Suggested Citation:
Wu, Shenglun, Christopher Alaimo, Yusheng Zhao, Peter G. Green, Thomas M. Young, Shang Liu, Toshihiro Kuwayama, Matthew M. Coggon, Chelsea E. Stockwell, Carsten Warneke, Jessica B. Gilman, Michael A. Robinson, Patrick R. Veres, J. Andrew Neuman, Michael J. Kleeman (2024)

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 NOx reductions will result in lower O₃ concentrations.