Atmospheric degradation of volatile organic compounds

. Kinetic and mechanistic data relevant to the tropospheric degradation of volatile organic compounds (VOC), and the production of secondary pollutants, have previously been used to define a protocol which underpinned the construction of a near-explicit Master Chemical Mechanism. In this paper, an update to the previous protocol is presented, which has been used to define degradation schemes for 107 non-aromatic VOC as part of version 3 of the Master Chemical Mechanism (MCM v3). The treatment of 18 aromatic VOC is described in a companion paper. The protocol is divided into a series of subsections describing initiation reactions, the reactions of the radical intermediates and the further degradation of first and subsequent generation products. Emphasis is placed on updating the previous information, and outlining the methodology which is specifically applicable to VOC not considered previously (e.g. a - and b -pinene). The present protocol aims to take into consideration work available in the open literature up to the beginning of 2001, and some other studies known by the authors which were under review at the time. Application of MCM v3 in appropriate box models indicates that the representation of isoprene degradation provides a good description of the speciated distribution of oxygenated organic products observed in reported field studies where isoprene was the dominant emitted hydrocarbon, and that the a -pinene degradation chemistry provides a good description of the time dependence of key gas phase species in a -pinene/NOX photo-oxidation experiments carried out in the European Photoreactor (EUPHORE). Photochemical Ozone Creation Potentials (POCP) have been calculated for the 106 non-aromatic non-methane VOC in MCM v3 for idealised conditions appropriate to north-west Europe, using a photochemical trajectory model. The POCP values provide a measure of the relative ozone forming abilities of the VOC. Where applicable, the values are compared with those calculated with previous versions of the MCM.

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Protocol for the development of the Master Chemical Mechanism, MCM v3 (Part A): tropospheric degradation of non-aromatic volatile organic compounds

Recent work has shown that the atmospheric oxidation of isoprene (2-methyl-1,3-butadiene, C5H8) leads to the formation of secondary organic aerosol (SOA). In this study, the mechanism of SOA formation by isoprene photooxidation is comprehensively investigated, by measurements of SOA yields over a range of experimental conditions, namely isoprene and NOx concentrations. Hydrogen peroxide is used as the radical precursor, substantially constraining the observed gas-phase chemistry; all oxidation is dominated by the OH radical, and organic peroxy radicals (RO2) react only with HO2 (formed in the OH + H2O2 reaction) or NO concentrations, including NOx-free conditions. At high NOx, yields are found to decrease substantially with increasing [NOx], indicating the importance of RO2 chemistry in SOA formation. Under low-NOx conditions, SOA mass is observed to decay rapidly, a result of chemical reactions of semivolatile SOA components, most likely organic hydroperoxides.

Secondary organic aerosol formation from isoprene photooxidation.

We propose, and quantify from first principles, two novel HOx-regenerating unimolecular reactions in isoprene oxidation, which are estimated to yield in pristine tropical forest conditions about 0.7 HO2 and 0.03 OH radicals per isoprene oxidized; it is further argued that the photolabile coproduct of HO2 can be a major source of OH, with a yield of the order of 1. The newly proposed chemistry could provide a rationalization for the unexpectedly high OH concentrations often observed in forested environments, such as over the Amazon forest in the recent Gabriel campaign.

HOx radical regeneration in the oxidation of isoprene

Organic peroxy radicals (often abbreviated RO2) play a central role in the chemistry of the Earth's lower atmosphere. Formed in the atmospheric oxidation of essentially every organic species emitted, their chemistry is part of the radical cycles that control the oxidative capacity of the atmosphere and lead to the formation of ozone, organic nitrates, organic acids, particulate matter and other so-called secondary pollutants. In this review, laboratory studies of this peroxy radical chemistry are detailed, as they pertain to the chemistry of the atmosphere. First, a brief discussion of methods used to detect the peroxy radicals in the laboratory is presented. Then, the basic reaction pathways – involving RO2 unimolecular reactions and bimolecular reactions with atmospheric constituents such as NO, NO2, NO3, O3, halogen oxides, HO2, and other RO2 species – are discussed. For each of these reaction pathways, basic reaction rates are presented, along with trends in reactivity with radical structure. Focus is placed on recent advances in detection methods and on recent advances in our understanding of radical cycling processes, particularly pertaining to the complex chemistry associated with the atmospheric oxidation of biogenic hydrocarbons.

