United States Environmental Protection Agency Atmospheric Sciences Research Laboratory Research Triangle Park NC 2771 1 / \ \" Research and Development EPA/600/S3-86/012 Mar. 1986 SERA Project Summary Development of CBM-X Mechanisms for Urban and Regional AQSMs G. Z. Whitten and M. W. Gery A series of chemical kinetic mechan- isms, each of which describes the formation of photochemical smog from nitrogen oxide and multiple organic precursors, has been developed. The most condensed version of the Carbon- Bond Mechanism series now available is known as the CBM-IV. The formula- tion and testing of the CBM-IV is the subject of this project summary. This mechanism has been carefully tested against results obtained from an ex- panded version of the CBM and several intermediate versions that represent steps of increasing condensation. This approach was used to (1) ensure that the CBM-IV did not contain compensat- ing errors, (2) verify mechanism per- formance under a range of conditions, and (3) verify the operating boundaries of the mechanism at each condensation step. The condensation of the expanded CBM mechanism to CBM-IV was tested in an incremental manner by simulating seven days of outdoor smog chamber data from two different experimental facilities. These data provided a wide range of time, temperature, light inten- sity, and concentration conditions for comparing simulation results. The de- velopment effort yielded a mechanism containing 28 species and 70 reactions. This Project Summary was developed by EPA's Atmospheric Sciences Re- search Laboratory, Research Triangle Park, NC, to announce key findings of the research project that is fully docu- mented in a separate report of the same title (see Project Report ordering infor- mation at back). Introduction Since 1976, Systems Applications has been developing Carbon Bond Mechan- isms (CBM) for use in urban and regional air quality simulation models. The series of mechanisms developed to date is given in Table 1. The main objective of the mechanism development effort reported here was to prepare a highly condensed version (CBM-IV) of the expanded Carbon Bond Mechanism (CBM-X) for use in complex atmospheric models such as the EPA Regional Oxidant Model (ROM). A further objective was to develop new methods for condensing and testing chemical mechanisms. Thus, in addition to documenting the development and testing of the CBM-IV mechanism, the Project Report also outlines a potential protocol for mechanism condensation and discusses a set of techniques (some of which are newly conceived) and tests that can be used to condense and test other chemical kinetic mechanisms. Condensation Technique As shown in Table 1, the most expand- ed Carbon Bond Mechanism developed to date is called the CBM-XR with 170 reactions and 78 species. The initial steps to condense this mechanism led to the CBM-RR version, which contains only 113 reactions and 47 species. However, these two versions of essentially the same chemistry did not require extensive condensation testing since most of the condensation techniques were simple algebraic transformations. Such trans- formations do not significantly affect the ------- Table 1. The Carbon-Bond Mechanism Series Mechanism CBM-I CBM-II CBM-III CBM-X CBM-XR CBM-RR CBM-IV Description Original CBM Update/ Expansion of CBM-I Update of CBM-II with improved aromatics chemistry Expanded version of CBM-III suitable for use in EKMA CBM-X with isoprene chemistry for regional scale modeling Condensed version of CBM-XR Highly condensed version of CBM-RR Number of Reactions/ Species 35/20 65/27 75/36 146/67 170/78 113/47 70/28 Reference Whitten and Hogo (1977) Whittenet al. (1980a) Whitten et al. (1980b) Killus and Whitten (1984) Whitten et al. (1985a) Whitten et al (1985b) Whittenet al. (1985b) This work Killus, J. P.. and G.Z. Whitten. 1984. Technical Discussion Relating to the Use of the Carbon-Bond Mechanism in OZIPM/EKMA. SYSAPP-84/117, Systems Applications, Inc., San Rafael, CA Whitten, G. 2., and H. Hogo. 1977. Mathematical Modeling of Simulated Photochemical Smog. EPA/600/3-77/011, U.S. Environmental Protection Agency, Research Triangle Park, NC Whitten, G. 2., H. Hogo, and J. P. Killus. 1980a. The Carbon-Bond Mechanism A Condensed Kinetic Mechanism for Photochemical Smog Environ. Sci. Techno/., 14:699. Whitten, G. Z, J. P. Killus, andH. Hogo. 1980b. Modeling of Simulated Photochemical Smog with Kinetic Mechanisms. EPA/600/3-80/028a, U.S. Environmental Protection Agency, Research Triangle Park, NC. Whitten, G. 2., J. P. Killus, and R. G. Johnson. 1985a Modeling of Auto Exhaust Smog Chamber Data for EKMA Development. EPA/600/3-85/025, U.S. Environmental Protection Agency, Research Triangle Park, NC. Whitten, G. 2., J. P. Killus, and R. G. Johnson. 