Assessing Near-Field and Downwind Impacts of Reactivity-Based Substitutions

Abstract

Three-dimensional chemical transport modeling of six different solvent substitution test scenarios was used to investigate possible transport effects of using volatile organic compound (VOC) reactivity scales for air quality management purposes with a particular focus on the northeastern United States. The primary issues analyzed are whether uses of reactivity-based substitutions adversely affect ozone concentrations downwind of the area in which they are applied and which reactivity scales appear most appropriate for areas where ozone transport between multiple cities is significant. VOC substitution scenarios were designed to assess biases in ozone metrics associated with substituting relatively highly reactive VOCs (as defined by the Maximum Incremental Reactivity [MIR] scale) associated with solvent use with less reactive VOCs that might be considered as possible substitutes. Aiming to balance industrially realistic and scientifically relevant constraints, the set of solvent emissions to be substituted included toluene, isomers of xylene, 2-butoxyethanol by the surrogate, and lower reactivity compounds 2-methylheptane and n-butyl acetate. For a 14-day episode in August 2002, seven scenarios were modeled including base-case emissions, removal of the selected higher reactivity solvent compounds, and substitution tests using the equivalent mass or equivalent reactivity-adjusted emissions based on the MIR and Maximum Ozone Incremental Reactivity (MOIR) reactivity scales. Results show that downwind increases in ozone concentrations are noticeable for the MIR-based substitution test scenarios although sensitivities demonstrate that these could be used to complement oxides of nitrogen (NO_x) controls. However, using the MOIR-scaled substitution test scenario led to results that were less biased, and the population-weighted metric showed little bias compared with the base case. Temporally and spatially extensive decreases are evident with the solvent mass substitution and the selected emissions removal test scenarios, supporting the conclusion that reactivity-based control can be used to regionally reduce ozone.