Reactions of C₁₂–C₁₄ n-Alkylcyclohexanes with Cl Atoms: Kinetics and Secondary Organic Aerosol Formation

Abstract

Long-chain alkanes are a type of intermediate volatility organic compound (IVOC) in the atmosphere and a potential source of secondary organic aerosols (SOAs). C 12 −C 14 nalkylcyclohexanes are important compositions of IVOCs, with considerable concentrations and emission rates. The reaction rate constants and SOA formation of the reactions of C 12 −C 14 nalkylcyclohexanes with Cl atoms were investigated in the present study. The reaction rate constants of the long-chain alkanes obtained via the relative-rate method at 298 ± 0.2 K (in units of ×10 −10 cm 3 molecule −1 s −1 ) were as follows: k hexylcyclohexane = 5.11 ± 0.28, k heptylcyclohexane = 5.56 ± 0.30, and k octylcyclohexane = 5.74 ± 0.31. The gas-phase products of the reactions were identified as mainly small molecules of aldehydes, ketones, and acids. The particlephase products were mostly monomers and oligomers, but there were still trimers even under high-NO x conditions. Moreover, under high-NO x conditions (urban atmosphere), the SOA yields of hexylcyclohexane are higher than that under low-NO x conditions (remote atmosphere), indicating that more attention should be given to the SOA formation of Cl-initiated n-alkylcyclohexane oxidations in polluted regions. This research can further clarify the oxidation processes and SOA formation of n-alkylcyclohexanes in the atmosphere.

Volatility of Cl-Initiated C12–C14 n-Alkylcyclohexane Secondary Organic Aerosol: Effects of NOx and Photoaging

“…Also, RO 2 unimolecular isomerization and the RO 2 + HO 2 reaction are the main pathways to produce ROOH, but the pathway of alkyl peroxy radical (RO 2 ) isomerization is much slower than that of isoprene . According to the average concentration of Cl atoms during the rapid oxidation period (4.25 × 10 5 molecules cm –3 ) and the reaction rate constant of RO 2 with Cl atoms (1.51 × 10 –10 cm 3 molecule –1 s –1 ), the RO 2 + Cl reactions could be ignored, and the RO 2 + RO 2 reactions are more important than RO 2 + HO 2 reactions in Cl-initiated oxidation without adding H 2 O 2 . Therefore, we repeated the experiments under both low- and high-NO x conditions but turned off the UV lights after the SOA mass concentrations reached the peak and then calculated the apparent wall loss rates in the dark as control groups.…”

“…To produce Cl atoms, Cl 2 (Dalian Special Gases, 1028 ppm) was introduced only once using a glass injector before the UV lights were turned on, and NO (National Institute of Metrology, 29.1 ppm) was added using a flowmeter. The concentration of Cl atoms was calculated using the decay trend of alkylcyclohexanes, and the decay of heptylcyclohexane is displayed in Figure S7 as an example. where [Cl] t and [alkylcyclohexane] t are the concentrations of Cl atoms and alkylcyclohexane at time t , respectively, and k alkylcyclohexane–Cl is the reaction rate constant of alkylcyclohexane with Cl atoms measured by Wang et al, 5.11 × 10 –10 cm 3 molecule –1 s –1 for hexylcyclohexane, 5.56 × 10 –10 cm 3 molecule –1 s –1 for heptylcyclohexane, and 5.74 × 10 –10 cm 3 molecule –1 s –1 for octylcyclohexane, respectively. The initial Cl atom concentration was ∼7.48 × 10 5 molecules cm –3 , and the average Cl atom exposure was ∼3.32 × 10 9 molecules cm –3 s using ∼4.25 × 10 5 molecules cm –3 as the average Cl atom concentration during the rapid oxidation period, which was equivalent to ∼18.44 h atmospheric oxidative aging (using 5 × 10 4 as the average atmospheric Cl atom concentration).…”

“…They are derived from the photolysis of active halogen compounds (e.g., ClNO 2 and Cl 2 ), which are widely distributed in the urban atmosphere . The rate constants of reactions of C 12 –C 14 alkanes with Cl atoms are ∼30 times faster than those of reactions with OH radicals. − Cl atoms and OH radicals are different from the H-abstraction activities on CH 3 , CH 2 , and CH, supported by theories and experiments, especially for the H-abstraction activity on CH 3 . − In addition, Cl atoms led to efficient SOA formation of C 8 –C 12 alkanes with higher SOA yields than that of OH radicals. − Except for long-chain alkanes, Cl-initiated oxidations of VOCs (α-pinene, β-pinene, d -limonene, monoterpenes, isoprene, and toluene − ) and IVOCs (such as acenaphthene and acenaphthylene) lead to high SOA yields.…”

Formation and impacts of nitryl chloride in Pearl River Delta

“…production enhanced by 0.70 ppbv h -1 (0.9%), 1.02 ppbv h -1 (1.9%), 0.24 ppbv h -1 (0.6%) on daytime accumulation in Type A, B, C, respectively, which is consistent with the nocturnal level of ClNO2 in the three air masses presented in Table 3. Previous studies suggest that chlorine radicals from ClNO2 photolysis may contribute significantly to the oxidation of some VOCs species, especially for long-chain alkanes (Shi et al, 2020;Wang et al, 2022b). The oxidation of long-chain alkanes (C10~14 n-alkanes) by chloride and OH radicals during the morning hour (08:00 -09:00) were also evaluated based on modeled oxidants concentration.…”

Atmospheric oxidation mechanism and kinetics of indole initiated by ●OH and ●Cl: a computational study

“…Besides reactions with q OH, reactions with chlorine radicals ( q Cl) have been proposed to be an important removal pathway for ONCs due to the identification of new q Cl continental sources and the high reactivity of q Cl (Wang et al, 2022;Li et al, 2021;Jahn et al, 2021;Xia et al, 2020;Young et al, 2014;Faxon and Allen, 2013;Riedel et al, 2012;Atkinson et al, 1989;Ji et al, 2013;Thornton et al, 2010;Le Breton et al, 2018). q Cl initiated atmospheric oxidation of ONCs can lead to the formation of N-centered radicals, once a strong 2-center-3-electron (2c-3e) bond complex has been formed between q Cl and NH x (x = 1, 2) (Mc-Kee et al, 1996;Xie et al, 2015Xie et al, , 2017Ma et al, 2018a).…”