Proposal to add compounds to exclusion list of volatile organic compound definition: appendix 1

Appendix I: Reactivity Levels for VOCs

The U.S. EPA uses three methods to determine if a compound is negligibly reactive:

• "The first method is based on the reaction rate constant of K_OH in the air. This reaction is the first step in a series of chemical reactions in the formation of ozone. If this reaction is slow (smaller K_OH value), the compound will not likely form ozone at a fast rate.
• The other two methods are based on the MIR, expressed either on a reactivity per gram basis (grams of ozone formed per gram of VOC) or reactivity per mole basis (grams of ozone formed per mole of VOC). The MIR methods consider the activities from all steps in the ozone formation process from a specific organic compound, as opposed to just the first step of the chemical reaction."²⁶

Comparing MIR values on a molar versus mass basis can lead to different conclusions about whether a compound is less or more reactive than ethane.

The U.S. EPA considers that the molar comparison is more consistent with the original smog chamber experiments, which compares equal molar concentrations of individual VOCs that underlie the selection of ethane as the threshold. The mass-based comparison is consistent with how MIR values and other reactivity metrics are applied in reactivity-based emission limits. The mass-based comparison is slightly less restrictive than the molar-based comparison in that a few more compounds qualify as negligibly reactive.

Environment Canada considered that these three methods are relevant to compare compounds to ethane.

Table 1 lists the fifteen compounds and compares the reactivity levels of each compound to that of ethane. TBAc, propylene carbonate, dimethyl carbonate HFO-1234ze, HFO-1234yf and Solstice^TM 1233zd(E) were excluded using the MIR methods, because the respective K_OH value of each compound was greater than the K_OH value of ethane.

Table 1: Reactivities of Ethane and Compounds Considered for Exclusion in Canada's Proposal²⁷

^*Currently listed on U.S. EPA’s VOC exclusion list

^**Proposed to be added on U.S. EPA’s VOC exclusion list

²⁶ Sacramento Metropolitan Air Quality Management District, Staff Report - Rule 101 - General Provisions and Definitions (PDF format, 58 KB), August 17, 2010.

²⁷ Data originate from the U.S. EPA's revision to the definition of VOC rules.