Investigations on the fate of selected peroxy radicals using synthetized precursors and isomeric speciation

Peroxy radicals (RO₂) play a central role in the atmospheric degradation of volatile organic compounds (VOC) whose atmospheric lifetime and further reactions depend strongly on the prevailing conditions. In air masses influenced by anthropogenic activities the fate of a peroxy radical is typically dominated by its reaction with NO which finally results in the formation of tropospheric ozone. Once NO no longer dominates, peroxy radical chemistry becomes more complex and includes reactions with HO₂, other peroxy radicals or unimolecular isomerization (H shift). The last can occur multiple times, each shift being followed by progressive addition of O₂. The resultant highly oxidized peroxy radicals either decompose or undergo bimolecular reactions. This process causes rapid formation of low-volatility vapours that contribute to new particle formation.

Apart from the progress made over the last decade in understanding peroxy radical chemistry and the formation of highly oxidized species in particular, the formation mechanisms and the influence of the peroxy radical structure on the reactivity are still not well-established. The detection of highly oxygenated organic molecules (HOM) was achieved by chemical ionisation mass spectrometry which provides only information on the chemical formula. Further, oxidation experiments on biogenic VOC (terpenes etc.) result in a complex mixture of peroxy radicals impeding to gain detailed information on formation mechanisms.

To address existing analytical and experimental shortcomings in characterizing the formation and fate of peroxy radicals, three iodo-carbonyl precursors (i.e., C₇H₁₃OI) were synthesized to produce specific peroxy radicals via photolysis. The experiments were performed in the QUAREC atmospheric simulation chamber (University of Wuppertal). The model reaction systems were monitored by Fourier-Transform infrared (FTIR) spectroscopy and molecular characterization of the resulting oxidation products was retrieved using the low pressure IMS-Tof-CIMS. The experimental set-up was carefully adjusted to differentiate between autoxidation processes, permutation reactions, and the reaction of peroxy radicals with HO₂. The influence of the peroxy radical structure on the reactivity will be discussed.