Highly oxygenated organic molecule (HOM) formation in the isoprene oxidation by NO3 radical

Highly oxygenated organic molecules (HOM) are found to play an important role in the formation and growth of secondary organic aerosol (SOA). SOA is an important type of aerosol with significant impact on air quality and climate. Compared with the oxidation of volatile organic compounds by ozone (O-3) and hydroxyl radical (OH), HOM formation in the oxidation by nitrate radical (NO3), an important oxidant at nighttime and dawn, has received less attention. In this study, HOM formation in the reaction of isoprene with NO3 was investigated in the SAPHIR chamber (Simulation of Atmospheric PHotochemistry In a large Reaction chamber). A large number of HOM, including monomers (C-5), dimers (C-10), and trimers (C-15), both closed-shell compounds and open-shell peroxy radicals (RO2), were identified and were classified into various series according to their formula. Their formation pathways were proposed based on the peroxy radicals observed and known mechanisms in the literature, which were further constrained by the time profiles of HOM after sequential isoprene addition to differentiate first-and second-generation products. HOM monomers containing one to three N atoms (1-3N-monomers) were formed, starting with NO3 addition to carbon double bond, forming peroxy radicals, followed by autoxidation. 1N-monomers were formed by both the direct reaction of NO3 with isoprene and of NO3 with first-generation products. 2N-monomers (e.g., C(5)H(8)N(20)n(n=7-13), C(5)H(10)N(2)On(n=8-14)) were likely the termination products of C(5)H(9)N(2)On which was formed by the addition of NO3 to C5-hydroxynitrate (C5H9NO4), a first-generation product containing one carbon double bond. 2N-monomers, which were second-generation products, dominated in monomers and accounted for similar to 34 % of all HOM, indicating the important role of second-generation oxidation in HOM formation in the isoprene + NO3 reaction under our experimental conditions. H shift of alkoxy radicals to form peroxy radicals and subsequent autoxidation (alkoxyperoxy pathway) was found to be an important pathway of HOM formation. HOM dimers were mostly formed by the accretion reaction of various HOM monomer RO2 and via the termination reactions of dimer RO2 formed by further reaction of closed-shell dimers with NO3 and possibly by the reaction of C5-RO2 with isoprene. HOM trimers were likely formed by the accretion reaction of dimer RO2 with monomer RO2. The concentrations of different HOM showed distinct time profiles during the reaction, which was linked to their formation pathway. HOM concentrations either showed a typical time profile of first-generation products, second-generation products, or a combination of both, indicating multiple formation pathways and/or multiple isomers. Total HOM molar yield was estimated to be 1.2 %(+1.3%)(-0.7%), which corresponded to a SOA yield of similar to 3.6 % assuming the molecular weight of C5H9NO6 as the lower limit This yield suggests that HOM may contribute a significant fraction to SOA yield in the reaction of isoprene with NO3.

Automated summary

Highly oxygenated organic molecules (HOM) are important in the formation of secondary organic aerosol (SOA), with the reaction of isoprene with nitrate radical (NO3) leading to the formation of various HOM species, including monomers, dimers, and trimers through peroxy radicals and autoxidation. The study shows that second-generation products, such as 2N-monomers, play a dominant role in HOM formation, with different HOM showing distinct time profiles indicating multiple formation pathways and isomers. Total HOM molar yield in the reaction was estimated to be 1.2% (+1.3%)(-0.7%), suggesting a significant contribution of HOM to SOA yield.