v>EPA United States Environmental Protection Agency Atmospheric Sciences Research Laboratory Research Triangle Park NC 27711 Research and Development EPA/600/S3-85/042 July 1985 Project Summary Evaluation of Chemical Reac- tion Mechanisms for Photo- chemical Smog: Part III. Sensi tivity of EKMA to Chemical Mechanism and Input Parameters Toby B. Shafer and John H. Seinfeld Six chemical reaction mechanisms for photochemical smog described in Parts I and II are used to study the effect of input parameters on volatile organic compound (VOC) control requirements needed to meet the National Ambient Air Quality Standard for ozone. The pa- rameters studied are initial VOC com- position, dilution rate, post-8-A.M. emissions, base-case (present-day) 03 levels, entrainment from aloft of VOC and ozone, initial MONO and initial VOC/NOX ratio. The Empirical Kinetic Modeling Approach (EKMA) was used to generate ozone isopleths for each chemical mechanism. The VOC control needed to reduce the maximum ozone concentration from, some present-day value to 0.12 ppm, assuming no IMOX control and a specified initial VOC/NOX ratio, was calculated using the six chemical reaction mechanisms. The ini- tial VOC/NOX ratio is found to have the largest effect of all the parameters stud- ied on VOC control requirements. Choice of chemical mechanism, ozone entrainment from aloft and the compo- sition of the initial VOC mixture also have a large effect on predicted control requirements. To reduce the degree of uncertainty in control predictions using EKMA it is necessary to establish as ac- curately as possible the composition of urban air in early morning. Also, be- cause of the substantial effect the choice of chemical mechanism has on the predicted control requirements using EKMA, it is important that future work continue to be directed toward evaluating candidate chemical mecha- nisms with respect to their ability to simulate atmospheric smog chemistry. This Project Summary was devel- oped by EPA's Atmospheric Sciences Research 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 Currently there are several chemical reaction mechanisms available for use in predicting hydrocarbon and NOX re- ductions needed to meet the National Ambient Air Quality Standard (NAAQS) for ozone for a particular region. Be- cause of the numerous organic species in the atmosphere, it is computationally infeasible to use a chemical reaction mechanism that contains every reaction of every species in carrying out a con- trol calculation. Each chemical mecha- nism reduces the number of organic species in a somewhat different way. This reduction, or "lumping," would not be a concern if each of the chemical mechanisms gave similar predictions under typical atmospheric conditions. As discussed in Part II, however, several investigators have shown that different ------- Dchemical smog reaction mecha- "nisHs predict significantly different control requirements under the same conditions. In addition, these investiga- tors have found that the sensitivity of the mechanism's predictions to changes in input parameters varies among mechanisms. Because of the im- portance of making intelligent hydro- carbon control decisions, it is necessary to understand in as fundamental way as possible how the choice of chemical mechanism, as well as the choice of in- put parameters, affects control predic- tions. The object of this work, therefore, was to present a comprehensive, sys- tematic study of the sensitivity of pre- dicted hydrocarbon control require- ments for photochemical smog to the choice of chemical mechanism and choice of all input parameters in the control calculation. To study the sensitivity of hydrocar- bon control predictions to choice of chemical mechanism and input param- eters, we used the U.S. Environmental Protection Agency's Empirical Kinetic Modeling Approach (EKMA) in which ozone isopleths are generated as a func- tion of initial volatile organic compound (VOC) and NOX concentrations using a chemical reaction mechanism. In the EKMA methodology, a chemical reac- tion mechanism for photochemical smog is used to predict the maximum ozone concentration achieved over a fixed time or irradiation, say 10 hours, as a function of initial precursor concen- trations (VOC and NOX). The maximum ozone concentrations corresponding to each set of initial concentrations are then represented as isopleths, from which control requirements are ob- tained graphically by assuming that the reductions in initial concentrations needed to lower the maximum ozone concentration from one isopleth (the base-case value) to another (usually the NAAQSof 0.12 ppmlare linearly related to the emissions of each precursor. The chemical reaction mechanism is inte- grated from early morning, e.g. 0800 hours, to calculate the evolution of species concentrations in a hypothetical air parcel that starts from center city and is advected by the wind. The air parcel may increase or decrease in size due to changes in the mixing height and ac- quire additional pollutants from both fresh source emissions and entrain- ment of aged pollutants from aloft. The simulations on which the isopleths are based can be performed at two different levels of detail: Level II, which requires 2 a specific meteorological description of the air parcel trajectory and emissions along the trajectory, and Level III, which makes standard assumptions about the meteorology and which uses a region- wide average for emission rates. It has been shown that the sensitivity of ozone predictions to choice of chemical mech- anism does not change appreciably when one imbeds the mechanisms in models that