O<sub>3</sub> Sensitivity to NO<sub><i>x</i></sub> and VOC During RECAP-CA: Implication for Emissions Control Strategies
Shenglun Wu, Christopher P. Alaimo, Yusheng Zhao, Peter G. Green, Thomas M. Young, Shang Liu, Toshihiro Kuwayama, Matthew M. Coggon, Chelsea E. Stockwell, Lu Xu, C. Warneke, J. B. Gilman, Michael Robinson, Patrick R. Veres, J. A. Neuman, Michael J. Kleeman
Abstract
High Resolution Image Download MS PowerPoint Slide 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 x -limited 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.