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Laboratory studies of organic peroxy radical chemistry: an overview with emphasis on recent issues of atmospheric significance

The laser flash photolysis/UV absorption spectrometry technique has been used to investigate the kinetics of the peroxy radical permutation reactions (i.e. self and cross reactions) arising from the OH-initiated oxidation of isoprene (2-methyl-1,3-butadiene), and of the simpler, but related conjugated dienes, 1,3-butadiene and 2,3-dimethyl-1,3-butadiene. The results of the two simpler systems are analysed to provide values of the rate coefficients for the 6 peroxy radical permutation reactions of the three types of isomeric peroxy radical produced in each system (T = 298 K, P = 760 Torr). The rate coefficients are all significantly larger than values estimated previously by extrapolation of structure-reactivity relationships based on the kinetics of a limited dataset of simpler radicals containing similar structural features. The results are discussed in terms of trends in self and cross reaction reactivity of primary, secondary and tertiary peroxy radicals containing combinations of allyl, β-hydroxy and δ-hydroxy functionalities. Since the peroxy radicals formed in these systems are structurally very similar to those formed in the isoprene system, the kinetic parameters derived from the results of the simpler systems are used to assist the assignment of kinetic parameters to the 21 permutation reactions of the six types of isomeric peroxy radical generated in the isoprene system. Kinetic models describing the OH-initiated degradation of all three conjugated dienes to first generation products in the absence of NOx are recommended, which are also consistent with available end product studies. The model for isoprene is considered to be a further improvement on that suggested previously for its OH-initiated oxidation in the absence of NOx. The mechanism is further extended to include chemistry applicable to ‘NOx-present’ conditions, and calculated product yields are compared with those reported in the literature.

Peroxy Radical Kinetics Resulting from the OH-Initiated Oxidation of 1,3-Butadiene, 2,3-Dimethyl-1,3-Butadiene and Isoprene

Rate constants for reactions of various peroxy radicals RO2 with HO2 have been measured at room temperature, using laser-flash photolysis and UV absorption spectrometry. The unique feature of those measurements is that they were performed under a large excess of HO2, so that the only significant reactions were those of HO2 with itself and with RO2. It results that the reaction mechanism was considerably simplified, because those reactions are terminating and the generally complex reaction sequence initiated by the RO2 self-reaction was of negligible importance. In addition, it was not necessary to know the RO2 concentration accurately, only the HO2 concentration. The measured rate constants agree with the literature values, when available. The results confirm the increase of rate constants with the size of RO2 radicals, from nearly 5 × 10-12 cm3 molecule-1 s-1 for CH3O2, up to a limit of about 2.2 × 10-11 cm3 molecule-1 s-1 for C10−C14 radicals. Interestingly, RO2 radicals originating from reactions of OH...

Rate Constants for RO2 + HO2 Reactions Measured under a Large Excess of HO2

The temperature dependence of the rate coefficients for β‐hydroxyperoxy radical self reactions

The laser flash photolysis technique has been used to study the kinetics of the following reactions of the allylperoxy radical at atmospheric pressure and as a function of temperature: CH2CHCH2O2+ CH2CHCH2O2→ products (1), CH2CHCH2O2+ HO2→ CH2CHCH2O2H + O2(2) The radicals were generated by the photolysis of suitable hexa-1,5-diene–O2–N2 and 3-chloropropene–HCHO–O2–N2 mixtures at λ= 193 nm, the resulting total absorbance being measured as a function of time by UV absorption spectrometry. Knowledge of the secondary chemistry and of radical and product absorption spectra, combined with reasonable assumptions of their variations with temperature, allowed the rate coefficients of interest to be estimated by an iterative procedure involving numerical integration of decay profiles recorded at appropriate analysis wavelengths. The resulting Arrhenius expression for reaction (1) is k1=(5.4 ± 1.1)× 10–14 exp[(760 ± 70)/T] cm3 molecule–1 s–1(T= 286–394 K), yielding k1(296 K)=(7.0 ± 0.2)× 10–13 cm3 molecule–1 s–1 and in very good agreement with the only other room-temperature measurement of this rate coefficient (M. E. Jenkin et al., J. Chem. Soc., Faraday Trans., 1993, 89, 433). The determination of k2 was limited by experimental conditions to T= 393–426 K, within which no variation with temperature could be distinguished, and giving k2=(5.6 ± 0.4)× 10–12 cm3 molecule–1 s–1. Extrapolation to 298 K then suggests k2≈ 1 × 10–11 cm3 molecule–1 s–1. The implications of these results for our understanding of isoprene degradation under conditions of low NOx concentrations and for general trends in peroxyl radical reactivity are discussed.

Kinetic studies of the allylperoxyl radical self-reaction and reaction with HO2

Self- and cross-reactions of β-hydroxyperoxy radicals of relevance to tropospheric monoterpene oxidation: structure–activity relationships for rate coefficients

Andrew A. Boyd

Papers

Peroxy Radical Kinetics Resulting from the OH-Initiated Oxidation of 1,3-Butadiene, 2,3-Dimethyl-1,3-Butadiene and Isoprene

Rate Constants for RO2 + HO2 Reactions Measured under a Large Excess of HO2

The temperature dependence of the rate coefficients for β‐hydroxyperoxy radical self reactions

Kinetic studies of the allylperoxyl radical self-reaction and reaction with HO2

Self- and cross-reactions of β-hydroxyperoxy radicals of relevance to tropospheric monoterpene oxidation: structure–activity relationships for rate coefficients