Abstract
Organic peroxy radicals (RO(2)) are important intermediates in aerobic systems such as Earth's atmosphere. The existence of a channel producing dialkyl peroxides (ROOR) in their self- and cross-reactions (i.e., between the same or different radicals) has long been debated and considered a theoretical "key problem in the atmospheric chemistry of peroxy radicals". Over the past decade, observations have suggested that this channel could be an important source of condensable compounds and, ultimately, new aerosol particles in Earth's atmosphere. However, experimental evidence for specific RO(2) reactions is scarce. In this work, the formation of ROOR in the self- and cross-reactions of eight RO(2) (CH(3)O(2), (13)CH(3)O(2), CD(3)O(2), C(2)H(5)O(2), 1- and iso-C(3)H(7)O(2), 1- and tert-C(4)H(9)O(2)) could be observed by modifying the ionisation conditions on a proton transfer mass spectrometer. The ROOR formation channel was confirmed to be in competition with the other product channels rather than precede them. For six of the RO(2) studied, the branching ratio, γ, for the ROOR channel of the self-reaction was quantified relative to these other channels. The results allowed for the first time to identify some trends in γ with respect to the RO(2) structure: γ decreases with increasing RO(2) chain length for the linear/primary radicals, ranging from (14.1 ± 7)% for CH(3)O(2) to (1.1 ± 0.5)% for 1-C(4)H(9)O(2), while branched radicals exhibit much higher γ than their linear counterparts, with γ = (17.2 ± 8.6)% for iso-C(3)H(7)O(2) and (46.6 ± 23.2)% for tert-C(4)H(9)O(2). The formation of ROOR products from RO(2) reactions in the atmosphere should thus be strongly dependent on the RO(2) structure.