Reactivity Scientific Advisory Committee (RSAC) Meeting
February 24, 1998

This page last reviewed August 1, 2008

Reactivity Scientific Advisory
Committee (RSAC) Meeting Information
February 24, 1998

The second meeting of the Reactivity Scientific Advisory Committee took place on February 24, 1998, at the Air Resources Board in Sacramento, California.
At the meeting, ARB staff provided a brief introduction of the Committee's role, the Board's needs concerning hydrocarbon reactivity, and a list of hydrocarbon reactivity topics that were to be discussed.
The Committee then offered comments on the discussion topics, and the public was given an opportunity to comment.

Results of this meeting will be posted as soon as they are available.

Meeting Topics Discussed

Uncertainties in Reactivity Values
Values for the maximum incremental reactivity (MIR) have been used as the basis for determining the reactivity adjustment factors in the ARB's Low-Emission Vehicles/Clean Fuels regulations. The MIR scale is also being considered for future reactivity-based programs at the ARB. Important to any regulatory approach that incorporates MIR values is the level of confidence we have that the MIR values are accurate, that is, that they will not change significantly as the science
improves.
Dr. William Carter of the University of California at Riverside has developed a method for ranking a compound's MIR value based on its chance to change with further research. The complete listing of the MIR values and their rankings are available as a PDF file. Basically, Dr. Carter proposes ten categories reflecting degrees of certainty based on the amount and quality of experimental data and scientific knowledge associated with each compound. Dr. Carter prefers the ranking-by-bins approach and believes that a numerical uncertainty bound is not particularly useful or relevant in most cases.
Specific questions discussed on this topic:

1.

Are the metrics that are used to rank the compounds appropriate?

2.

Are there other metrics that should be considered in developing the rank of a MIR value?
Estimation of Upper-Limit Default MIR Values
A generally accepted methodology for estimating default MIR values for compounds with little or no experimental reactivity information could speed development of reactivity-based regulations. Additional flexibility in the regulation would be realized by allowing for the use of these unstudied compounds. However, it is important that the estimation methodology provide results that estimate the upper limit of a compound's likely ozone-formation potential.
Dr. Carter has developed an estimation technique based on deriving upper limits for the two factors that determine a compound's reactivity -- the fraction of the compound's emissions that react (kinetic reactivity), and the amount of ozone formed per molecule of compound that reacts (mechanistic reactivity). The kinetic reactivity will range from 0 to 1, while the mechanistic reactivity can be significantly greater than 1. A paper describing Dr. Carter's technique is available as a PDF file for FTP download.
Specific Questions on This Topic:

1.

Are the estimation techniques used to calculate the kinetic and mechanistic reactivity appropriate?

2.

Do better estimation techniques exist?

3.

What are the scientific advantages and disadvantages of using this method rather than an arbitrary
default MIR value?

4.

Are there specific compounds for which or instances when the technique is not appropriate?
Reactivity and the New 8-hour Ozone Standard
The new National Ambient Air Quality Standard (NAAQS) for ozone is based on an 8-hour average (the existing standard is based on a 1-hour average). To attain the new standard, overall reduction in the afternoon ozone peak may be necessary, and a reduction in the peak ozone value alone may not suffice. It is also expected that certain rural areas may have difficulty in attaining the new standard due to sustained high ozone concentrations in these areas. Thus, it is expected that innovative control strategies would be required for compliance with the new standard.
Questions Specific to Reactivity and the New 8-Hour Ozone NAAQS:

1.

Are traditional reactivity measures such as MIR and MOIR geared only toward attaining the old 1-hour standard?

2.

What additional measures of reactivity would be needed to ensure that the concept of reactivity can be used in
designing strategies to attain the 8-hour standard?

3.

Would the ordering of compounds in the reactivity scale for the 8-hour standard be different from that for the
1-hour standard?

4.

Are compounds with low or negligible reactivities and high mass emission rates more important with the
longer averaging time of the new standard?
Reactivity Scales for Aerosol Formation
The new national standards for PM2.5 and regional haze will require the reduction of both inorganic and organic secondary aerosols. The most direct way to control the formation of secondary aerosols is to reduce the emission of their precursors. The present models for the formation of secondary aerosols are useful for inorganic aerosols but are not as well developed for organic aerosols.
Questions Specific to Reactivity Scales for Aerosol Formation are:

1.

Is it possible, based on current scientific knowledge, to develop a comprehensive reactivity scale for the formation of secondary organic aerosols?

2.

Would the same reactivity scale be applicable to both PM2.5 and regional haze strategies?

3.

Some compounds could have negligible ozone reactivities and large aerosol reactivities. Hence, the ordering
of a given set of compounds on an ozone reactivity scale could be different from that on the aerosol reactivity
scale. What is the best way to reconcile this possible difference?

4.

What is the current status of computer models for simulating the formation of secondary aerosols, and what improvements (if any) need to be incorporated before the models can be used for regulatory purposes?
SAPRC vs. Carbon Bond IV
The MIR values used in developing reactivity-based regulations, such as the Low-Emission Vehicles / Clean Fuels regulations and a possible reactivity-based consumer product regulation, are based on the SAPRC chemical mechanism. However, the U.S. Environmental Protection Agency recommends that the Carbon Bond IV (CBIV) chemical mechanism be
used as the regulatory mechanism for State Implementation Plan (SIP) evaluation. Therefore, the effects of control strategies
based on alternative formulations that are deemed to meet reactivity limits are being evaluated using the CBIV embedded in
the gridded3-D Urban Airshed Model.
Questions Specific to This Topic are:

1.

Are the benefits of a regulation based on SAPRC-based MIRs (e.g., California's State Implementation
Plan for Ozone) appropriately evaluated using the CBIV mechanism?

2.

In what ways would the results be expected to differ if the same mechanism were used for both MIR
determination and SIP modeling evaluations?



Reactivity Scientific Advisory Committee

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