Resolving Isomer and Interference Biases in PTR-ToF-MS Measurements of Atmospheric VOCs and Photochemical Impacts

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 isobaric interferences, fragmentation, and ionization byproducts. Here, a thermal desorption preconcentration gas chromatography (GC) coupled with Vocus PTR-ToF-MS was deployed at a suburban site in Hong Kong to resolve isomers and quantify interferences for ambient VOCs measurement. We identified and quantified 48 compounds using GC-PTR measurements and resolved their isomer profiles in real-time PTR data based on GC-derived fractions. Our analysis revealed that real-time (RT) PTR measurements substantially underestimate long-chain aldehydes (e.g., C5–C8 aldehydes) due to extensive fragmentation, while overestimating isoprene, benzene, styrene, and phenol by 14-60% because of interference from other species. These biases propagate into photochemical modeling, leading to overestimation of daytime ozone production by ~40% and of biogenic VOCs’ OH reactivity. Correcting isomer distributions and interference effects reduces modeled ozone production rates and alters 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.