Abstract. 

While proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) is widely used for ambient volatile organic compounds (VOCs) quantification, its accuracy is limited by isomeric speciation and ionization byproducts. Here, gas chromatography (GC) coupled with PTR-ToF-MS was deployed at a suburban site in Hong Kong to resolve isomers and quantify interferences for ambient VOCs measurement. We identified 48 compounds using GC-PTR and resolved their isomer profiles in direct-PTR data. Our analysis revealed that direct-PTR measurements substantially underestimated long-chain aldehydes (C₅–C₈) due to extensive fragmentation, while overestimating isoprene, benzene, styrene, and phenol by ∼14–60% because of interference from other species. The biogenic VOCs’ OH reactivity was overestimated by up to 31% for monoterpenes and 241% for isoprene, depending on the seasonality of biogenic emissions. Propagating these biases into the photochemical model resulted in a 46% overestimation in daytime net O₃ production under VOC-limited and low-isoprene conditions, whereas the overestimation decreased to 19% under VOC-saturated, high-isoprene conditions. Correcting isomer distributions and interference effects reduced modeled ozone production rates and altered precursor sensitivities, revealing a larger role for oxygenated VOCs in ozone formation than previously recognized. Our results highlight the necessity for isomer-resolved measurements and interference-aware calibration to improve VOC-based assessments of photochemical air pollution.