The gas-phase ozonolysis of α-humulene

Abstract

α-Humulene contains three double bonds (DB), and after ozonolysis of the first DB the first-generation products are still reactive towards O(3) and produce second- and third-generation products. The primary aim of this study consisted of identifying the products of the three generations, focusing on the carboxylic acids, which are known to have a high aerosol formation potential. The experiments were performed in a 570 litre spherical glass reactor at 295 K and 730 Torr. Initial mixing ratios were 260-2090 ppb for O(3) and 250-600 ppb for α-humulene in synthetic air. Reactants and gas-phase products were measured by in situ FTIR spectroscopy. Particulate products were sampled on Teflon filters, extracted with methanol and analyzed by LC-MS/MS-TOF. Using cyclohexane (10-100 ppm) as an OH-radical scavenger and by monitoring the yield of cyclohexanone by PTR-MS, an OH-yield of (10.5 ± 0.7)% was determined for the ozonolysis of the first DB, and (12.9 ± 0.7)% of the first-generation products. The rate constant of the reaction of O(3) with α-humulene is known as k(0) = 1.17 × 10(-14) cm(3) molecule(-1) s(-1) [Y. Shu and R. Atkinson, Int. J. Chem. Kinet., 1994, 26, 1193-1205]. The reaction rate constants of O(3) with the first-generation products and the second-generation products were, respectively, determined as k(1) = (3.6 ± 0.9) × 10(-16) and k(2) = (3.0 ± 0.7) × 10(-17) cm(3) molecule(-1) s(-1) by Facsimile-simulation of the observed ozone decay by FTIR. A total of 37 compounds in the aerosol phase and 5 products in the gas phase were tentatively identified: 25 compounds of the first-generation products contained C13-C15 species, 9 compounds of the second-generation products contained C8-C11 species, whereas 8 compounds of the third-generation products contained C4-C6 species. The products of all three generations consisted of a variety of dicarboxylic-, hydroxy-oxocarboxylic- and oxo-carboxylic acids. The formation mechanisms of some of the products are discussed. The residual FTIR spectra indicate the formation of secondary ozonides (SOZ) in the gas phase, which are formed by the intramolecular reaction of the Criegee moiety with the carbonyl endgroup. These SOZ revealed to be stable over several hours and its formation was shown not to be affected by the addition of Criegee-radical scavengers such as HCOOH or H(2)O. This suggests that in the ozonolysis of α-humulene at atmospheric pressures the POZ will decompose rapidly, and that a large fraction of the formed exited Criegee Intermediate will be stabilized to form stable SOZ, while the formation of OH-radicals via the hydroperoxide channel will be a minor process.

CCN activity of secondary aerosols from terpene ozonolysis under atmospheric relevant conditions

“…As can be seen, in the absence of scavengers, the observed maximum total number concentrations of particles formed from ozonolysis were 1.1 × 10 4 , 8.0 × 10 4 , 8.2 × 10 4 , and 8.3 × 10 5 particle cm −3 for α‐pinene, limonene, α‐cedrene, and α‐humulene, respectively, suggesting that α‐humulene is the most efficient in generating new particles among the four terpenes. In the presence of cyclohexane as an OH scavenger, particle number concentrations slightly increased during the ozonolysis of α‐pinene, α‐cedrene, and α‐humulene, suggesting that the dominant pathway generating SOA is the reaction with O 3 , which is consistent with the results from other studies [ Johnson and Marston , ; Beck et al , ; Yao et al , ]. In contrast, the addition of an OH scavenger led to a statistically significant decrease in particle number concentration for limonene, suggesting that OH radicals may play an important role in SOA formation from this reaction system.…”

“…Previous chamber and field studies have generally assumed a density varying from 1.0 g cm −3 to 1.25 g cm −3 when calculating the mass yield of SOA [ Turpin and Lim , ; Lee et al , ; Beck et al , ]. In our study, the effective density of aerosols was derived from real‐time measurements by using the DMA‐APM according to equation .…”

“…The oxidation of terpenes plays an essential role in atmospheric chemical processes [Neeb et al, 1997;Kanakidou et al, 2005]. A large variety of products, including carbonyl compounds, hydroxyl compounds, organic acids, organosulfates, organic nitrates, and some highly oxygenated, extreme low-volatile organic compounds, have been identified in SOA generated from the ozonolysis of terpenes, such as α-pinene, β-pinene, α-cedrene, α-humulene, and limonene [Froyd et al, 2010;Beck et al, 2011;Wells and Ham, 2014;Yao et al, 2014;Ehn et al, 2014;He et al, 2014;Jokinen et al, 2015;Tröstl et al, 2016]. The reaction mechanisms involving Criegee intermediates (CIs) to form SOA precursors have been investigated; however, the mechanisms of SOA formation and transformation are still not fully understood [Johnson and Marston, 2008;McVay et al, 2016].…”

The atmospheric impacts of monoterpene ozonolysis on global stabilised Criegee intermediate budgets and SO<sub>2</sub> oxidation: experiment, theory and modelling

“…However, a major difference in ozonolysis at endocyclic 2 bonds is that, on decomposition of the POZ, the carbonyl oxide and carbonyl moieties are 3 tethered as part of the same molecule, providing the potential for further interaction of the 4 two. These can react together to form secondary ozonides (SOZ), which may be stable for 5 several hours (Beck et al, 2011). However, while this has been shown to be potentially the 6 major fate in the atmosphere for SCI derived from sesquiterpenes (C 15 Decomposition of the α-pinene POZ yields four different C 10 Criegee intermediates (Scheme 11 2: CI-1a, 1b, 2a, 2b), with the carbonyl oxide moiety at one end and a carbonyl group at the 12 other.…”

COMPASS – COMparative Particle formation in the Atmosphere using portable Simulation chamber Study techniques

“…By contrast, the sesquiterpene reacts with ambient ozone, forming a large variety of products e.g. the Criegee intermediates (Criegee, 1975) and the intramolecularly formed secondary ozonide (Beck et al, 2011). Both biogenic volatile organic compounds (BVOCs) were added in a small bypass air flow of (10 ± 1) cm 3 min −1 to COMPASS2 using a permeation oven at a defined temperature.…”