Reconstructed VOC emissions reveal hidden ozone precursors: Overlooked roles of primary OVOCs and unmeasured species

Abstract. Ambient volatile organic compounds (VOCs), including non-methane hydrocarbons (NMHCs) and oxygenated VOCs (OVOCs), are critical precursors of tropospheric ozone (O3). However, conventional estimates of the ozone formation potential (OFP) derived from observed VOC concentrations may introduce substantial biases, as they neglect the photochemical degradation of primary VOCs and the concurrent generation of secondary OVOCs during atmospheric transport. This study quantified the sources of ambient OVOCs at a suburban site in Shanghai, China during summer 2020 to reconstruct their initial emission concentrations. Together with the reconstructed initial concentrations of NMHCs, we estimated the OFP of freshly emitted VOCs. In addition, the sources and the OFP of unmeasured VOCs were inferred by concurrent measurements of missing OH reactivity. Our results demonstrate that photochemical reactions substantially altered the composition and source characteristics of VOCs, leading to pronounced discrepancies in the OFP estimation between observed and reconstructed initial concentrations. Specifically, the OFP contributions from reconstructed primary emitted NMHCs (52.3 %) were underestimated by 31.7 % when derived from observed concentrations for this site, whereas those from reconstructed primary emitted OVOCs (33.2 %) were overestimated by 42.6 %. Reconstructed VOC emissions indicated that anthropogenic sources dominated total emissions (71.5 %), whereas OVOCs constituted a substantial fraction of total VOC emissions (40.8 %). Unmeasured VOCs, primarily of biogenic origin, contributed an additional 12.6 %. Collectively, OVOCs and unmeasured species exhibited an OFP comparable to NMHCs, underscoring their critical role in O3 production and the necessity of incorporating these species into the design of comprehensive and effective O3 control strategies.