Composition and reactivity of volatile organic compounds in the South Coast Air Basin and San Joaquin Valley of California

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 calculated gas-phase hydroxyl radical reactivity (cOHR(TOTAL)) was quantified to be 6.1 and 4.6 s(-1) for the SoCAB and SJV, respectively. VOCs accounted for similar to 60 %-70 % of the cOHR(TOTAL) in both basins. In particular, oxygenated VOCs (OVOCs) contributed >60 % of the cOHR of total VOCs (cOHR(VOC)) and 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 cOHR(VOC) in the SoCAB and SJV, respectively. Furthermore, the contribution of BVOCs to the cOHR(VOC) increased with increasing cOHR(VOC) in the SoCAB, suggesting that BVOCs were important ozone precursors during high ozone episodes. Spatially, the trace gases were heterogeneously distributed in the SoCAB, with their mixing ratios and cOHR being significantly greater over the inland regions than the coast, while their levels were more evenly distributed in 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.