United States Environmental Protection Agency Environmental Sciences Research Laboratory Research Triangle Park NC 27711 Research and Development EPA-600/S3-84-063 June 1984 &EPA Project Summary Evaluation of Chemical Reaction Mechanisms for Photochemical Smog Part II: Quantitative Evaluation of the Mechanisms Joseph A. Leone and John H. Seinfeld Six chemical reaction mechanisms for photochemical smog are analyzed to determine why, under identical conditions, they predict different maximum ozone concentrations. To perform the analysis, a counter species analysis technique is used to determine the contributions of individual reactions or sets of reactions to the overall behavior of a chemical reaction mecha- nism. Using this technique, we can obtain answers to previously inacces- sible questions such as the relative contributions of individual organic species to photochemical ozone forma- tion. Based on the results of the analysis, we have identified specific aspects of each mechanism that are responsible for the discrepancies with other mechanisms and with a master mechanism based on the latest understanding of atmospheric chemistry. For each mechanism, critical areas are identified that, when altered, bring the predictions of the various mechanisms into much closer agreement. Thus, we have been able to identify why the predictions of the mechanisms are different, and have recommended research efforts that are needed to eliminate most of the dis- crepancies. This Project Summary was developed by EPA's Environmental Sciences Research Laboratory, Research Triangle Park, NC. to announce key findings of the research project that is fully documented in a separate report of the same title (see Project Report order- ing information at back.) Introduction In determining emission control levels, one must be able to predict how changes in emission levels will affect ambient air quality. An important component of such an approach is a description of atmos- pheric organic chemistry. Unfortunately, the development of a chemical reaction mechanism that accurately describes atmospheric chemistry and, at the same time, is computationally tractable, is a difficult undertaking. Since typical urban atmospheres contain hundreds of different organic species, it is not feasible to write a mechanism that includes each individual species. Thus, these reaction mechanisms must maintain a balance between the level of chemical detail and, for numerical reasons, the number of species and reaction pathways. Currently, several chemical reaction mechanisms exist that describe the organic chemistry of the urban atmos- phere and that attempt to maintain a balance between chemical detail and mechanism length. These mechanisms are all based on the same body of experimental rate constant data, and each mechanism has been evaluated against data from various smog chamber facilities. In each mechanism, the detailed atmospheric chemistry has been greatly simplified by a process referred to ------- as lumping. However, because this simplification, or lumping process, has been carried out in different ways, no two mechanisms are the same. The differences among mechanisms would not be of any concern if each of the mechanisms gave similar predictions over a range of atmospheric conditions. However, several recent investigations have shown that different mechanisms predict substantially different degrees of emission controls to achieve the same desired air quality under identical conditions. Since tremendous expenses are involved in implementing hydrocarbon and oxides of nitrogen (NOx) emission controls predicted by such mechanisms, there is an urgent need to understand the fundamental reasons for these discrepancies. This report represents the second part of a three-part study of lumped reaction mechanisms for photochemical smog. Part I (EPA-600/3-83-086) contains information concerning the various approaches to lumping, and how the particular mechanisms chosen for this study were selected. Also included in Part I is a detailed description of each lumped mechanism. Part II presents a quantitative analysis aimed at determining why these mechanisms, under identical conditions predict the formation of substantially different amounts of ozone (03). In Part III we will analyze the emission control require- ments predicted by the various mech- anisms under conditions approximating those occurring in the real atmosphere. Method of Analysis To determine why the O3 yields predicted by these lumped mechanisms are so different, we would like to determine how much of the total 03 production can be attributed to each of the initially present organic species in each mechanism. With this information, we could determine the relative contribu- tions of individual organic species (or reactions, reaction pathways, etc.) to the overall behavior of each mechanism. Unfortunately, analyzing the behavior of these