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 cm3 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 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 either NO2 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.
Highlights
Biogenic sources dominate the global emission budget of volatile organic compounds into the atmosphere, with monoterpenes accounting for a significant fraction of nonmethane hydrocarbons emitted (Guenther et al, 1995; Schurgers et al, 2009; Guenther et al, 2012; Lathière et al, 2006; Sindelarova et al, 2014)
Mackenzie-Rae et al.: Ozonolysis of α-phellandrene cal reactivity (Calvert et al, 2000; Atkinson and Arey, 2003), monoterpenes are thought to play an important role in the chemistry of the atmosphere, influencing its oxidative capacity, the tropospheric ozone budget and producing secondary organic aerosol (SOA) with impacts on both health and climate (Hoffmann et al, 1997; Griffin et al, 1999a; Chung and Seinfeld, 2002; Hallquist et al, 2009; Pye et al, 2010)
A basic overview of the reaction pathways is provided in Fig. 1, with a comprehensive discussion of the reaction mechanism of α-phellandrene with ozone based on findings of MackenzieRae et al (2016) pertinent to this study provided in the Supplement
Summary
Biogenic sources dominate the global emission budget of volatile organic compounds into the atmosphere, with monoterpenes accounting for a significant fraction of nonmethane hydrocarbons emitted (Guenther et al, 1995; Schurgers et al, 2009; Guenther et al, 2012; Lathière et al, 2006; Sindelarova et al, 2014). The ozonolysis of monoterpenes is thought to be one of the major sources of organic SOA in the atmosphere (Griffin et al, 1999b; Ortega et al, 2012)
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