Highly oxygenated organic molecules are a key component of atmospheric secondary organic aerosol. However, the origin and formation mechanism of highly oxygenated organic molecules with high unsaturation (HU-HOMs), 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 can be an important source of the unexplained HU-HOMs widely observed in the atmosphere. The team was led by Yafang Cheng from the Max Planck Institute for Chemistry and Chuncheng Chen from the Institute of Chemistry, Chinese Academy of Sciences. Their results are based on molecular-level investigations of the photochemical aging of soot by O2. The PAH-derived HU-HOMs exhibit lactone and anhydride functional groups and can substantially increase the hydrophilicity of soot.