Comparative analysis for the impacts of VOC subgroups and atmospheric oxidation capacity on O3 based on different observation-based methods at a suburban site in the North China Plain

Abstract

The persistent O₃ pollution in the Beijing-Tianjin-Hebei (BTH) region remains unresolved, largely due to limited comprehension of O₃-precursor relationship and photochemistry drivers. In this work, intraday O₃ sensitivity evolution from VOC-limited (volatile organic compound) regime in the forenoon to transition regime in the late afternoon was inferred by relative incremental reactivity (RIR) in summer 2019 at Xianghe, a suburban site in BTH region, suggesting that VOC-focused control policy could combine with stringent afternoon NO_x control. Then detailed impacts of VOC subgroups on O₃ formation were further comprehensively quantified by parametric OH reactivity (K^OH), O₃ formation potential (OFP), as well as RIR weighted value and O₃ formation path tracing (OFPT) approach based on photochemical box model. O₃ episode days corresponded to stronger O₃ formation, depicted by higher K^OH (10.4 s^-1), OFP (331.7 μg m^-3), RIR weighted value (1.2), and F(O₃)-OFPT (15.5 ppbv h^-1). High proportions of isoprene and OVOCs (oxygenated VOCs) to the total K^OH and the OFPT method were demonstrated whereas results of OFP and RIR-weighted presented extra great impacts of aromatics on O₃ formation. The OFPT approach captured the process that has already happened and included final O₃ response to the original VOC, thus reliable for replicating VOC impacts. The comparison results of the four methods showed similarities when utilizing K^OH and OFPT methods, which reveals that the potential applicability of simple K^OH for contingency VOC control and more complex OFPT method for detailed VOC- and source-oriented control during policy-making. To investigate propulsion of VOC-involved O₃ photochemistry, atmospheric oxidation capacity (AOC) was quantified by two atmospheric oxidation indexes (AOI). Both AOIp_G (7.0 × 10⁷ molec cm^-3 s^-1, potential AOC calculated by oxidation reaction rates) and AOIe_G (8.5 μmol m^-3, estimated AOC given redox electron transfer for oxidation products) were stronger on O₃ episode days, indicating that AOC promoted the radical cycling initiated from VOC oxidation and subsequent O₃ production. Result-oriented AOIe_G reasonably characterized actual AOC inferred by good linear correlation between AOIe_G and O₃ concentrations compared to process-oriented AOIp_G. Therefore, with continuous NO_x abatement, AOIe_G should be considered to represent actual AOC, also O₃-inducing ability.

Keywords: K(OH); OFP; Ozone formation path tracing approach; Photochemical box model; Quantitative index of AOC; RIR(-Weighted).