Reactivity Scientific Advisory
Committee (RSAC) Meeting
February 3, 1997

This page last reviewed August 1, 2008

Meeting Summary

February 3, 1997

The first meeting of the Reactifity Scientific Advisory Committee was held at the California Institute of Technology on February 3, 1997. Each member received a briefing paper of discussion topics to help guide the committee in considering the needs of the ARB. Three general technical issues were outlined in the briefing paper.
The first, and perhaps the most fundamental, issue is the determination of the reactivity values for each hydrocarbon.
The second issue concerns negligibly reactive compounds, that is, the question as to when a hydrocarbon's reactivity is low enough to be exempt from regulation.
The third issue relates to emissions trading for compounds of different reactivities.
Specific questions were offered for each general topic and were used as starting points for discussion. It was recognized that the topics being discussed were complex and although definitive answers were desired, very few were expected.

Reactivity Values

Atmospheric chemist Dr. Bill Carter from the College of Engineering Center for Environmental Research and Technology, University of California at Riverside, was asked to give some background on how he developed the maximum incremental reactivity (MIR) scale. He described methods of determining MIRs for individual compounds and the weaknesses with the methods. The major points are summarized below:


The need for determination of uncertainty in the chemical mechanism.


The need to determine which of the more advanced chemical mechanisms should be used in estimating MIRs.


The fact that the MIR concept is based on high NOx conditions (conditions most sensitive to hydrocarbon controls) and does not adequately represent rural (low NOx) areas.


The potential for using exisiting sophisticated airshed models to estimate MIR values.


The choice within the spectrum of ambient concentrations to use as the "base" mixture in determining
the MIR value.

Negligibly Reactive Compounds


The committee felt that all hydrocarbon compounds should be inventoried regardless of their reactivity, with the exception of methane. Tracking all compounds ensures better understanding of ozone chemistry, and provides more complete data for modeling runs and validation studies.


A threshold reactivity level below which a compound would be exempt from regulation is a policy decision, not necessarily a technical one. With a reactivity scale, there is no need to exempt compounds from regulations less reactive compounds would naturally receive less attention. If threshold values are mandated, mass emissions and reactivity should be considered together. The threshold value should be tied to a specific compound (e.g., ethane), rather than an absolute value. A mechanism would also need to be put in place to periodically check that the mass emissions of exempt compounds are low enough for the compound to continue to be exempt.


More research is needed before determination of the effect of a compound's volatility on ozone formation or particulate formation can be adequately assessed.

Emissions Trading


Emission trades should be put on a reactivity, rather than a mass, basis. A consistent scale for reactivity is needed for trading. Reactivity values used in the trades must take into account the uncertainties associated with the values. A suggestion to accomplish this is to discount the reactivity value by its uncertainty (e.g., one or two standard deviations). There are methods available for calculating uncertainties associated with the reactivity values.


When trades are being considered, all of the air quality aspects that may result from the trade -- for example, the effect of the trade on other pollutants such as secondary particulate formation -- should be considered.

Future Research


Efforts should be continued to improve chemical mechanisms, kinetic rate constants, uncertainty analyses, biogenic emissions estimates, analytical techniques and the emissions inventory.


It is important to determine whether the presence of particulate matter has some effect on ozone levels. Aqueous aerosols have been shown to be effective at removing reactive nitrogen and hydroperoxy radicals, both of which are important to ozone formation. Reduction of aerosol levels might lead to an increase in ozone concentrations. These interactions should be investigated.

Reactivity Scientific Advisory Committee