atmospheric reaction mecha- nisms is a demanding task because of the large number of species and reactions that each contains, and because of the interwoven nature of the free radical chain reactions characterizing each mechanism. Thus, there is no direct way of calculating the relative amounts of 03 that each organic species is respon- sible for producing in a mechanism. We can, however, using a method described below, keep track of the number of NO to N02 conversions that arise as a result of the presence of each organic species. The amount of 03 attributable to a given organic species is well established to be directly proportional to the number of NO oxidations affected by the species. With this information, corresponding species or reactions in the various mechanisms can be compared directly to one another. The technique we use to determine the number of NO to NO2 conversions attributable to individual species is called counter species analysis. To illustrate the usefulness of such an analysis, consider the following simple mechanism that describes the photooxidation of formalde- hyde and acetaldehyde in the presence of NOx. N02 + hv-NO + 03 (1) NO + 03 - N02 + 02 (2) H02-+ NO - NO2 + OH + C3 (3) HCHO + hv -2HO2-+ CO + C4 (4) 20 2 CH3O(O)02- + NO ~NO2 + CH3O2-+CO+ C9 °2 (9) CH3C(0)02- + NO2 - CH3C(0)02N02 (PAN) (10) PAN - CH3C(O)O2' + N02 (11) CH3O2 • + NO - CH30- + N02 + C12 (12) CH30 • + O2 - HCHO + HO2- + C13 (13) N02 + OH - HNO3 + C14 (14) HCHO +hv - H2 + CO HCHO+OH^HO U2 CH3CHO + hvo7s + C7 2°2 (5) C6 (6) HO2.+ CO (7) CH3CHO + C8 0.080 0.060 •§ 0.040 s I o 0.020 OH - CH3C(O)02- °2 H20 (8) In the atmosphere these aldehydes react to produce HO2, CH3O2, and CH3C(O)O2 radicals that can convert NO to NO2, and thus cause [NO2]/[NO], and consequently O3, to increase. Ozone formation will continue as long as aldehydes and NOx are both present. NOx is consumed via reactions 10 and 14, so ultimate 03 yields are limited by NOx availability as well as by how fast the aldehydes lead to O3 formation through the conversion of NO to N02- The species Ci are fictitious products that are used to count the number of times that reaction i has occurred. By counting these fictitious species, we can determine the number of NO to N02 conversions that each reaction is responsible for. I I I NO - /V02 by CH3CHO + OH • NO- /VO2 by HCHO + OH NO - /VO2 by CH3CHO +hv I I I + Otf^* 60 120 180 240 Time (Minutes) 300 360 Figure 1 . Counter species results for the formaldehyde/acetaldehyde/NO* simulation. Number of NO to NOi conversions due to the photolysis and OH reactions of I formaldehyde and acetaldehyde. ------- The results of analyzing this simple mechanism with the counter species technique are shown in Figure 1. The amount of NO to N02 conversions due to the four key reactions is shown as a function of time. The OH reaction of acet- aldehyde is the most important reaction from an NO to N02 conversion (and O3 production) point of view. With simple mechanisms, such as the aldehyde mechanism shown above, there are other ways of obtaining the desired informa- tion. One method is to make use of the pseudo-steady state approximation to eliminate the fast-reacting species. Unfortunately, all of the useful chemical reaction mechanisms describing photochemical smog are much more complicated than the simple aldehyde mechanism presented above. With these mechanisms, it becomes extremely difficult to eliminate the fast-reacting species using the pseudo-steady state approximation. Nevertheless, the counter species analysis provides an effective means to examine the properties of these complex mechanisms The Master Mechanism The counter species technique described above is used to compare the structure and behavior of each of the six lumped mechanisms. Since each of these mechanisms represents an attempt to simulate atmospheric organic/NOx chemistry, it is also desirable to compare the structure and behavior of each mechanism to a fully explicit, detailed mechanism containing as many of the important organic species as possible. We have constructed such a mechanism which contains the detailed chemistry of 12 of the most important atmospheric organic species. We call this mechanism the "master mechanism." Results The results of applying the counter species analysis to the six lumped mech- anisms and the master mechanism are shown in Figure 2. Shown for each mechanism is the amount of NO to N02 conversions attributable to each of the initially present organic species. One can see that the amount of NO to N02 conver- sions due to a given species can vary substantially between the various mechanisms. Based on results like these, critical areas of each mechanism are identified which are most responsible for the observed discrepancies. When these critical areas are modified, the predictions of the various mechanisms 1000 800 600 ^ s^ O ! 