Michael E. Jenkin

The tropospheric degradation of volatile organic compounds: a protocol for mechanism development

Kinetic and mechanistic data relevant to the tropospheric oxidation of volatile organic compounds (VOCs) are used to define a series of rules for the construction of detailed degradation schemes for use in numerical models. These rules are intended to apply to the treatment of a wide range of non-aromatic hydrocarbons and oxygenated and chlorinated VOCs, and are currently being used to provide an up-to-date mechanism describing the degradation of a range of VOCs, and the production of secondary oxidants, for use in a model of the boundary layer over Europe. The schemes constructed using this protocol are applicable, however, to a wide range of ambient conditions, and may be employed in models of urban, rural or remote tropospheric environments, or for the simulation of secondary pollutant formation for a range of NOx or VOC emission scenarios. These schemes are believed to be particularly appropriate for comparative assessments of the formation of oxidants, such as ozone, from the degradation of organic compounds. The protocol is divided into a series of subsections dealing with initiation reactions, the reactions of the radical intermediates and the further degradation of first and subsequent generation products. The present work draws heavily on previous reviews and evaluations of data relevant to tropospheric chemistry. Where necessary, however, existing recommendations are adapted, or new rules are defined, to reflect recent improvements in the database, particularly with regard to the treatment of peroxy radical (RO2) reactions for which there have been major advances, even since comparatively recent reviews. The present protocol aims to take into consideration work available in the open literature up to the end of 1994, and some further studies known by the authors, which were under review at that time. A major disadvantage of explicit chemical mechanisms is the very large number of reactions potentially generated, if a series of rules is rigorously applied. The protocol aims to limit the number of reactions in a degradation scheme by applying a degree of strategic simplification, whilst maintaining the essential features of the chemistry. These simplification measures are described, and their influence is demonstrated and discussed. The resultant mechanisms are believed to provide a suitable starting point for the generation of reduced chemical mechanisms.

Photochemical ozone creation potentials for a large number of reactive hydrocarbons under European conditions

A photochemical trajectory model is used to describe the ozone production from the oxidation of methane and 95 other hydrocarbons in the presence of sunlight and NOx in air parcels advected across north west Europe towards the British Isles. By adding a small additional mass emission of each hydrocarbon in turn, additional ozone production was stimulated. A photochemical ozone creation potential (POCP) index was generated from the model results showing the relative importance of each hydrocarbon in ozone formation, on a mass emitted basis. Aromatic and olefinic hydrocarbons showed the highest POCP values with halocarbons the lowest. Using the POCP index, motor vehicle exhaust is seen to exhibit the highest ozone-forming potential of all the hydrocarbon emission source categories evaluated. Toluene, n-butane, ethylene and the xylenes, alone, account for over one third of the ozone forming potential of European emissions. Certain hydrocarbons, including acetone and methyl acetate, show significantly lower POCPs and have considerable potential as candidates for substitution in industrial or chemical processes and as solvents.

Photochemical ozone creation potentials for organic compounds in northwest Europe calculated with a master chemical mechanism

Abstract Master Chemical Mechanism containing over 2400 chemical species and over 7100 chemical reactions is employed here to describe the atmospheric degradation of 120 organic compounds and the associated regional scale ozone and PAN formation under conditions appropriate to the polluted boundary layer over northwest Europe. Photochemical ozone and PAN creation potentials (POCP and PPCP) are derived for each organic compound from their propensities to form ozone and PAN relative to ethylene and propylene, respectively. The robustness of these POCP values to changes in the NOx emission densities across Europe is tested and the values are compared with previous studies. The POCP values are reviewed and rationalised against the background of our current understanding of the intrinsic properties of each organic compound, their atmospheric degradation pathways and other mechanistic data. These POCPs should assist policy-makers in defining realistic and robust pollution control strategies which focus on those organic compounds which contribute most to regional scale ozone formation across northwest Europe.

