Comment on acp-2022-85

Abstract

Nighttime nitrate radical(NO3)-initiated oxidation of biogenic volatile organic compounds (BVOC) such as monoterpenes is important for the formation and growth of secondary organic aerosol (SOA), which has significant impact on climate, air quality and human health. In SOA formation and growth from the oxidation of monoterpenes by NO3, highly oxygenated organic molecules (HOM) may be crucial, but their formation pathways and role in aerosol formation have yet to be clarified. Among monoterpenes, limonene is of research interest for its high emission globally and high SOA yield. In this work, HOM formation in the reaction of limonene with nitrate radical was investigated in the SAPHIR chamber (Simulation of Atmospheric PHotochemistry In a large Reaction chamber). About 280 HOM products were identified, grouped into 6 monomer series (each including 3 families) and one family, 11 dimer families and 3 trimer families. Both closed-shell products and open-shell peroxy radicals (RO2•) were observed, and many of them have not been reported previously. Monomers and dimers accounted for over 90 % of HOM concentrations. In the most abundant monomer series – C10H15–17NO6–14, carbonyl products outnumbered hydroxyl products, indicating the importance of the unimolecular RO2• termination pathway. Both RO2• autoxidation and alkoxy-peroxy pathways were found to be important processes leading to HOM. Time-dependent concentration profiles of monomer products containing nitrogen showed mainly second-generation formation patterns. Dimers were likely formed via the accretion reaction of two monomer RO2•, and HOM-trimers via the accretion reaction between monomer RO2• and dimer RO2•. Trimers are suggested to play an important role in new particle formation (NPF) observed in our experiment. A HOM yield of 1.5 % (+1.7 %/−0.7 %) was estimated considering only first-generation products. SOA mass growth could be reasonably explained by HOM condensation on particles assuming irreversible uptake of extremely low volatility organic compounds (ELVOC) and low volatility organic compounds (LVOC). This work provides evidence for the important role of HOM formed via the limonene + NO3 reaction in NPF and SOA growth.