10 Jul 2023
 | 10 Jul 2023

Oxygenated organic molecules produced by low-NOx photooxidation of aromatic compounds and their contributions to secondary organic aerosol

Xi Cheng, Yong Jie Li, Yan Zheng, Keren Liao, Tong Zhu, Chunxiang Ye, Xinghua Qiu, Theodore K. Koenig, Yanli Ge, and Qi Chen

Abstract. Oxygenated organic molecules (OOMs) produced by the oxidation of aromatic compounds are key components of secondary organic aerosol (SOA) in urban environments. The steric effects of substitutions and rings and the role of key reaction pathways on altering the OOM distributions remain unclear because of the lack of systematic multi-precursor study over a wide range of OH exposure. In this study, we conducted flow-tube experiments and used the nitrate adduct time-of-flight chemical ionization mass spectrometer (NO3-TOF-CIMS) to measure the OOMs produced by the photooxidation of six key aromatic precursors under low-NOx conditions. For single aromatic precursors, the detected OOM peak clusters show one or two oxygen-atom difference, indicating the involvement of multi-step auto-oxidation and alkoxy radical pathways. Multi-generation OH oxidation is also needed to explain the diverse hydrogen numbers in the observed formulae. Especially for double-ring precursors at higher OH exposure, multi-generation OH oxidation may have significantly enriched the dimer formulae. Methyl substitutions in precursor may lead to less fragmented products in the OOMs, while the double-ring structure corresponds to less efficient formation of closed-shell monomeric and dimeric products, both highlighting significant steric effects of precursor molecular structure on the OOM formation. The estimated accretion reaction rate constants for key dimers formed from the benzene oxidation are much greater than those formed from the naphthalene oxidation (7.0 vs. 0.9×10−10 cm3 molecules−1 s−1). Naphthalene-derived OOMs however have lower volatilities and greater SOA contributions than the other-types of OOMs, which may be more important in initial particle growth. Overall, the OOMs identified by the NO3-TOF-CIMS perhaps consist of 3–11 % of the SOA mass. Our results highlight the key roles of progressive OH oxidation, methyl substitution and ring structure in the OOM formation from aromatic precursors, which needs to be considered in future model developments to improve the model performance on organic aerosol.

Xi Cheng et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-1215', Anonymous Referee #1, 26 Jul 2023
  • RC2: 'Comment on egusphere-2023-1215', Anonymous Referee #2, 29 Aug 2023
  • AC1: 'Comment on egusphere-2023-1215', Xi Cheng, 27 Oct 2023

Xi Cheng et al.


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Short summary
In this study we conducted laboratory measurements to investigate the formation of gas-phase oxygenated organic molecules (OOMs) from six aromatic volatile organic compounds (VOCs). We provide a thorough analysis on the effects of precursor structure (substituents and ring numbers) in product distribution, and highlight from a laboratory perspective that heavy (e.g., double-ring) aromatic VOCs are important in initial particle growth during secondary organic aerosol formation.