Highly Oxygenated Organic Nitrates Formed from NO₃ Radical-Initiated Oxidation of β-Pinene

Abstract

The reactions of biogenic volatile organic compounds (BVOC) with the nitrate radicals (NO 3 ) are major nighttime sources of organic nitrates and secondary organic aerosols (SOA) in regions influenced by BVOC and anthropogenic emissions. In this study, the formation of gas-phase highly oxygenated organic molecules-organic nitrates (HOM-ON) from NO 3 -initiated oxidation of a representative monoterpene, β-pinene, was investigated in the SAPHIR chamber (Simulation of Atmosphere PHotochemistry In a large Reaction chamber). Six monomer (C = 7−10, N = 1−2, O = 6−16) and five accretion product (C = 17−20, N = 2−4, O = 9−22) families were identified and further classified into firstor second-generation products based on their temporal behavior. The time lag observed in the peak concentrations between peroxy radicals containing odd and even number of oxygen atoms, as well as between radicals and their corresponding termination products, provided constraints on the HOM-ON formation mechanism. The HOM-ON formation can be explained by unimolecular or bimolecular reactions of peroxy radicals. A dominant portion of carbonylnitrates in HOM-ON was detected, highlighting the significance of unimolecular termination reactions by intramolecular H-shift for the formation of HOM-ON. A mean molar yield of HOM-ON was estimated to be 4.8% (−2.6%/+5.6%), suggesting significant HOM-ON contributions to the SOA formation.

Identification of highly oxygenated organic molecules and their role in aerosol formation in the reaction of limonene with nitrate radical

“…This high ratio is also consistent with findings in the ozonolysis of alkenes (Mentel et al, 2015), where unimolecular termination reactions were also proposed to be the likely explanation (Crounse et al, 2013;Rissanen et al, 2014). Our result thus further emphasizes that unimolecular termination reactions of RO2 radicals are important pathways in the formation of HOM monomers derived from the reactions of monoterpenes with NO3 (Shen et al, 2021). Scheme 1c shows this intramolecular termination process using a C10H16NO9• radical as an example.…”

“…C10H16NOx• with both even and odd number of oxygen atoms as well as their termination products had comparable abundance, which suggests that the alkoxy-peroxy pathway was important for RO2• formation in this reaction. This finding is analogous to the findings in the reaction of a number of alkenes with O3 and in the reaction of isoprene and β-pinene with NO3 (Mentel et al, 2015;Zhao et al, 2021;Shen et al, 2021). may not be the dominant pathways.…”

“…This finding is likely attributed to unimolecular termination reactions of RO2•. The importance of unimolecular termination reactions of HOM-RO2• and the resulting high ratio of carbonyl compounds to hydroxyl/hydroperoxyl compounds has also been found in the reaction system of β-pinene + NO3 (Shen et al, 2021). This high ratio is also consistent with findings in the ozonolysis of alkenes (Mentel et al, 2015), where unimolecular termination reactions were also proposed to be the likely explanation (Crounse et al, 2013;Rissanen et al, 2014).…”

“…A set of instruments were used to measure other gas-phase species, including VOC, NOx, O3, NO3 and N2O5 (Shen et al, 2021). Concentrations of NO3 and N2O5 were measured in-situ using a home-built diode laser-based, cavity ring-down spectrometer similar to the instrument described in the work by Wagner et al (2011).…”

“…HOM concentrations were obtained from the normalized signals to the total signals of the mass spectra (nc, normalized counts) by applying a calibration coefficient (C) of 2.5×10 10 molecule cm -3 nc -1 . C was determined using H2SO4 as the charging efficiency of HOM and H2SO4 are considered to be equal (Ehn et al, 2014;Pullinen et al, 2020;Shen et al, 2021). The details of determination of the calibration coefficient are shown in the supplement S1.…”