Effects of Chemical Mechanism Uncertainties on the Reactivity Quantification of Volatile Organic Compounds Using a Three-Dimensional Air Quality Model

Accurate quantification of the ozone-forming potential, termed “reactivity”, of voltaile organic compounds (VOCs) is critical for correctly assessing the impacts of emissions on air quality. As reactivity-based regulations are being more carefully considered for urban ozone control strate gies, the uncertainties in our ability to quantify reactivity are gaining importance. This study utilized a three-dimensional air quality model to examine the uncertainty in reactivity quantification resulting from a set of reaction rate constant uncertainties. A previous study identified the set of rate constants that were most critical for single-cell model ozone predictions. With the detailed airshed model, uncertainties in rate constants for aldehyde photolysis, nitric acid formation, and decomposition of peroxy acetyl nitrate (PAN) and peroxy propionyl nitrate plus higher PAN analogues (PPN) exhibited the greatest impact on relative compound reactivity values. For the compounds and reactions examined, the combined responses to 2σ changes in reaction rate constants were approximately 15% of the predicted relative reactivity values, with the reactivities of ethylbenzene and toluene exhibiting the greatest response. The choice of reactivity quantification measures and the air quality models used had a greater impact on relative reactivity predictions than did the rate constant uncertainties.