the fact that much of their relatively high atmospheric reactivity is due to secondary reactions of highly photoreactive but poorly characterized ring-opening products (Calvert et al, 2002 and references therein). Aromatics are important compounds in airshed models because of their relatively high reactivity combined with their relatively large emissions, so appropriate representation of their reactions in the mechanisms is a priority. The earlier versions SAPRC, such as SAPRC-90 (Carter, 1990) and SAPRC-99 (Carter, 2000a) use highly parameterized model species whose yields and photolysis rates are adjusted to fit chamber data to represent the photoreactive ring-opening products, and also used highly parameterized and adjusted mechanisms for the phenols and other major ring-retaining products. The availability of at least some data concerning the reactions of unsaturated dicarbonyl aromatic products resulted in SAPRC-07 having a slightly less parameterized representation of these products than the earlier versions, but significant uncertainties remain and photodecomposition quantum yields of the model species representing these products still have to be adjusted based on model simulations of chamber data. The importance of chamber data in the development of aromatic mechanisms means that the predictive capabilities of this important aspect of the overall mechanism are highly dependent on the quality and comprehensiveness of the available data. The development of SAPRC-07 utilized results of over 2500 environmental chamber experiments, of which 226 were single aromatic - NO x experiments that served as the basis for the adjustments in the aromatics mechanisms (Carter, 2010a). Because of uncertainties in characterizing light intensity and spectra of outdoor chamber runs, the adjustments were based on only indoor chamber runs with well-characterized blacklight or solar simulator arc light sources, with runs with significant measurement or characterization uncertainties or whose results appeared to be outliers compared to comparable runs not being utilized. Data were available for benzene, toluene, ethylbenzene, and all the xylene and trimethylbenzene isomers, with mechanisms for other alkylbenzenes being estimated based on the mechanisms derived for the most structurally similar compound where data were available. Most of the experiments used in the SAPRC-07 aromatics mechanism development were carried out at our environmental chamber laboratories at the University of California at Riverside (UCR), with a majority of these being carried out in the older UCR chambers (Carter et al, 1995a) at NO x levels of greater than ~200 ppb. The dataset did include 5 experiments carried out in the TVA chamber (Simonaitis and Bailey, 1995; Bailey et al, 1996; Simonaitis et al, 1997; Carter, 2004), most at lower NO x levels, and ~60 were carried out at using the new UCR-EPA chamber (Carter, 2004; Carter et al, 2005) at NO x levels of less than 100 ppb. However, the parameters derived for the SAPRC-07 aromatics mechanisms, like those in earlier version of SAPRC, reflect primarily results of older UCR chamber experiments carried out at generally higher than ambient NO x levels. Although the initial aromatics experiments carried out at lower NO x levels in the UCR EPA chamber suggested that the mechanism could simulate results of very low NO x experiments reasonably well (Carter, 2004), more recent low NO x experiments, carried out under a wider variety of conditions, indicate that this may not be the case. Azzi et al (2010) reported results of model simulations of new toluene and m-xylene - NO x experiments carried out in a new indoor chamber at the Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia (Hynes et al, 2005; White et al, 2010) using the SAPRC-07 mechanism, and found that it consistently underpredicted O 3 formation in the toluene experiments, and had an underprediction bias in the m-xylene experiments at lower xylene / NO x ratios. In addition, a large number of new aromatic - NO x experiments were carried out in the UCR EPA chamber for the purpose of studying secondary organic aerosol (SOA) formation from aromatics (Carter et al, 2012), and the results also indicated significant biases in SAPRC-07 ozone predictions at lower NO x or lower VOC/NO x levels. 5
Page 1 and 2: DEVELOPMENT OF REVISED SAPRC AROMAT
Page 3 and 4: ACKNOWLEDGEMENTS AND DISCLAIMERS Th
Page 5 and 6: TABLE OF CONTENTS (continued) APPEN
Page 7 and 8: LIST OF FIGURES (continued) Figure
Page 9 and 10: EXECUTIVE SUMMARY Background and Pr
Page 11: However, this issue is probably not
Page 15 and 16: New UCR EPA Experiments New CSIRO E
Page 17 and 18: Table 1. List of model species in t
Page 19 and 20: Table 2. Reactions that were modifi
Page 21 and 22: Table 2 (continued) Phot Set = name
Page 23 and 24: mechanism using the generic lumped
Page 25 and 26: Table 3 (continued) 4 Average of 5.
Page 27 and 28: Hydroperoxides (R6OOH) Phenolic Pro
Page 29 and 30: Table 5 (continued) 3 Yields of pro
Page 31 and 32: Table 6. Summary of yields of lumpe
Page 33 and 34: thus the model species AFG3 used to
Page 35 and 36: simulations of the chamber data usi
Page 37 and 38: separately primarily because they a
Page 39 and 40: the photoreactive α-dicarbonyls an
Page 41 and 42: MECHANISM EVALUATION Methods Chambe
Page 43 and 44: Table 9 (continued) ID TVA CSI Brie
Page 45 and 46: Modeling Methods The procedures use
Page 47 and 48: irradiation time would not yield a
Page 49 and 50: Adjustments to Mechanisms to Fit Da
Page 51 and 52: Compound ∆([O 3 ]-[NO]) Formation
Page 53 and 54: Benzene Number of Runs 14 Average M
Page 55 and 56: Toluene Plots and tables of selecte
Page 57 and 58: CTC108B ETC101 ETC103 CTC127B IR Su
Page 59 and 60: Ethyl Benzene (Model "A") Number of
Page 61 and 62: n-Propyl Benzene Number of Runs 4 5
Page 63 and 64:
m-Xylene Number of Runs 128 Average
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m-Xylene (continued) o-Xylene p-Xyl
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IntOH Model Error 100% 50% 0% -50%
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p-Ethyl toluene Number of Runs 7 Av
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1,3,5-trimethylbenzene Model Error
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Phenol Number of Runs 5 50% Model E
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Although there is somewhat greater
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∆([O3]-[NO]) Model Error Full Sur
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constants and product yield paramet
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25 20 10 SAPRC-11 Reactivity 15 10
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there is an inconsistency between t
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e obtained from the AFG1 / (AFG1+AF
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higher NO x experiment. Note that t
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in this evaluation, as well as the
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develop chemically detailed or expl
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laboratory data and theories of atm
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REFERENCES Anderson, R.S., Czuba, E
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Carter, W. P. L. (2000a): “Docume
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IUPAC (2008e): http://www.iupac-kin
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White, S.J., M. Azzi, D. E. Angove,
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Table A-1 (continued) Type and Name
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Table A-1 (continued) Type and Name
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Table A-2. Listing of reactions and
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Table A-2 (continued) Label Reactio
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Table A-2 (continued) Phot Set = na
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Table B-1 (continued) Run New [a] C
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Table B-2. Summary of incremental r
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Table B-3 (continued) Run ID Surrog
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Table B-3 (continued) Run ID Surrog
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Table B-4. (continued) Cham. Set(s)
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Table B-4. (continued) Cham. Set(s)
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DEVELOPMENT OF REVISED SAPRC AROMATICS MECHANISMS

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