Accretion product formation from acyl peroxy radicals

Acyl peroxy radicals (RC(=O)O₂) are a particular class of organic peroxy radicals formed in the troposphere predominantly by OH-initiated oxidation of aldehydes or the photolysis of ketones. One of the key features is the formation of acyl peroxy nitrates by reaction with NO₂ which act as reservoir species for nitrogen oxides (NO_x = NO + NO₂) and enable the long-range transport of NO_x. When NO_x concentrations fall below critical levels, acyl peroxy radical chemistry exhibits substantially an increased complexity. Reactions with HO₂ were shown to produce OH. Particularly for larger acyl peroxy radicals (> C₄) unimolecular H shift reactions are rapid and yield the formation of highly oxygenated organic molecules (HOMs). More recently, it has been proposed that reactions of acyl peroxy radicals with unsaturated organics such as terpenes finally result in the formation of low-volatility vapours that act as aerosol precursors.

To further elucidate acyl peroxy radical chemistry at conditions where the peroxy radical loss is no longer dominated by reactions with NO we performed experiments in the QUAREC atmospheric simulation chamber (University of Wuppertal) using α,β-dicarbonyl photolysis as clean acyl peroxy radical sources. Based on the results of two methods (FTIR spectroscopy, NH₄⁺-CIMS) we provide evidence for the formation of accretion products from acyl peroxy radical self- and cross-reactions.