Comparison of Chemical Transport Model and Observation Derived Ozone-NO x ‑VOC Isopleths for the South Coast Air Basin of California.

We developed, applied, evaluated, and compared chemical transport model-based and observation-based methods for constructing ozone isopleths used to understand ozone-emission relationships, focusing on the South Coast Air Basin of California (SoCAB). The High-order Decoupled Direct Method (HDDM) embedded in the Community Multiscale Air Quality (CMAQ) model was applied under different emission levels to map out the sensitivities of ozone levels to anthropogenic emissions of nitrogen oxides (NO x ) and volatile organic compounds (VOCs). Ozone concentrations and sensitivities to emissions simulated by CMAQ-HDDM were used to develop ozone isopleth diagrams for monitoring locations across the SoCAB, and those isopleths were compared with observation-derived isopleths. The CMAQ-HDDM- and observation-derived isopleths reproduced the observed ozone concentrations (R 2 = 0.70 and 0.96, respectively) and showed better agreement, especially at inland locations, which typically experience higher ozone levels than central or coastal sites. The responses of ozone to NO x and VOC emissions were compared over time, based on historical VOC and NO x emission levels. The sensitivities to NO x emissions showed a stronger correlation (R 2 = 0.75) between the CMAQ-HDDM- and observation-derived methods than the sensitivities to VOC emissions (R 2 = 0.18). For most regions in SoCAB, both the CMAQ-HDDM- and observation-derived isopleths indicate that the ozone regime has transitioned from VOC-limited to NO x -limited around the late 2000s. As a result, for equal absolute emission changes, further reductions in NO x emissions are more effective than comparable VOC controls in achieving future ozone decreases, with greater confidence for inland cities in the SoCAB.