Applying the CMAQ-Direct Decoupled Method to the Tri-Cities Ozone Precursor Study (T-COPS)

Localized high ozone episodes affecting the Tri-Cities area in eastern Washington were initially detected via the AIRPACT5 air quality forecast system for the Pacific Northwest region. These episodes provided the motivation for the Tri-Cities Ozone Precursor Study (T-COPS) to investigate the temporal and spatial patterns of ozone precursors in the area. In T-COPS, measurements were made during a three-week period (July 27 to August 18, 2016) and included detailed VOC, NOx and other precursor and product concentration measurements. For policy guidance, we need to explore how ozone concentrations respond to fractional changes in precursor emissions. The Community Multiscale Air Quality (CMAQ) Decoupled Direct Method in 3 Dimensions (DDM-3D) is one of the most widely-used tools to calculate ozone sensitivities with respect to precursor emissions for regulatory purposes and scientific investigations. Calculating the sensitivity coefficients by DDM enables evaluating the effectiveness of emission controls at a relatively low computational cost. Sensitivity of ozone to precursor emissions also provides a basis for diagnosing and mapping NOx and VOC limited areas to help guide potential control strategies. Results from running CMAQ5.0.2-DDM for a 20-gridcell box around the Tri-Cities region suggests a highly VOC-sensitive region. Further DDM runs for larger regions around the Tri-Cities will be used to account for the contributions of upwind sources emissions to elevated ozone levels.