Minutes of the Reactivity Scientific Advisory Committee (RRAC)
October 8, 1999

I Introduction
  The fourth meeting of the Reactivity Scientific Advisory Committee (RSAC) was held on October 8, 1999 in Riverside, California. The meeting was chaired by Dr. John Seinfeld. Dr. Roger Atkinson, Dr. Jack Calvert, Dr. Harvey Jeffries, Dr. Jana Milford, and Dr. Armistead Russell also participated. Mr. Don Ames, Assistant Division Chief of the Stationary Source Division of the Air Resources Board (ARB), opened the meeting with a short welcome. Next Mr. Andrew Chew, of the Stationary Source Division, gave a short presentation on the use of reactivity in the proposed California Low Emissions and Reactivity Regulation for Aerosol Coatings. Dr. William Stockwell then provided a description of his peer review of SAPRC-99. The review included evaluations of the base mechanism; volatile organic compounds (VOCs) represented, using the assigned mechanistic parameter method; use of the Lumped Molecule approach; handling of uncertainty; and SAPRC-99 documentation.
II Discussion of the Final Report for the contract "Review of the Updated Maximum Incremental Reactivity Scale Published by Dr. William Carter in August 1998" and the SAPRC-99 chemical mechanism.
  Dr. Seinfeld opened the discussion to the committee of both the SAPRC-99 mechanism and the peer review. Much of the discussion centered around the topic of changes to Dr. Carter’s chemical mechanism and the resultant uncertainty in reactivity values. Some of the topics discussed included:
  • Uncertainty of a typical simulation with respect to ozone concentrations.
  • Estimates of uncertainty changes if relative reactivity is examined.
  • Changes to the base mechanism when going from SAPRC-94 to SAPRC-99.
  • Temperature dependence of the base mechanism.
  • Changes to some maximum incremental reactivity (MIR) values (e.g., glyoxal) between SAPRC90 and SAPRC-99.
  After discussing these items and other issues, the committee stated that, as additional research is performed, the chemical mechanism has proven to be robust. For this reason, objective estimates of the uncertainty associated with MIR values have gone down. Members of the committee approved of Dr. Stockwell’s analysis of changes in MIRs values based on the historical record. The committee asked that Dr. Stockwell recalculate the comparison using Dr. Carter’s most recent MIRs values. The committee also requested that the compounds that showed the greatest change in MIR value should be examined to try to understand the reason for the change. The committee stated it is necessary that ARB continue to support fundamental research in the area of atmospheric chemistry, so that areas of the mechanism which are still uncertain may be understood.
  Next Dr. Seinfeld opened the discussion to members of the public. Topics addressed included:
  •  A question as to whether the scenarios for MIRs are realistic. The committee responded that the relative reactivity doesn’t change between scenarios and that, in a study which examined an exposure metric calculated by a 3-D model, the relative reactivities correlated well with MIRs.
  • It was suggested that, if absolute rather than relative MIR values are used, a cushion of 50% should be added to the standards. The committee replied that the use of absolute rather than relative MIRs does not matter.
  • It was suggested improvement to the mechanism would occur if data were collected in an outdoor chamber, in addition to an indoor chamber. Dr. Carter agreed.
  • Dr. Richard Derwent stated that the good agreement between MIRs and Photochemical Ozone Creation Potentials calculated with the Master Chemical Mechanism provides confidence in both mechanisms. However, several compounds show significant differences in their reactivity estimates. He believes the uncertainty associated with reactivity may be significantly underestimated because of currently unsuspected chemical reactions.
  • The question was asked if MIR conditions are appropriate for California. The committee’s response was that is a policy call.
  • Dr. Carter stated that the model does a poor job of representing halogen chemistry. This may be a concern due to the large number of swimming pools and the associated use of chlorine in the South Coast Air Basin.
  • While reactivity values may increase from one version of SAPRC to the next, it is possible that a future version of SAPRC may include changes that cause reactivity values to go down. Therefore, is it appropriate that uncertainty factors are used to increase the absolute MIR values? The committee stated that, in their opinion, while this a policy question, it is better to be conservative.
  • What fundamental research still needs to be done? Is research into the base mechanism needed? The committee’s response was that research in the last five years has clarified a number of assumptions that had been made regarding the reactions of VOCs in the atmosphere. As future research investigates later generation products of the initial reactions of VOCs, the mechanism will continue to improve.
  After the lunch break, Dr. Seinfeld asked for the committee’s conclusions regarding Dr. Stockwell’s peer review and ARB’s use of SAPRC-99. Members of the committee stated that SAPRC-99 was a state-of-the-art chemical mechanism and that the peer review was excellent. The committee approved of both the peer review and SAPRC-99. Drs. Milford and Russell requested that the analysis of uncertainty be updated to the 1999 MIRs values, changes in relative reactivity be examined, and compounds that changed the most be scrutinized.
III Use of SAPRC-99 instead of Carbon Bond IV in modeling performed as part of the State Implementation Plan (SIP).
  The U.S. Environmental Protection Agency recommends that the Carbon Bond IV (CBIV) chemical mechanism be used as the regulatory chemical mechanism for State Implementation Plan (SIP) evaluations. The RSAC was asked to comment on the possible use of SAPRC-99 instead of CBIV in ARB’s SIP modeling. Dr. Jeffries recommended that, since SAPRC-99 is the most up-to-date chemical mechanism available, and as it has been thoroughly peer-reviewed, the ARB should use SAPRC-99 in future SIP modeling. He cited the example of North Carolina, which has successfully petitioned the U.S. Environmental Protection Agency (USEPA) to use a chemical mechanism other than CBIV. Members of the RSAC agreed unanimously with Dr. Jeffries' recommendation that ARB use SAPRC-99 instead of CBIV in future SIP modeling.
IV Public Comment Period
  The final item of the meeting was a public comment period. Topics discussed included:
  • The effect of particulate matter on MIRs.
  • USEPA’s proposed use of a mass-based reactivity comparison as the determinant for federal VOC exemptions, instead of the previously used mole-based reactivity comparison.