United States Environmental Protection Agency Office of Research and Development National Exposure Research Laboratory Research Abstract Government Performance Results Act (GPRA) Goal #1 Annual Performance Measure #124 Significant Research Findings: Updated CB4 Chemical Mechanism Included in Models-3/CMAQ Scientific Problem and Policy Issues Air quality (AQ) models describe the meteorological transport, atmospheric chemistry, and fate of air pollutants like ozone, particulate matter, and air toxics. These AQ models are used by EPA and state agencies to predict current and future concentrations of these pollutants and to develop strategies to bring pollutant concentrations below harmful levels. One critical component of AQ models is the description of the complex atmospheric photochemistry - the chemical mechanism. All chemical mechanisms in AQ models are approximations of the real atmospheric processes, condensing the myriad of atmospheric chemicals and chemical reactions into a smaller number of representative chemicals and reactions - in order to maintain computational speed while still representing the atmospheric chemistry well. This research describes a major upgrade to the most widely used AQ chemical mechanism, the implementation of the upgraded mechanism in EPA's Community Multiscale Air Quality (CMAQ) Model, and release of the upgraded mechanism for public use. The Carbon Bond 4 mechanism (CB4) has been the most widely used AQ chemical mechanism since its release in 1989. CB4 has been used in a wide variety of AQ models and forms the basis for many of EPA's regulatory policies for ozone and particulate matter (PM), and for State Implementation Plans (SIPs). CB4 has also been a cornerstone of research models that predict and analyze the production of secondary gas- and aerosol-phase pollutants. Despite its widespread use, CB4 has not seen a major update since 1989. Although minor modifications have been made to it, the majority of reaction rates are no longer consistent with current research. Since its original development, researchers have also discovered important pathways for ozone and PM production that are not adequately represented by the existing condensed species in CB4. Because of the importance of this mechanism to EPA and the public, we have performed a major update, creating the CB05 mechanism. We have also implemented the updated ------- mechanism in the newest version of EPA's air quality model, CMAQ. With this updated description of chemistry in CMAQ and other air quality models, EPA will be able to predict concentrations of criteria pollutants, in urban and non-urban areas, using our most up-to-date knowledge of atmospheric chemistry. While the original intent of this work was to improve ozone predictions, the updated organic and inorganic chemistry will also improve predictions of concentrations of particulate matter as well as toxic air pollutants, such as formaldehyde, mercury, and benzene. Research The major objective of this work was to provide a comprehensive update of Approach the CB4 chemical mechanism, consistent with the most recent international research, and to compare updated mechanism predictions against experimental data. We strived to maintain a balance maintaining the low computational cost of CB4, and enhancing the chemical detail that is needed to accurately characterize the chemistry that occurs throughout a wide range of atmospheric conditions. All reaction rates were reviewed and updated to be consistent with values published by the International Union of Pure and Applied Chemistry (IUPAC) and National Aeronautics and Space Administration-Jet Propulsion Laboratory (NASA-JPL) data evaluations. At each step of the CB05 mechanism development, the work was reviewed by both EPA/NERL and external scientists. All reaction rates were calculated at three sets of temperature/pressure conditions and compared with rates in the CB4 as well as in the SAPRC-99 mechanism, a second widely used condensed chemical mechanism for AQ modeling. The implementation of CB05 in CMAQ was tested by comparing results against two separate box model implementations, one conducted by EPA staff and another by external reviewers. The final CB05 mechanism was implemented in the full CMAQ version 4.5 (September 2005 release). Simulations were performed using two 1-month periods (January and July of 2001) in different seasons. The CB05 results were compared with those obtained using the CB4. Results and Not only does CB05 use the most up to date reaction rates and mechanisms, Impact but it has significantly greater capacity to represent the large variety of conditions found in ambient atmospheres - from urban to remote regions. Some major features are: • All inorganic reaction rates have been modified - some minimally, but many others substantially - so that they are now consistent with the most recently published IUPAC and NASA-JPL values. • CB05 increases NOx recycling and provides more detail about ------- peroxy radicals to improve simulations covering multiple days and to describe the atmospheric chemistry that occurs under rural and remote conditions much more accurately. • Increasing the number of species used to track aldehydes, peroxyacyl nitrates, and internal olefins helps