Evaluated kinetic and photochemical data for atmospheric chemistry: Volume VI – heterogeneous reactions with liquid substrates

Atmospheric chemistry is the study of the complex network of thermal and photochemical processes occurring in the gas and condensed (cloud droplets, aerosol particles, ice crystals) phase as well as in multiphase processes. Chemical kinetic modeling is required to interpret observations in the field. This necessitates the availability of a robust, reliable, and regularly updated database of elementary reactions for use in chemical-radiative-transport models. Combustion chemistry and plasma processing for semiconductor applications require the same type of database.

Analysis of the relationship between ambient levels of O3, NO2 and NO as a function of NOx in the UK

Abstract Monitoring data from the UK Automatic Urban and Rural Network are used to investigate the relationships between ambient levels of ozone (O3), nitric oxide (NO) and nitrogen dioxide (NO2) as a function of NOx, for levels ranging from those typical of UK rural sites to those observed at polluted urban kerbside sites. Particular emphasis is placed on establishing how the level of ‘oxidant’, OX (taken to be the sum of O3 and NO2) varies with the level of NOx, and therefore to gain some insight into the atmospheric sources of OX, particularly at polluted urban locations. The analyses indicate that the level of OX at a given location is made up of NOx-independent and NOx-dependent contributions. The former is effectively a regional contribution which equates to the regional background O3 level, whereas the latter is effectively a local contribution which correlates with the level of primary pollution. The local oxidant source has probable contributions from (i) direct NO2 emissions, (ii) the thermal reaction of NO with O2 at high NOx, and (iii) common-source emission of species which promote NO to NO2 conversion. The final category may include nitrous acid (HONO), which appears to be emitted directly in vehicle exhaust, and is potentially photolysed to generate HOx radicals on a short timescale throughout the year at southern UK latitudes. The analyses also show that the local oxidant source has significant site-to-site variations, and possible reasons for these variations are discussed. Relationships between OX and NOx, based on annual mean data, and fitted functions describing the relative contributions to OX made by NO2 and O3, are used to define expressions which describe the likely variation of annual mean NO2 as a function of NOx at 14 urban and suburban sites, and which can take account of possible changes in the regional background of O3.

Photochemical ozone formation in north west Europe and its control

A photochemical trajectory model together with a Master Chemical Mechanism and a highly speciated emission inventory for organic compounds have been used to describe the formation of ozone in north west Europe and to identify the most prolific ozone-forming organic compounds. Observations are reviewed to assess the impact of emission controls on their urban volatile organic compound (VOC) concentration trends with time. The observed trends are then used to deduce the likely trends in episodic peak ozone concentrations and to compare them with observed trends in peak ozone concentrations. It is concluded that it is likely that motor vehicle emission controls have brought about a substantial reduction in episodic peak ozone concentrations in north west Europe during the 1990s.

Laboratory studies of the kinetics of formation of nitrous acid from the thermal reaction of nitrogen dioxide and water vapour

Abstract The kinetics of the formation of nitrous acid (MONO) and removal of NO2 during the heterogeneous reaction of NO2 and H2O were investigated using infra-red diode laser spectroscopy and u.v./visible spectroscopy. The majority of experiments were performed at temperatures of 292 and 296 K, and at pressures of less than 10 Torr, but a limited number were performed at 277 and 311 K and at pressures of up to 297 Torr. The 19.8-l pyrex reaction vessel used in this work ( s v = 13 m −1 ) had stainless steel end plates and contained gold coated multi-reflection ‘White’ optics. The initial kinetics of HONO production and NO2 removal were found to be 1st order in both [NO2] and [H2O] and the observed 2nd order rate constant, independent of pressure at 296 K, was: k= d[ HONO ] dt [NO 2 ][H 2 O] = 3.2 × 10 −22 cm 3 molecule −1 s −1 . The yield of HONO during the early stages of reaction was ~ 50 % relative to NO2 removed, but as the reaction proceeded the kinetics became more complicated, and the yield was reduced. HNO3 is almost certainly produced, but could not be detected in the gas phase by infra-red measurement at 890 cm−1. HONO may be an important precursor to OH radicals in the atmospheric boundary layer. An estimate of the production rate of HONO from the reaction of NO2 and H2O in the night-time boundary layer has been made using the results of this work, and a comparison is made with those based on field observations.

