Ozonolysis of <i>α</i>-phellandrene – Part 1: Gas- and particle-phase characterisation

Abstract

Abstract. The ozonolysis of α-phellandrene, a highly reactive conjugated monoterpene largely emitted by Eucalypt species, is characterised in detail for the first time using a smog chamber at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. Gas-phase species were monitored by a proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF), with yields from a large number of products obtained, including formaldehyde (5-9 %), acetaldehyde (0.2-8 %), glyoxal (6-23 %), methyl glyoxal (2-9 %), formic acid (22-37 %) and acetic acid (9-22 %). Higher m/z second-generation oxidation products were also observed, with products tentatively identified according to a constructed degradation mechanism. OH yields from α-phellandrene and its first-generation products were found to be 35 ± 12 and 15 ± 7 %, respectively, indicative of prominent hydroperoxide channels. An average first-generation rate coefficient was determined as 1.0 ± 0.7 × 10 −16 cm 3 molecule −1 s −1 at 298 K, showing ozonolysis as a dominant loss process for both α-phellandrene and its first-generation products in the atmosphere. Endocyclic conjugation in α-phellandrene was also found to be conducive to the formation of highly condensible products with a large fraction of the carbon mass partitioning into the aerosol phase, which was monitored with a scanning mobility particle sizer (SMPS) and a high-resolution timeof-flight aerosol mass spectrometer (AMS). Nucleation was observed almost instantaneously upon ozonolysis, indicating the rapid formation of extremely low-volatility compounds. Particle nucleation was found to be suppressed by the addition of either NO 2 or a Criegee scavenger, with it being proposed that stabilised Criegee intermediates are important for new particle formation in the system. Aerosol yields ranged from 25 to 174 % depending on mass loadings, with both first-and second-generation products identified as large contributors to the aerosol mass. In short, with a high chemical reactivity and aerosol-forming propensity, α-phellandrene is expected to have an immediate impact on the local environment to which it is emitted, with ozonolysis likely to be an important contributor to the significant blue haze and frequent nocturnal nucleation events observed over Eucalypt forests.

Ozonolysis of α-phellandrene – Part 2: Compositional analysis of secondary organic aerosol highlights the role of stabilised Criegee intermediates

“…In total, 21 prominent products or isomeric product groups were identified using both positive and negative ionisation modes, with potential formation mechanisms discussed. The aerosol was found to be composed primarily of polyfunctional first-and second-generation species containing one or more carbonyl, acid, alcohol and hydroperoxide functionalities, with the products significantly more complex than those proposed from basic gas-phase 20 chemistry in the companion paper (Mackenzie-Rae et al, 2017a). Mass spectra show a large number of dimeric products are also formed.…”

“…This paper extends the current discussion by analysing filter samples collected during the α-phellandrene ozonolysis chamber experiments (as described in Mackenzie-Rae et al, 2017a). Samples were analysed using a combination of liquid chromatography with soft-ionisation electrospray mass spectrometry.…”

“…The complete experimental design and details are provided in the companion paper (Mackenzie-Rae et al, 2017a). Briefly and relevant to the current discussion, α-phellandrene (10 -175 ppb, Aldrich Chemical Company, Inc., USA) and ozone (56 -500 ppb) were mixed in a background matrix of purified air.…”

“…Because all first-and second-generation products of α-phellandrene ozonolysis contain at least one functional group that is capable of ionisation (Mackenzie-Rae et al, 2016, 2017a, it is reasonable to assume that a high proportion of water soluble SOA components will be observed, with the analyte R being observed as [R-H] -ion in the negative mode and [R+Na] + ion 25 in the positive mode. Negative mode analysis leads to formation of deprotonated ions; hence molecules containing functional groups that readily lose a proton, such as carboxylic acids, are frequently observed in this mode.…”

“…In the companion paper (Part 1, Mackenzie-Rae et al, 2017a), it was found that α-phellandrene yields a large amount of self-25 nucleated SOA upon reaction with ozone, concluding that the ozonolysis of α-phellandrene is likely an important contributor to the intense and frequent nocturnal nucleation events observed in Eucalypt forests (Lee et al, 2008;Ortega et al, 2012;Suni et al, 2008). However, postulated gas-phase species could not explain the properties of the SOA observed.…”