Highly oxygenated molecules (HOMs) and secondary organic aerosol (SOA) formation from the oxidation of <i>α</i> - and <i>β</i> -phellandrenes by NO ₃ radicals

Abstract. Nighttime NO3-initiated oxidation of monoterpenes plays a crucial role as source of organic nitrates (ONs) and secondary organic aerosol (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 β-phellandrene generating significantly more SOA than α-phellandrene. Both monoterpenes produce large quantities 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 the formation of the detected products. 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 the NO3 radical.