Detection of RO2 radicals and other products from oxidation of VOCs in PAM chamber with NH4+ ionization mass spectrometry

Organic peroxy radicals (RO2), derived from volatile organic compounds (VOCs) oxidation by hydroxyl radical (OH), ozone (O3), and nitrate radical (NO3) etc., play an important role in atmospheric chemistry. Direct measurements of speciated RO2 are challenging but necessary for understanding their atmospheric fate and impact. In this study, we introduce a newly developed Proton Transfer Reaction measurement method (PTR3-TOF) using NH4+ as the ion source to measure the multifunctional complex RO2 and related reactive oxidation products based on the principle of chemical ionization mass spectrometry (CIMS). We establish a calibration system that combines a home-built OH calibration source with various standard VOC gases, enabling to quantify both RO2 radicals and closed-shell species with a detection limit of about 1.6×107 molecule cm-3 (2σ, 60 s) and an uncertainty of 10%. This technique is first applied to study the gas-phase ozonolysis of cyclohexene in our laboratory, with experiments carried out using the O3 OFR mode in a PAM-OFR system under near-real atmosphere conditions. As a consequence, a total of 30 cyclohexene ozonolysis reaction products are detected in the first step of cyclohexene ozonolysis, including 9 types of RO2 radicals and 21 types of closed-shell species, with oxygen atoms ranging from 1 to 8. We also establish a mechanistic model for the first-generation products of ozonolysis of cyclohexene, and the simulation and measurement results agree within a factor of 2-3 for the respective formulas except for some differences in a few species.