1985b Development of a Chemical Kinetic Mechanism for the U.S. EPA Regional Oxidant Model. EPA/600/3-85/026, U.S. Environmental Protection Agency, Research Triangle Park, NC. numerical output of the integrated dif- ferential equations based on the chemical reaction set. However, much of the descriptive nature of the original expand- ed reaction set (CBM-XR) becomes lost in the algebraic transformation. The more extensive condensation employed during this project involved assumptions con- cerning the bounding conditions on var- ious reactions and species. This type of procelure is not trivial and requires stepwise verification over a range of possible conditions. The techniques used to condense the CBM-XR to the CBM-RR involved elim- ination of (1) stable products, (2) constant concentration species, (3) mass balance species, (4) unit stoichiometric factors, (5) species that only rapidly decay unimolec- ularly, and (6) the addition of universal peroxy radicals which convert NO to NC"2 or nitrates. With the exception of the last technique, no testing of the results is required except to verify the algebra and arithmetics involved. For the last tech- nique, the use of universal peroxy rad- icals, rather obvious tests were i mplicated since the maximum errors would be expected for conditions of near zero NO concentrations because the main reac- tions of peroxy radicals are with NO. Smog chamber data were therefore used from experiments in which NO approach- ed zero several times during runs up to four days. For this project the new techniques used for condensation were (1) elimina- tion of unimportant species, (2)the use of implicit steady-state approximations, and (3) combinations or extensions of previous techniques. When species and reactions were eliminated, much care was taken to first search for and simulate the condi- tions where such species were most important to ensure that, even under such conditions, these species were still unimportant to the overall process of oxidant formation. For some species a boundary of importance was noted. For example, peroxynitric acid (PNA) and methylperoxynitric acid (MPNA) were found to be important species below 290 K under conditions of high oxidant forma- tion such as in a pure propene oxidation system. Hence, it might be wise to include PNA and MPNA if winter conditions are to be simulated where significant ozone formation occurs. Tests of Mechanism Condensation Since the expanded Carbon-Bond Mechanisms (CBM-X and CBM-XR) had already been validated against smog chamber data, the goal in this part of the project was to ensure that the condensa- tion steps produced a condensed version (CBM-IV) that gives model results similar to the expanded versions. To ensure that one condensation step did not produce compensating effects that might cancel some previous condensation step, tests were performed at each step in the condensation process rather than only with the completely condensed CBM-IV mechanism. Three multi-component outdoor smog chamber experiments encompassing a wide range of chemical, temporal, and meteorological conditions were used to demonstrate most of the condensation steps. Two of these experiments were multi-day runs performed at the Univer- sity of California at Riverside and the third experiment was performed at the Univer- sity of North Carolina (UNC). All these experiments involved multicomponent, urban-like mixtures of hydrocarbons. Temperature ranges were tested using two propene outdoor experiments per- formed at the UNC facility in December and October. An isoprene experiment at UNC was used to test the isoprene condensation steps. Summary The project report provides the details of the condensed versions (CBM-IV) of the extended Carbon-Bond Mechanisms (CBM-X and CBM-XR). The report des- cribes the various methods of mechanism ------- condensation employed and how each of these methods were tested. The conden- sation of the expanded CBM to CBM-IV was tested in an incremental manner by simulating seven days of outdoor smog chamber data from two different experi- mental facilities. These data provided a wide range of time, temperature, light intensity, and concentration conditions for comparing simulation results from increasingly condensed mechanisms with the expanded CBM results. This approach was enacted to minimize com- pensating errors and to verify the operat- ing boundaries of the mechanism at each condensation step. This development yielded a mechanism containing 28 spec- ies and 70 reactions (including isoprene chemistry) compared with 78 species and 170 reactions contained in the CBM-XR. Because the CBM-IV includes isoprene chemistry, the mechanism is suitable for use in gridded regional oxidant models. With the further addition of sulfur chem- istry, along with PNA, MPNA, and organic acid chemistry, it may also be suitable for modeling homogeneous gas phase chem- istry in acid rain models. Moreover, additional product species that account for organic nitrates, organic peroxides, or peroxy acids can easily be included in the CBM-IV. However, the CBM-IV must be used with caution under conditions for which it has not yet been tested. Although the CBM-IV may provide the performance of a highly detailed mechan- ism in an efficient and compact form, all the chemistry is not visible and mechan- ism updates may require both recalcula- tion of parameters and retesting. G. Z. WhinenandM. W. Geryare with Systems Applications, Inc.. SanRafael, CA 94903. Marcia C. Dodge is the EPA Project Officer (see below). The complete report, entitled "Development ofCBM-X Mechanisms for Urban and Regional AQSMs," (Order No. PB 86-155 033/AS; Cost: $16.95, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Atmospheric 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 EPA/600/S3-86/012 00003Z9 PS U S ENVIR PROTECTION AGENCY REGION 5 LIBRARY 230 S DEARBORN STREET CHICAGO IL 60604 ------- |