Detectionof RO2 radicalsand other products in the oxidation of VOCs using NH4+chemicalionization mass spectrometry

Volatile organic compounds (VOCs) in the atmosphere undergo oxidation by atmospheric oxidants such as hydroxyl radical (OH), ozone (O-3), and nitrate radical (NO3) to form organic peroxy radicals (RO2). The RO2 radicals then degrade into secondary pollutants such as O-3 and fine particulate matter, which would have significant impacts on human health and ecological environment. Accurately identifying and quantifying the oxidation intermediates of VOCs (including closed-shell products and RO2 radicals) are crucial for elucidating the degradation mechanisms of VOCs, achieving precise simulation of secondary species, and implementing refined control measures. Chemical ionization mass spectrometry (CIMS) is currently the most widely used technique for the simultaneous measurement of RO2 radicals and other oxidation products. However, the most popular CIMS technique (e.g., chemical ionization atmospheric pressure interface time-of-flight mass spectrometry, CI-APi-TOF) has limited measurement range which primarily focuses on the measurement of highly oxygenated compounds. In this study, a highly selective ammonium ion-adduct mode based on the latest proton transfer reaction-time-of-flight mass spectrometry was developed named NH4+-CIMS (ammonium chemical ionization mass spectrometry), enabling high-sensitivity detection of various RO2 radicals and oxygenated VOCs. Additionally, we established calibration systems for closed-shell products and RO2 radicals, respectively. The calibration system achieved the direct calibration of RO2 radicals based on CIMS for the first time, effectively reducing measurement uncertainties arising from sensitivity substitutions. This system was first applied to study the gas-phase ozonolysis of alpha-pinene in our laboratory under near-real atmospheric conditions. In the first-generation oxidation products of alpha-pinene ozonolysis, a total of thirteen reaction products were detected, including five RO2 radicals and eight closed-shell species containing 3 to 7 oxygen atoms. The peroxy radical C10H17O3, generated from OH chemistry, exhibited the highest proportion among the oxidation products. The peroxy radical C10H15O4, produced from direct ozone oxidation, underwent multi-step autoxidation and bimolecular reactions in subsequent reactions, confirming the importance of the alpha-pinene autoxidation pathway. The distribution of products from alpha-pinene ozonolysis remained unchanged with variations in precursor concentrations. The current atmospheric chemistry mechanisms could generally capture the product distribution of the alpha-pinene ozonolysis reaction. However, there is still significant uncertainty regarding the relative proportions of products from different oxidation pathways, influenced by the reaction rates and branching ratios set in the model. Additionally, the hydrogen abstraction pathway might play a crucial role in the oxidation of alpha-pinene by OH radicals, but the specific mechanisms require further exploration. The presence of isoprene altered the distribution of oxidation products in the system, potentially resulting in an increased formation of carbonyl-containing compounds. These results indicate that NH4+-CIMS is a promising measurement technique with high sensitivity, high resolution, and low detection limits, especially for moderately oxidized closed-shell products and RO2 radicals. With further advancements in the technology, it can be utilized to investigate reactions under more complex atmospheric conditions, aiding in elucidating the evolution patterns, degradation mechanisms, and environmental impacts of VOCs in diverse atmospheric environments.