Composition and Reactivity of Volatile Organic Compounds in the South Coast Air Basin and San Joaquin Valley of California

Abstract. Comprehensive aircraft measurements of volatile organic compounds (VOCs) covering the South Coast Air Basin (SoCAB) and San Joaquin Valley (SJV) of California were obtained in the summer of 2019. Combined with the CO, CH4, and NOx data, the total measured gas-phase hydroxyl radical reactivity (OHRTOTAL) was quantified. VOCs accounted for ~60 %−70 % of the OHRTOTAL in both basins. In particular, oxygenated VOCs (OVOCs) contributed > 60 % of the OHR of total VOCs (OHRVOC) as well as the total observed VOC mixing ratio. Primary biogenic VOCs (BVOCs) represented a minor fraction (< 2 %) of the total VOC mixing ratio but accounted for 21 % and 6 % of the OHRVOC in the SoCAB and SJV, respectively. Furthermore, the contribution of BVOCs to the OHRVOC increased with increasing OHRVOC in the SoCAB, suggesting that BVOCs was important ozone precursors during high ozone episodes. Spatially, the trace gases were heterogeneously distributed in the SoCAB with their mixing ratios and OHR significantly greater over the inland regions than the coast, while their levels were more evenly distributed in the SJV. The results highlight that a better grasp of the emission rates and sources of OVOCs and BVOCs is essential for a predictive understanding of the ozone abundance and distribution in California.