Ozonolysis of α-phellandrene, Part 1: Gas- and particle-phase characterisation
Mackenzie-Rae, Felix A.
Saunders, Sandra M.
The ozonolysis of <i>α</i>-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 <i>m/z</i> second-generation oxidation products were also observed, with products tentatively identified according to a constructed degradation mechanism. OH yields from <i>α</i>-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<sup>−16</sup> cm<sup>3</sup> molecule<sup>−1</sup> s<sup>−1</sup> at 298 K, showing ozonolysis as a dominant loss process for both <i>α</i>-phellandrene and its first-generation products in the atmosphere. Endocyclic conjugation in <i>α</i>-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, monitored with a scanning mobility particle sizer (SMPS) and a high-resolution time-of-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 a Criegee scavenger, suggesting that stabilised Criegee intermediates are important for new particle formation in the system. Aerosol yields ranged from 25–174 % dependant on mass loadings, with both first- and second-generation products identified as large contributors to the aerosol mass. Effective densities ranged from 1.29–1.90 g cm<sup>−3</sup>. The aerosol oxidation state was also found to be dependent on mass loadings, with parametrisation of the bulk mass indicating a large contribution of highly functionalised low- and semi-volatile organic compounds to the aerosol phase. With a high chemical reactivity and aerosol forming propensity <i>α</i>-phellandrene is expected to have an immediate impact on the local environment to which it is emitted, with ozonolysis therefore likely to be an important contributor to the significant blue haze and frequent nocturnal nucleation events observed over Eucalypt forests.