Post Date: 8 January 2025

Observation of volatile organic compounds by in situ Gas Chromatography coupled with Vocus PTR-Tof-MS at a suburban site in Hong Kong

Abstract:

Proton transfer reaction time-of-flight mass spectrometry (PTR-Tof-MS) has been extensively employed to quantify ambient Volatile organic compounds (VOCs) in a multitude of environments. However, PTR-Tof-MS is constrained by its ability to detect only isobaric ions and interference from fragmentations, water clusters, and other ionization products. Therefore, a thermal desorption preconcentration gas chromatography (GC) coupled with Vocus PTR-Tof-MS was deployed to measure the ambient VOCs at a suburban station in Hong Kong and explore the isomers and interference for PTR measurement. The system alternated between Vocus-only detection for higher time-resolution (RT-Vocus) and GC-Vocus for molecular speciation. We identified 50 compounds in GC Chromatogram and decomposed the RT-Vocus signal to their isomers. The real-time PTR measurement underestimated the concentration of un-calibrated OVOCs (pentanal, hexanal, heptanal, octanal) due to large fragmentation ratios of long-chain aldehydes in PTR and overestimated sensitivities. Interferences affecting the RT signal of isoprene, benzene, styrene, and phenol are also identified by GC chromatogram, showing that the interference may contribute 60.2%, 41.0%,14.3%, and 44.6% of their RT signal, respectively. Our findings demonstrate that the LOH of aldehydes was also underestimated by RT-Vocus measured data, and certain monoterpenes, particularly γ-terpinene, play a crucial role in OH reactivity due to their high reaction rates. Accurate O3 production rate simulation requires corrections in VOC concentrations and isomer abundances, underscoring the importance of coupling GC system with the PTR measurement.