New study challenges current understanding of formation mechanism

Particle-phase photochemical oxidation of polycyclic aromatic hydrocarbons on soot can produce a substantial amount of highly unsaturated and oxygenated organic aerosols. This changes soot to better serve as cloud condensation nuclei and affects its impact on climate – these are the findings of a new study by Yafang Cheng, the independent research group Minerva “Aerosols, air quality air and climate” at the Max Planck Institute for Chemistry in Mainz.

Highly oxygenated organic molecules are a key component of atmospheric secondary organic aerosols. However, the origin and the mechanism of formation of highly oxygenated organic molecules with high unsaturation (HU-HOM) remain unknown. But now an international team of researchers has found that photooxidation of large polycyclic aromatic hydrocarbons (PAHs) on soot by singlet oxygen and superoxide anion radicals may be a significant source of the unexplained HU-HOMs widely observed in the atmosphere. . The team was led by Yafang Cheng from the Max Planck Institute of Chemistry and Chuncheng Chen from the Institute of Chemistry of the Chinese Academy of Sciences. Their findings are based on molecular-level studies of photochemical aging of soot by O2. PAH-derived HU-HOMs exhibit lactone and anhydride functional groups and can significantly increase the hydrophilicity of soot.

The increase in soot hydrophilicity after photochemical weathering should further influence the fate and effects of soot aerosols in the atmosphere: for example, becoming better cloud condensation nuclei, being more readily involved in chemistry and aging in the aqueous phase, altering its wet deposition process. etc

Decipher molecular formulas

Researchers characterized the change in molecular composition during soot photoaging by applying laser desorption ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry (LDI FT-ICR MS), a technique for ultra-high resolution mass spectrometry allowing a reliable assignment of molecular formulas. In situ attenuated total internal reflection (ATR-IR) was used to study the evolution of functional groups during soot oxidation. They find that highly oxygenated, high unsaturation organic molecules (HU-HOM) are formed by a multigenerational photochemical oxidation pathway, where ketones, aldehydes and acids are produced by the photooxidation of large polycyclic aromatic hydrocarbons on soot at the initial stage, followed by the formation and accumulation of lactones and anhydrides during further oxidation.

“In this heterogeneous photochemical oxidation, O2 The molecules are the initial oxidant, which is then photosensitized to form reactive oxygen species such as singlet oxygen and superoxide anion radicals,” said Meng Li, postdoctoral fellow in Yafang Cheng’s group and first author of the study.2 in the troposphere, this oxidation pathway is expected to be a very important aging process for PAHs and soot particles, especially in clean and remote regions,” Meng Li added.

“This novel HU-HOM formation pathway could be a characteristic evolutionary pathway of primary organic aerosols from various combustions, due to the widespread existence of PAHs there, thus contributing to a more complete understanding of the chemical evolution of organic aerosols,” Yafang Cheng said. who leads the independent research group Minerva at MPIC.

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Materials provided by Max Planck Institute for Chemistry. Note: Content may be edited for style and length.

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