Project at a Glance

Title: Assessment of effects of chemical mechanism uncertainty on airshed model results.

Principal Investigator / Author(s): Kumar, Narseh

Contractor: UC Riverside

Contract Number: 93-716

Research Program Area: Atmospheric Processes

Topic Areas: Chemistry & Reactivity, Modeling, Monitoring


The objective of this phase of the program was to determine the effect on Urban Airshed Model (UAM) simulations of using a different and more up-to-date chemical mechanism than the Carbon Bond IV mechanism which is presently incorporated in the regulatory model. The approach was to implement updated version of the SAPRC-90 mechanism of Carter (1990), designated the SAPRC-93 mechanism, in the UAM Flexible Chemical Mechanism (FCM) solver developed by Sonoma Technology, Inc (STI) in Phase I of this program (Kumar et aI, 1995) and use it to simulate the August SCAQS episode. Significant differences were found in the results. A discussion and interpretation of these results is beyond the scope of this report.

Work Carried Out Mechanism Implementation - The updated version of the SAPRC-90 chemical mechanism of Carter (1990), modified as described by Carter et al (1993) and Carter (1995), is listed in Appendix A. Files implementing this mechanism in the format required by the FCM solver of Kumar et al (1995) were prepared and made available for downloading from the Internet, as described in Appendix B. Some minor modifications had to be made to the emissions processing routines associated with the FCM software to implement this version of the mechanism. This updated mechanism is referred to as the SAPRC-93 mechanism in the subsequent discussion. The UAM program for this mechanism was recompiled and the chemical parameter input files were prepared as described by Kumar et. al. (1995).

UAM Simulations - UAM emissions input files were created for the August SCAQS episode for both the SAPRC-93 and the UAM 6.21 version of the Carbon Bond IV (CB4) mechanisms. These were generated with the 1987 on-road motor vehicle exhaust hydrocarbons scaled up by a factor of four. This scale-up was done previously to obtain what is considered acceptable performance of the UAM 6.21 model simulations to the chamber data, and is not to imply that the authors necessarily consider that this is an appropriate representation of the actual emissions. A new set of initial and boundary condition files were created for both mechanisms for this episode. UAM-FCM was first performed using the photolysis rates calculated the actinic fluxes distributed with the mechanisms. Since actinic fluxes are part of the scenario and not the mechanism, this is not an appropriate comparison of mechanism effects. Therefore, these simulations were repeated with the photolysis rates calculated using the same actinic fluxes.

Subsequently, emissions files were recreated using the SAPRC-93 mechanism with the 1987 on-road motor vehicle exhaust emissions scaled by a factor of two and UAM simulations were performed using these new emissions files. This was done to develop a more reasonable baseline case for sensitivity calculations, which were not carried out as part of this program and will thus not be discussed further here.

UAM input files for both chemical mechanisms, along with the modified UAM-FCM software incorporating the updates developed during this phase, were transmitted to the CARB technical support division staff.

For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

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