Source apportionment of Volatile Organic Compounds (VOCs) in the South Coast Air Basin (SoCAB) During RECAP-CA

Ozone (O₃) concentrations in the South Coast Air Basin (SoCAB) surrounding Los Angeles remain at unhealthy levels despite multiple decades of control programs designed to reduce emissions of precursor Volatile Organic Compounds (VOCs). Here we report on comprehensive VOC measurements made at Redlands, which has the highest measured O₃ concentrations in SoCAB, as part of the Re-Evaluating the Chemistry of Air Pollutants in California (RECAP-CA) field campaign (July–October 2021). Positive matrix factorization (PMF) analysis was applied to identify nine VOC factors. A photochemical chamber model initialized by field measurements was configured with a tagging technique to quantify the VOC factor contributions to O₃ formation in Redlands. Biogenic VOCs (BVOCs) made the largest contribution (26.6%) to O₃ formation, 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 that concentrated VOCs from all sources and high temperature days that enhanced O₃ formation efficiency. This implies that VOC controls optimized to reduce O₃ concentrations would look similar in both the NO_x-limited and VOC-limited regimes that can occur at Redlands. These results suggest that control strategies that reduce VOC and NO_x emissions from the on-road vehicle fleet, such as increasing electrification, may yield O₃ reductions on days in both the NO_x-limited and VOC-limited chemical regimes at Redlands.