Abstract
Abstract. A degradation mechanism for β-caryophyllene has recently been released as part of version 3.2 of the Master Chemical Mechanism (MCM v3.2), describing the gas phase oxidation initiated by reaction with ozone, OH radicals and NO3 radicals. A detailed overview of the construction methodology is given, within the context of reported experimental and theoretical mechanistic appraisals. The performance of the mechanism has been evaluated in chamber simulations in which the gas phase chemistry was coupled to a representation of the gas-to-aerosol partitioning of 280 multi-functional oxidation products. This evaluation exercise considered data from a number of chamber studies of either the ozonolysis of β-caryophyllene, or the photo-oxidation of β-caryophyllene/NOx mixtures, in which detailed product distributions have been reported. This includes the results of a series of photo-oxidation experiments performed in the University of Manchester aerosol chamber, also reported here, in which a comprehensive characterization of the temporal evolution of the organic product distribution in the gas phase was carried out, using Chemical Ionisation Reaction Time-of-Flight Mass Spectrometry (CIR-TOF-MS), in conjunction with measurements of NOx, O3 and SOA mass loading. The CIR-TOF-MS measurements allowed approximately 45 time-resolved product ion signals to be detected, which were assigned on the basis of the simulated temporal profiles of the more abundant MCM v3.2 species, and their probable fragmentation patterns. The evaluation studies demonstrate that the MCM v3.2 mechanism provides an acceptable description of β-caryophyllene degradation under the chamber conditions considered, with the temporal evolution of the observables identified above generally being recreated within the uncertainty bounds of key parameters within the mechanism. The studies have highlighted a number of areas of uncertainty or discrepancy, where further investigation would be valuable to help interpret the results of chamber studies and improve detailed mechanistic understanding. These particularly include: (i) quantification of the yield and stability of the secondary ozonide (denoted BCSOZ in MCM v3.2), formed from β-caryophyllene ozonolysis, and elucidation of the details of its further oxidation, including whether the products retain the "ozonide" functionality; (ii) investigation of the impact of NOx on the β-caryophyllene ozonolysis mechanism, in particular its effect on the formation of β-caryophyllinic acid (denoted C137CO2H in MCM v3.2), and elucidation of its formation mechanism; (iii) routine independent identification of β-caryophyllinic acid, and its potentially significant isomer β-nocaryophyllonic acid (denoted C131CO2H in MCM v3.2); (iv) more precise quantification of the primary yield of OH (and other radicals) from β-caryophyllene ozonolysis; (v) quantification of the yields of the first-generation hydroxy nitrates (denoted BCANO3, BCBNO3 and BCCNO3 in MCM v3.2) from the OH-initiated chemistry in the presence of NOx; and (vi) further studies in general to improve the identification and quantification of products formed from both ozonolysis and photo-oxidation, including confirmation of the simulated formation of multifunctional species containing hydroperoxide groups, and their important contribution to SOA under NOx-free conditions.
Highlights
It is well established that the degradation of emitted volatile organic compounds (VOCs) has a major influence on the chemistry of the troposphere, contributing to the formation of ozone, secondary organic aerosol (SOA) and other secondary pollutants (e.g., Haagen-Smit and Fox, 1954; Went, 1960; Andreae and Crutzen, 1997; Jenkin and Clemitshaw, 2000; Hallquist et al, 2009)
Its atmospheric degradation has been the subject of a number of experimental and theoretical mechanistic appraisals (Calogirou et al, 1997; Jaoui et al, 2003; Lee et al, 2006b; Kanawati et al, 2008; Winterhalter et al, 2009; Nguyen et al, 2009; Zhao et al, 2010; Li et al, 2011; Chan et al, 2011), and several established oxidation products have been used in tracer studies to show that β-caryophyllene-derived SOA makes potentially important contributions to ambient fine particulate matter at a number of locations (Jaoui et al 2007; Kleindienst et al 2007; Parshintsev et al 2008)
The performance of the mechanism has been evaluated in chamber simulations in which the gas phase chemistry was coupled to a representation of the gas-to-aerosol partitioning of 280 multi-functional oxidation products
Summary
It is well established that the degradation of emitted volatile organic compounds (VOCs) has a major influence on the chemistry of the troposphere, contributing to the formation of ozone, secondary organic aerosol (SOA) and other secondary pollutants (e.g., Haagen-Smit and Fox, 1954; Went, 1960; Andreae and Crutzen, 1997; Jenkin and Clemitshaw, 2000; Hallquist et al, 2009). Atmospheric modelling studies in which the oxidation of β-caryophyllene (and/or other sesquiterpenes) has been treated, have invariably used highly simplified or parameterized representations of the chemistry (e.g., Lane et al, 2008; Sakulyanontvittaya et al, 2008; Carlton et al, 2010; Zhang and Ying, 2011), with SOA formation represented by assigning empirically-derived yields and partitioning coefficients to notional products, based on the results of chamber studies Whilst such approaches are practical and economical, it is generally acknowledged that the gas phase formation and evolution of low volatility products of VOC oxidation are sensitive to the prevailing atmospheric conditions (e.g., level of NOx, relative humidity), and that it is ideally necessary to understand and represent the competitive reactions involved over several generations of oxidation, if SOA formation, and its dependence on conditions, is to be represented rigorously (e.g., Kroll and Seinfield, 2008; Hallquist et al, 2009). For the photo-oxidation and ozonolysis conditions considered in the present paper, the systems are insensitive to the NO3-initiated chemistry, which is not discussed further
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