UC Davis

Lack of recent progress in reducing ground-level ozone (O3) concentrations to comply with health-based standards in California has motivated a reanalysis of emissions control strategies. Effective O3 control strategies account for the O3 sensitivity of its two primary precursors: volatile organic compounds (VOCs) and nitrogen oxides (NOx=NO+NO2), and then target the anthropogenic sources that emit the limiting precursor. In this dissertation, we developed a transportable smog chamber system to directly measure the response of O3 to perturbations in precursor NOx and VOCs under ambient atmosphere in the field. The chamber measurements were combined with field measurements, statistical analysis and box modeling to analyze the O3 photochemistry in California.

Long-term measurements of O3 sensitivity at Sacramento, CA in 2020 observed seasonal trends that shifted from VOC-limited in winter towards NOx-limited in summer and back to VOC-limited in fall. These trends were confirmed to be widespread across California using satellite measurements of O3 sensitivity (TROPOMI HCHO/NO2). A comprehensive measurement of O3 sensitivity and VOCs were made at two sites (Pasadena and Redlands) in Southern California as part of the Re-Evaluating the Chemistry of Air Pollutants in California (RECAP-CA) field campaign (July ~ October, 2021). Redlands observed a similar seasonal trend as observed in Sacramento, where O3 sensitivity was VOC-limited in July and October and transitioned towards the NOx-limited in August and September, but Pasadena was stable in the VOC-limited regime. An observation-constrained box model was developed to calculate the O3 isopleth for Pasadena and Redlands. The isopleth suggests that an additional ~40% NOx reduction is needed to move 95% of the days with O3 concentrations above 70 ppb to the NOx-limited regime where further NOx reductions will result in lower O3 concentrations.

VOC concentrations measured at Redlands were apportioned to nine source types using a Positive Matrix Factorization (PMF) model. The observation-constrained model was then configured with a tagging technique to quantify the VOC factor contributions to O3 formation. Biogenic VOCs (BVOCs) made the largest contribution (26.6%) to O3 formation in Redlands, followed by traffic VOCs (21.2%), Volatile chemical products (VCPs) (19%), and plant decomposition (14.9%). High O3 episodes were not driven by increased VOC emissions from any single source, but rather were associated with stagnation events. This implies that VOC controls optimized to reduce O3 concentrations would look similar in both the NOx-limited and VOC-limited regimes.