O₃ Sensitivity to NO_x and VOC During RECAP-CA: Implication for Emissions Control Strategies

Lack of recent progress in reducing ground-level ozone (O₃) concentrations to comply with health-based standards in the South Coast Air Basin (SoCAB) has motivated a reanalysis of emission control strategies. Here we used two parallel transportable smog chamber systems to measure the sensitivity of O₃ to volatile organic compounds (VOCs) and nitrogen oxides (NO_x = NO + NO₂) in Pasadena and Redlands, California from July to October, 2021. The transportable smog chamber system measures the ambient O₃ sensitivity and the ambient O₃ chemical regime by comparing O₃ formation in a basecase chamber and a perturbed chamber. The monthly median observed O₃ sensitivity in Pasadena was stable in the VOC-limited regime, but showed a seasonal trend in Redlands, where median O₃ sensitivity was VOC-limited in July and October and transitioned towards the NO_x-limited regime in August and September. Day-specific O₃ sensitivity at both Pasadena and Redlands could be either NO_x-limited or VOC-limited on O₃-nonattainment days. Calculated O₃ isopleths for Pasadena and Redlands were constructed using a photochemical box model based on comprehensive measurements of NO_x and VOCs during the Re-Evaluating the Chemistry of Air Pollutants in California (RECAP-CA) campaign. Calculated O₃ isopleths were in good agreement with the chamber measurements. The calculations suggest that an additional ∼40% NO_x reduction is needed for Pasadena and Redlands to move 95% of the days with O₃ concentrations above 70 ppb to the NO_x-limited regime where further NO_x reductions will result in lower O₃ concentrations.