Comparison of oxidation products generated from the reaction of α-pinene with hydroxyl radicals, chlorine atoms, and bromine atoms measured using ammonium adduct chemical ionization mass spectrometry

Abstract

Halogen atoms play important but under characterized roles in atmospheric oxidation chemistry. Here, we report laboratory measurements of gas- and condensed-phase products formed from the oxidation of α-pinene by hydroxyl radicals (OH), chlorine atoms (Cl), and bromine atoms (Br) in an oxidation fow reactor (OFR). Products were detected using a Vocus proton-transfer time-of-flight reaction mass spectrometer (PTR-ToF-MS) operated with low-pressure ammonium adduct (NH₄⁺) ionization and a Vaporization Inlet for Aerosols (VIA). We applied Positive Matrix Factorization (PMF) to classify precursor and product ions into early and later-generation oxidation products. While some common products were observed cross all oxidants, significant compositional differences were also apparent. Vocus:VIA signal ratios were used to estimate volatility trends, revealing that more highly oxygenated compounds and many halogenated products contributed to SOA formation. Cl and Br oxidation led to the formation of oxygenated volatile organic compounds (OVOCs) and secondary organic aerosol (SOA), which retained halogen atoms, with Br-derived products exhibiting the lowest carbon oxidation state and the highest halogen retention. Halogenated oxidation products were less volatile than their non-halogenated counterparts. Photochemical modeling suggests that the fates of organic peroxy radicals (RO₂) were primarily influenced by RO₂ + HO₂ reactions for α-pinene/OH, RO₂ + Cl, RO₂ + HO₂, and potentially RO₂ isomerization/autooxidation reactions for α-pinene/Cl, and RO₂ + Br reactions for α-pinene/Br.