Gas phase precursors to anthropogenic secondary organic aerosol: Using the Master Chemical Mechanism to probe detailed observations of 1,3,5-trimethylbenzene photo-oxidation

doi:10.1016/j.atmosenv.2009.09.043

Atmospheric Environment

Volume 44, Issue 40, December 2010, Pages 5423-5433

https://doi.org/10.1016/j.atmosenv.2009.09.043 Get rights and content

Abstract

A detailed gas-phase photochemical chamber box model, incorporating the Master Chemical Mechanism (MCMv3.1) degradation scheme for the model anthropogenic aromatic compound 1,3,5-trimethylbenzene, has been used to simulate data measured during a series of aerosol chamber experiments in order to evaluate the mechanism under a variety of VOC/NO_x conditions.

The chamber model was used in the interpretation of comprehensive high (mass and time) resolution measurements of 1,3,5-trimethylbenzene and its photo-oxidation products recorded by a Chemical Ionisation Reaction Time-of-Flight Mass Spectrometer (CIR-TOF-MS). Supporting gas and aerosol measurements have also enabled us to explore the ‘missing link’ between the gas and aerosol phases. Model-measurement comparisons have been used to gain insight into the complex array of oxygenated products formed, including the peroxide bicyclic ring opening products (α,β-unsaturated-γ-dicarbonyls and furanones) and the O₂-bridged peroxide bicyclic ring-retaining products. To our knowledge this is the first time such high molecular weight species, corresponding to various peroxide bicyclic products represented in the MCMv3.1, have been observed in the gas-phase. The model was also used to give insight into which gas-phase species were participating in SOA formation, with the primary and secondary peroxide products, formed primarily under low NO_x conditions, identified as likely candidates.

Introduction

Aromatic volatile organic compounds are known to be highly reactive, to have large emission rates and high photochemical ozone creation potentials – substantially contributing to photochemical ozone formation (Derwent et al., 1998). It has been estimated that aromatic compounds, whose major sources are from petrol vehicle exhaust and solvent usage, contribute ca. 10% to global anthropogenic non-methane hydrocarbon emissions (Houweling et al., 1998). In the United Kingdom, models predict that total aromatics contribute ca. 30% to regional ozone production (Derwent et al., 2007). Therefore, it is important to understand the detailed photochemical degradation mechanisms of aromatic compounds, under a variety of atmospheric conditions, for use in air quality models.

Aromatic photo-oxidation mechanisms contain a complex array of reaction pathways leading to a wide range of secondary oxygenated and nitrated volatile, semi-volatile and non-volatile organic products which can partition to the aerosol phase (Calvert et al., 2002). Indeed, photo-oxidation of aromatics has been shown to significantly contribute to anthropogenic secondary organic aerosol (SOA) formation (e.g. Hallquist et al., 2009). However, despite its crucial impacts on air quality, climate and associated consequences for human health (Mauderly and Chow, 2008), SOA physical and chemical properties, composition and formation mechanisms are poorly understood, partly owing to a distinct lack of knowledge with regards to gas-phase precursors (Fuzzi et al., 2006).

In the work presented, a chamber optimised photochemical box model was used to simulate the evolution of 1,3,5-trimethylbenzene (TMB) gas-phase photochemistry during a number of comprehensive simulation chamber experiments in order to evaluate the mechanism used under a variety of VOC/NO_x conditions. More specifically, the model was used in the interpretation of data recorded by a Chemical Ionisation Reaction Time-of-Flight Mass Spectrometer (CIR-TOF-MS), a novel instrument used to provide comprehensive, high (mass and time) resolution measurements of the organic gaseous oxidation products formed from the TMB precursor (Wyche et al., 2009). Model-measurement comparisons using such highly detailed simultaneous measurements of the complex array of multi-functional products formed, enables us to pin down the mechanism in detail and can also provide key insight for guiding the directions of future laboratory experiments.

The box model used is based around a detailed TMB degradation mechanism which has been extracted from the Master Chemical Mechanism (MCMv3.1; http://mcm.leeds.ac.uk/MCM). MCMv3.1 is a near-explicit chemical mechanism originally conceived to model ozone formation in Europe but now also employed as a benchmark mechanism in a wide variety of applications where chemical detail is required. The MCM currently describes the detailed gas-phase tropospheric degradation of 135 primary emitted non-methane hydrocarbons (NMHCs) leading to a mechanism containing ca. 5900 species and 13 500 reactions (Jenkin et al., 1997, Jenkin et al., 2003, Saunders et al., 2003). The update from MCMv3.0 to MCMv3.1 was carried out in order to incorporate recent kinetic and mechanistic improvements in the understanding of aromatic photo-oxidation (Bloss et al., 2005b). MCMv3.1 has previously been evaluated using a high quality, comprehensive chamber dataset designed to focus in on key aspects of the photochemical oxidation of benzene, toluene, p-xylene and TMB (Bloss et al., 2005a). However, although the model/measurement agreement was improved in some areas, significant deficiencies were identified in the mechanisms, in particular concerning the ozone formation potential and oxidative capacity of the aromatic hydrocarbon systems.

