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

Abstract

Lack of recent progress in reducing ground-level ozone (O 3 ) 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 O 3 to volatile organic compounds (VOCs) and nitrogen oxides (NO x = NO + NO 2 ) in Pasadena and Redlands, California from July to October, 2021. The transportable smog chamber system measures the ambient O 3 sensitivity and the ambient O 3 chemical regime by comparing O 3 formation in a basecase chamber and a perturbed chamber. The monthly median observed O 3 sensitivity in Pasadena was stable in the VOC-limited regime, but showed a seasonal trend in Redlands, where median O 3 sensitivity was VOC-limited in July and October and transitioned towards the NO x -limited regime in August and September. Day-specific O 3 sensitivity at both Pasadena and Redlands could be either NO xlimited or VOC-limited on O 3 -nonattainment days. Calculated O 3 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 O 3 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 O 3 concentrations above 70 ppb to the NO x -limited regime where further NO x reductions will result in lower O 3 concentrations.