Speciated Measurement of Bicyclic Peroxy Radicals via Iodide-CIMS and its Implication on OH-Initiated Aromatic Oxidation

Abstract. Bicyclic peroxy radicals (BPRs) from aromatics hydrocarbons oxidation play increasingly recognized roles in the formation of secondary air pollutants. However, their reaction mechanisms remain poorly constrained, largely due to the lack of direct measurement techniques. In this study, we developed a method for  quantitative measurement of BPRs using an iodide chemical ionization mass spectrometer (Vocus AIM). Following instrument optimization, the sensitivity for BPRs reached 0.2–0.4 ncps/pptv, with a detection limit of ~1 pptv and an uncertainty of ~28 %. Our flow reactor experiments revealed that the bicyclic pathway dominates the OH-initiated oxidation of aromatics under low-NO_x conditions, accounting for 58.6 % and 72.0 % of the oxidation products of toluene and m-xylene, respectively. Comparative analysis further demonstrated that conventional product-yield-based approaches underestimate the branching ratio of the bicyclic pathway by 5–12 % relative to direct BPR quantification. This discrepancy suggests the presence of unaccounted reaction channels in current chemical mechanisms, even when autoxidation and accretion reactions are considered. By directly quantifying BPRs, this study provides new insights into the atmospheric oxidation of aromatics and highlights the need for further mechanistic investigation. Moreover, the reaction-pathway-controlled quantification approach proposed here effectively reduces the challenges associated with measuring functionalized RO₂ radicals and demonstrates strong potential for sensitive, speciated RO₂ detection using Vocus AIM in both laboratory and ambient environments.