the mechanism better represent the chemistry of these species - all of which are critically important to oxidant formation in both urban and remote areas. • Initial tests show approximately 13 percent increase in ozone, while the timing of peak concentration is unchanged. Average aerosol production in the test (nitrate, sulfate, and organic) generally decreases on average between 3 and 9 percent. These changes should improve the AQ simulations, and further testing is being performed against observations. • Two options have been included as part of the CB05; either one or both can be added to the base mechanism. These include an explicit chlorine mechanism and an explicit air toxics mechanism. • The CB05 has been evaluated against two large databases of smog chamber experiments at the University of North Carolina, Chapel Hill and the University of California, Riverside. CB05 simulations matched the observed data better than other mechanisms for many of the experiments. The CB05 mechanism has been successfully implemented in the latest release of CMAQ and made available to the public. This work contributes toward the long-term goal of producing advanced and accurate modeling tools that can be used by the Agency to support the implementation of current ozone standards and to improve the science used in reviewing these standards. Accurate and defensible chemical mechanisms are a key component of air quality models used to develop SIPs demonstrating compliance with National Ambient Air Quality Standards (NAAQS). Improving the predictions of criteria pollutants, ozone and PM, as well as air toxics, will help EPA and states develop more scientifically sound approaches for assessing and characterizing environmental exposure and risk, and developing strategies for reducing concentrations of harmful pollutants. The Office of Air Quality Planning and Standards will benefit from improvements in the scientific basis by which they evaluate air pollutant control strategies. Because CB4 is in widespread use both nationally and internationally, we expect that CB05 will be embraced by many in the research and regulatory community. External groups impacted by this research include academia, the NARSTO program, private consultants, and anyone performing air quality modeling. ------- The scoping effort for this work, technical direction, error checking, implementation of CB05 into CMAQ, and initial analysis was done by EPA/NERL scientists. The inclusion of updated rate constants, new species (split from lumped model species), and emissions mapping was performed under contract #RT-04-00675 with Yocke and Co. Emissions processing for CB05 was provided under contract #68-W-01-032 with Computer Sciences Corporation. The updated CB05 mechanism was presented to the public at the Community Modeling and Analysis System Center's 4th Annual CMAS Models-3 User's Conference, Sept. 26-28, 2005. The presentation at the conference described the results of the CB05 implementation into CMAQ, together with an initial analysis of differences between CB4 and CB05. This conference traditionally draws hundreds of participants in air quality modeling for three days of intensive discussion and interaction on application, improvements, and results from atmospheric models. The products of this work include the CB05 mechanism, a detailed contractor report describing the mechanism and evaluations, and files to implement CB05 in CMAQ, which have been archived in the CMAQ model archive. The entire package is scheduled for a formal, public, beta-release in October 2005. Recent publications and presentations associated with this research task include the following: Sarwar, G., D. Luecken, G. Yarwood, G.Whitten and W. Carter, 2005. "Implementing an Updated Carbon Bond Mechanism into the Community Multiscale Air Quality Model," presented at the 4th Annual CMAS Models-3 User's Conference, Chapel Hill, NC, September 26, 2005. Yocke and Co., 2005. Updates to the Carbon Bond Mechanism: CB05. RT-04-0065. July 29, 2005. Future Research Future research efforts must focus on identifying and understanding differences in concentrations and relative reduction factors between CB4 and CB05. Since previous SIP implementation and regulatory development within EPA relied on CB4, it is critical to understand the implications of switching to an alternative mechanism. We must focus on evaluating air quality models using the CB05 mechanism against all available monitoring data, over a variety of conditions, temperatures, seasons, and regions. Additional research will evaluate both the impacts of the new mechanism on computational speed and the improved accuracy in the chemical profiles and predictions. Research Collaboration and Research Products Contacts for Questions and inquiries can be directed to: Additional Information ------- Deborah J. Luecken U.S. EPA, Office of Research and Development National Exposure Research Laboratory Atmospheric Modeling Division, E243-03 Research Triangle Park, North Carolina 27711 Phone: 919/541-0244 E-mail: luecken.deborah@epa.gov Funding for this project was through the U.S. EPA's Office of Research and Development, National Exposure Research Laboratory, and the work was conducted by the Atmospheric Modeling Division. ------- |