are more sophisticated than the simple Lagrangian box model on which EKMA is based. Therefore, we expect that the results of the current study, which we obtained by use of EKMA, are a valid indication of the ef- fects of chemical mechanism and input parameters on predicted organic con- trol requirements regardless of the level of sophistication of the air quality model in which the mechanism is imbedded. Results We have examined the sensitivity of predicted VOC control requirements to the choice of chemical mechanism and to the variation of parameters associ- ated with the chemical and meteorolog- ical nature of the atmosphere. Each chemical reaction mechanism was found to respond to a variation of the parameters in a somewhat different way; in many cases the relative sensitiv- ity of the mechanism can be explained from an analysis of the intrinsic chemi- cal features of the mechanism. The sen- sitivity calculations can be summarized concisely as follows: Initial VOC Composition The choice of chemical mechanism has a large effect on VOC control predic- tions and on the sensitivity of the pre- dictions to a change in VOC composi- tion. The ERT mechanism is most sensitive to a change in composition; the CBM least sensitive. Aldehyde Content Predicted VOC control is quite sensi- tive to the aldehyde content of the initial VOC mixture. Generally, the ERT mech- anism exhibits the greatest sensitivity to changes in aldehyde content. VOC/NOX Ratio The VOC/NOX ratio was generally the parameter having the greatest influence on VOC control predictions. Generally, the ERT mechanism showed the great- est sensitivity to a change in the VOC/NOX ratio, and the CBM exhibited the least sensitivity. In addition, the sen- sitivity of predictions to a change in the VOC composition is greatest at low VOC/NOX ratios. Base Case Ozone Effects of changes in VOC composi- tion are more pronounced the lower the base case ozone. The DEM mechanism is most sensitive to the choice of base case ozone. Dilution Differences among the predictions of the mechanisms are somewhat smaller at higher dilution rates than they are at low dilution rates. Emissions When emissions are assumed to be present, a given percent VOC reduction represents a larger reduction in total VOC in the atmosphere than in the ab- sence of emissions. VOC Entrainment from Aloft Predicted VOC control requirements change by between 4 and 21% when VOC entrainment from aloft is included, regardless of the reactivity of the mix- ture assuming a base case ozone con- centration of 0.24 ppm, and between 6 and 40% when a base case ozone con- centration of 0.18 is assumed. Ozone Entrainment from Aloft All mechanisms exhibit a strong sen- sitivity to ozone entrainment from aloft. Ozone aloft is therefore a key variable in control studies. The sensitivity of VOC control to the VOC/NOX ratio is influ- enced more by the entrainment of VOC than by the entrainment of ozone. Initial MONO Predictions obtained with the ERT mechanism are most sensitive to initial MONO; predictions from the PW mecha- nism are least sensitive. Summary and Recommenda- tions The results obtained in the series of reports, of which this is the third, paint a consistent picture of the behavior of chemical reaction mechanisms for pho- tochemical smog. Several key issues emerged that are suggestive of future work: (1) In much of the existing laboratory smog chamber data base, chamber wall effects are important enough to make accurate discrimination among rival chemical reaction mechanisms difficult. Continuing extensive analysis of data from out-1 door chamber facilities is recom- " ------- mended as these systems seem to be characterized by smaller wall ef- fects than indoor facilities. (2) We have found that predicted VOC control requirements are highly sensitive to the assumed VOC/NOX ratio and the VOC composition of the initial mixture and emissions. Therefore, it is important that these variables be as accurately character- ized as possible for areas for which control strategies are to be de- signed. (3) Because predicted VOC control can be quite sensitive to entrainment from aloft, the organic and espe- cially the ozone concentration of en- trained air aloft must be better char- acterized in areas where control strategies are to be designed. (4) Continued effort should be placed on the evaluation of the perfor- mance of chemical reaction mecha- nisms in simulating both laboratory and outdoor smog chamber experi- ments. These experiments should include a wide range of conditions in order to gain a better understand- ing of the effects of input parame- ters (for example, initial VOC/NOX, emissions and initial VOC composi- tion) on control predictions. (5) When determining control strate- gies, regulatory agencies should use several chemical mechanisms because at this time it is impossible to determine which chemical mech- anism best simulates atmospheric chemistry. (6) It is most important to know the in- put parameters as accurately as possible since the sensitivity of one input parameter may depend on the values assumed for other parame- ters. T. B. Shafer and J. H. Seinfeld are with California Institute of Technology, Pasadena. CA91125. Marcia C. Dodge is the EPA Project Officer (see below). The complete report, entitled "Evaluation of Chemical Reaction Mechanisms for Photochemical Smog: Part III. Sensitivity of EKMA to Chemical Mechanism and Input Parameters," (Order No. PB 85-210 888; Cost: $11.50, 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/S3-85/042 0000329 PS U S ENVIR PR£I££TION &U.S. GOVERNMENT PRINTING OFFICE.-1985—559-016/2711 It IS I I II It II I ------- |