| 11 Mar 2026
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 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 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 OFP estimation between observed and reconstructed initial concentrations. Specifically, OFP contributions from reconstructed NMHCs (52.3 %) were underestimated by 31.7 % when derived from observed concentrations for this site, whereas those from reconstructed 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 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.
Summary
In this study, the authors reconstruct VOC emissions at a suburban site in Shanghai and estimate ozone formation potential based on reconstructed results. The topic is within the scope of ACP. However, in my point of view, this study does not provide sufficient novel insights into VOC emissions for promoting the understanding of ozone pollution in China. The overall framework remains largely an extension of conventional OFP calculations based on MIR by Carter et al, while the observations are limited by discontinuous sampling and a single-site dataset.
Major comments:
1) The most significant concern is that this work relies on MIR-based OFP estimation derived from Carter et al, which remains a highly simplified reactivity metric rather than a process-resolving representation of ozone production. In recent years, many studies in China have already highlighted the limitations of conventional MIR/OFP approaches and have proposed more localized or observation-constrained methods to diagnose ozone precursor importance under region-specific chemical regimes. Although the authors attempt to reconstruct initial VOC concentrations and correct biases arising from photochemical aging, the analysis still builds on a simple OFP framework rather than a more rigorous assessment.
2) Another major concern is the very short sampling period and the quality of the VOC observations. As shown in Figure 1, VOC observations contain substantial missing periods, especially during the first three weeks of August, where only intermittent measurements are available and the time series is highly discontinuous. I have serious concerns regarding the quality of the dataset and the robustness of the source apportionment as well as reconstruction results. When the observational basis is fragmented to this extent, it becomes difficult to support strong quantitative statements regarding the relative contributions of OVOCs, NMHCs, and unmeasured species to ozone precursor budgets.
3) Another key issue is that the importance of OVOCs in Chinese urban and suburban atmospheres, including their sources, formation pathways, and contribution to ozone chemistry, has already been extensively investigated in previous studies, including many conducted in the Pearl River Delta, Yangtze River Delta, and other heavily polluted regions of China. Compared with prior studies, this analysis is based on a single suburban site with limited available data points. The authors rely on only approximately three weeks of available VOC observations for their analysis. It remains unclear whether the key conclusion regarding the contribution of directly emitted OVOCs would be changed if the sampling period were extended to a full season or even an annual scale.
4) In the Atmospheric implications section, the authors suggest that this work supports a shift toward reactivity-based VOC management. However, a critical issue is overlooked: how important are these OVOCs for ozone pollution? In other words, what is their contribution to ozone levels in Shanghai under ambient conditions? This is a central question that the study should address. A more rigorous analysis, such as using an MCM-based box model, is essentially needed to quantify and elucidate the contribution of OVOCs to ozone formation.
5) In terms of the comprehensiveness of the data and the scientific insights presented in the manuscript, I think this paper does not qualify as a research article in its current form in ACP.