HOMs and SOA formation from the oxidation of α- and β-phellandrenes by NO₃ radicals

Abstract. Nighttime NO₃-initiated oxidation of monoterpenes plays a crucial role as source of organic nitrates (ONs) and secondary organic aerosols (SOA), impacting climate, air quality, and human health. Nevertheless, monoterpene reactions with NO₃ remain poorly understood. This study provides an in-depth investigation of the NO₃-initiated oxidation of α- and β-phellandrenes, through simulation chamber experiments and a combination of various analytical techniques (FTIR, PTR-ToF-MS, ACSM, nitrate-CI-APi-ToF-MS, Orbitrap, SMPS). SOA yields were measured, and oxidation products, including highly oxygenated organic molecules (HOMs), were investigated in gas and aerosol phases. Numerical simulations were also performed to investigate the dominant chemical regimes for RO₂ radicals. We found that α- and β-phellandrenes are efficient SOA precursors with yields reaching up to 35 % and 60 %, respectively, with b-phellandrene generating significantly more SOA than α-phellandrene. Both monoterpenes produce large amounts of ONs in gas and aerosol phases with total molar yields of 40–60 %. Similar gas-phase products were detected for α- and β-phellandrenes. In particular, carbonyl nitrates, dicarbonyl nitrates and dicarbonyls were detected as first-generation products. Autooxidation processes were also shown to occur with numerous gas-phase HOM monomers and dimers detected. Chemical mechanisms have been proposed to explain products formation. Since gas-phase products were similar for both monoterpenes, they do not explain the differences in SOA yields. However, some differences in aerosol-phase composition were observed which may explain why β-phellandrene is a more efficient SOA precursor. This study is the first mechanistic investigation of the reactions of α- and β-phellandrenes with NO₃ radical.