The modelling work discussed here both supports and complements the companion paper by Wyche et al. (2009), in which the detailed measurements provided by the CIR-TOF-MS (and other instrumentation) are discussed in greater detail. In combination, the work presented here and in Wyche et al. (2009) provides comprehensive information on the gas-phase photochemical degradation of TMB and the identification of potential gas-phase precursors to SOA formation and growth. More specifically, the model has been used to give insight into which gas-phase species are likely to participate in SOA formation, including the high mass (multi-functional) oxygen-bridged bicyclic ring-retaining and primary ring opening products.

Section snippets

Experimental

The experimental section of this work was conducted at the Paul Scherrer Institut (PSI) aerosol chamber facility in Switzerland. This is a temperature controlled indoor chamber comprising a 27 m³ Teflon bag illuminated by four 4 kW xenon arc lamps (Paulsen et al., 2005). The chamber was equipped with a suite of instrumentation, including: standard NO_x and O₃ monitors, a Chemical Ionisation Reaction Time-of-Flight Mass Spectrometer (CIR-TOF-MS), using proton transfer reaction (PTR) from the

Model construction

The composition and evolution of the gas phase components of the TMB-SOA chamber system were simulated using a chamber optimised photochemical box model incorporating the comprehensive TMB photo-oxidation scheme extracted from MCMv3.1. The TMB mechanism employed (along with an appropriate inorganic reaction scheme) contains ca. 145 species and 390 reactions.

The box model used in this study also includes a series of ‘chamber specific’ auxiliary reactions adapted from Bloss et al. (2005a) and

Model-measurement comparison of TMB oxidation and the evolution of the major inorganic species

Fig. 1(a) and 1(b) present the modelled and measured profiles for TMB, NO, NO₂ and O₃ for experiments 6 and 7, respectively, i.e. typical examples of the ‘high initial’ and ‘low’ NO_x scenarios described above (see Table 1). From initial inspection of these figures it is clear that the model is able to simulate the oxidation of the precursor TMB with some level of success, with some under prediction in the loss rate, particularly in the latter stages of each experiment, indicating an under

Conclusions

In the work presented, a detailed chamber specific photochemical box model, incorporating the MCMv3.1 degradation mechanism for 1,3,5-trimethylbenzene, has been used to simulate data measured during a series of chamber experiments in order to evaluate the mechanism under a variety of conditions. More specifically, the model was used in the interpretation of detailed measurements of organic gaseous oxidation products from a Chemical Ionisation Reaction Time-of-Flight Mass Spectrometer

Acknowledgements

ARR and MJP would like to acknowledge support from the EU EUROCHAMP program (RII3-CT-2004-505968). KPW, PSM and AME would like to acknowledge and thank ACCENT, NERC and EPSRC for providing funds in order to conduct the experimental section of this work.

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Gas phase precursors to anthropogenic secondary organic aerosol: Using the Master Chemical Mechanism to probe detailed observations of 1,3,5-trimethylbenzene photo-oxidation

Abstract

Introduction

Section snippets

Experimental

Model construction

Model-measurement comparison of TMB oxidation and the evolution of the major inorganic species

Conclusions

Acknowledgements

Atmospheric Environment

Environmental Science and Policy

Atmospheric Environment

Atmospheric Environment

Demonstration of proton-transfer reaction time-of-flight mass spectrometry for real-time analysis of trace volatile organic compounds

Analytical Chemistry

Proton-transfer reaction mass spectrometry

Chemical Reviews

Evaluation of detailed aromatic mechanisms (MCMv3 and MCMv3.1) against environmental chamber data

Atmospheric Chemistry and Physics

Development of a detailed chemical mechanism (MCMv3.1) for the atmospheric oxidation of aromatic hydrocarbons

Atmospheric Chemistry and Physics

The Mechanisms of Atmospheric Oxidation of Aromatic Hydrocarbons

Laboratory observation of oligomers in the aerosol from isoprene/NOx photooxidation

Geophysical Research Letters

Identification of organic acids in secondary organic aerosol and the corresponding gas-phase from chamber experiments

Analytical Chemistry

Critical assessment of the current state of scientific knowledge, terminology, and research needs concerning the role of organic aerosols in the atmosphere, climate, and global change

Atmospheric Chemistry and Physics

The formation, properties and impact of secondary organic aerosol: current and emerging issues

Atmospheric Chemistry and Physics

Measurements of photo-oxidation products from the reaction of a series of alkyl-benzenes with hydroxyl radicals during EXACT using comprehensive gas chromatography

Atmospheric Chemistry and Physics

A method for real-time detection of PAN, PPN and MPAN in ambient air

Geophysical Research Letters

Gas/particle partitioning of carbonyls in the photooxidation of isoprene and 1,3,5-trimethylbenzene

Atmospheric Chemistry and Physics

Laboratory observation of oligomers in the aerosol from isoprene/NO_x photooxidation