400 200 - CHiCHO HCHO butane propane ethene tpropanes n -butane 2,3- dimethyl _butane toluene m-xylene r~ other •"• CH3CHO HCHO butene propene ethene : propane ~ n-butane 2.3- butane toluene m-xylene (^ CH3CHO HCHO butene propene ethene propane n-butane 2,3- dimethyl butane m-xylene H3CH2CHO CH3CHO HCHO butene propene ethene '.propane^ n-butane 2.3- dimethyl butane toluene m-xylene other CH3CHO HCHO butene propene ethene 'propane" n-butane 2,3- dimethyl butane toluene m-xylene other carbonyls HCHO butene mpropene_ ethene propane n-butane 2,3- dimethyl butane toluene m-xylene acetone + CzHsCHO CH3CHO HCHO butene propene ethene 1 propane* n-butane 2,3- dimethyl butane toluene m-xylene 0) 1 01 I Q :i n Mechanism Figure 2. Counter species results showing the amount of NO to NOz conversions attributable to each of the initially present organics. are in much better agreement. This is shown in Figure 3. Conclusions In this report we have presented a quantitative analysis of six lumped mechanisms describing photochemical smog. We determined why these mechanisms, under identical conditions. gave rise to widely different predictions of peak-03 levels. Based on the results of this analysis, we have identified specific areas in each mechanism that are most responsible for the observed discrepan- cies in O3 predictions. For each mechan- ism, several recommendations have been made that are aimed at eliminating these discrepancies. Some of these recom- mendations amount merely to updating ------- 7000 SOO I i 3 o o i 600 400 200 .h II Mechanism ll I* II figure 3. A comparison of the total amount of NO to NOsConversions predicted by the original (unshaded) and modified (shaded) versions of five lumped mechanisms. area needing improvement concerns the mechanism of aromatic ring opening. The products formed during aromatic ring opening must be eluci- dated before any faith can be placed in the predictions of photochemical smog mechanisms. The photolysis products of the important a-dicarbo- nyls such as methyl glyoxal also represents a critical area of uncertainty in aromatic chemistry. • much greater resources be devoted to the determination of the fundamental mechanism leading to chamber radical sources. It is an unfortunate fact that mechanisms cannot be unambiguously evaluated using chamber data or compared to each other until the radical source issue is resolved. It now appears that the only way to resolve this controversy is to determine the mechanism which gives rise to this radical source. • the counter species analysis tech- nique be used by each investigator when future versions of these mech- anisms are developed. We have only applied this analysis to a single primary hydrocarbon distribution, at 3 RHC to NOx ratios. Individual investigators should make use of counter species analysis to test their mechanisms with a variety of initial hydrocarbon distributions and RHC to NOx ratios. • future work in evaluating these lumped mechanisms include an analysis of the methods for estab- lishing initial conditions when detailed hydrocarbon composition profiles are not available. The emission control requirements predicted by these lumped mecha- nisms should be evaluated under real world conditions of continuous pollutant emissions, continuous dilution, and in the presence of background pollutants. Problems of this type will be addressed in Part III of this study. rate constants, while others involve developing completely new reaction sequences. When the lumped mechan- isms are modified to include our sugges- tions, their predictions are in much closer agreement. However, changes such as those that we have suggested should not be adopted until the performance of the entire mechanism is reevaluated. Several recommendations for future work are apparent upon completing this study. These would include recommend- ing that: • future work be directed at several important areas of atmospheric chemistry where our knowledge is lacking. Perhaps the most critical that a significant effort be devoted toward measuring the composition and amounts of organic species emitted into urban environments. Without this information, the uncertainties involved in specifying input data could nullify any improve- ments in the reaction mechanisms themselves. ------- J. A. Leone and J. H. Seinfeld are with the California Institute of Technology, Pasadena, CA 91125. Marc/a C. Dodge is the EPA Project Officer (see below). The complete report, entitled "Evaluation of Chemical Reaction Mechanisms for Photochemical Smog. Part II. Quantitative Evaluation of the Mechanisms," (Order No. PB 84-196 740; Cost: $19.00, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Environmental Sciences Research Laboratory U.S. Environmental Protection Agency Research Triangle Park, NC 27711 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 Official Business Penalty for Private Use $300 & nt r u, ^-» -J o"!:> U.S. GOVERNMENT PRINTING OFFICE: 1984-759-102/10605 ------- |