Halogen oxides: Radicals, sources and reservoirs in the laboratory and in the atmosphere

Abstract The central topic of this review concerns the species XO, where X is F, Cl, Br or I. These molecules are thus the radicals FO, ClO, BrO and IO, but attention is also given to some of their precursors in the laboratory and the atmosphere, as well as to their reservoirs, sinks, and other related species of potential atmospheric importance. Laboratory data on the physics and chemistry of the species and atmospheric determinations of their concentrations are both considered. One aim of the review is to highlight the relationship between the laboratory investigations and the atmospheric studies. The emphasis of the review is on gas-phase processes. After a brief introductory section, the review continues with an examination of laboratory techniques for the study of the halogen-oxide species. This section fast looks at the general properties of the oxides and sources of them for laboratory experiments, then discusses the detection and measurement of the monoxide radicals in the laboratory, and ends with a description of the kinetic tools that have been harnessed in the various studies. The spectroscopy, structure, photochemistry and thermochemistry, of the halogen oxides are discussed in Section III. Both experimental and theoretical aspects are presented. The objectives of the work described are on the one hand to establish the basis for the detection of the radical and the measurement of its concentration in the laboratory and in the atmosphere, and on the other to provide the framework for interpreting pathways, mechanisms and efficiencies of photochemical and thermal reactions. Sections IV, V and VI of the review address the main issues of observed chemistry and its kinetics. Section IV gathers together available kinetic and mechanistic information on gas-phase reactions of FO, ClO, BrO and IO radicals, and the available data are summarized in appropriate tables. Section V reports on the corresponding data available for the gas-phase reactions of certain species containing the XO grouping, which include most of the so-called atmospheric reservoirs of XO radicals. There are three sub-sections, which deal in turn with oxide species, HOX, and XONO2. Heterogeneous processes are introduced in Section VI. Heterogeneous chemistry in the atmosphere is that which occurs on or in ambient condensed phases that are in contact with the gas phase, such as aerosols, clouds, surface waters, and so on. It is becoming increasingly clear that such processes are of importance not only in the stratosphere, but also in the troposphere. Section VII of the review is concerned directly with the atmosphere. The sources and sinks of the compounds, the reaction pathways, temporary and permanent reservoirs, observational evidence, the involvement of the species in atmospheric chemistry, and modelling studies are considered for the troposphere and the stratosphere in turn. The section concludes with a more detailed exposition of the role of modelling of the halogen compounds in the stratosphere. The review concludes with an examination of issues in regard to the halogen oxide species that are unresolved, uncertain, or in need of further research. Further data are required, for example, on the spectroscopy and photochemistry of reservoir compounds, on potential organic sources of atmospheric iodine, and even on the channels for photolysis of compounds such as OClO. Within the field of reaction kinetics, there is a need for further study of the kinetics of dimer formation, and of certain other reactions of the radicals themselves (especially of IO) and some of their reservoirs. A substantial number of problems in heterogeneous chemistry of the species remain to be solved. Not only are some key physical measurements missing, but most of what has been achieved in both chemistry and physics is limited to chlorine-containing species, so that the work needs to be extended to the other halogens. There is also a need for a search for novel reactions occurring on conventional surfaces and for all types of reaction occurring on surfaces that exist within the atmosphere but which have not yet been the subject of laboratory study. So far as the atmosphere itself is concerned, there are important issues to be resolved. They include (i) the involvement of halogen species in episodic tropospherec ozone depletion in the Arctic (and a further question about whether or not such depletion is more widespread); (ii) the role of an active halogen chemistry in the oxidation of VOC; (iii) the significance and detail of stratospheric iodine and iodine-catalysed ozone removal; and (iv) the quantitative description of heterogeneous stratospheric chemistry.

Photochemical ozone creation potentials for oxygenated volatile organic compounds: sensitivity to variations in kinetic and mechanistic parameters

The sensitivity of Photochemical Ozone Creation Potentials (POCP) to a series of systematic variations in the rates and products of reactions of radical intermediates and oxygenated products is investigated for the C4 alcohols, 1-butanol (n-butanol) and 2-methyl-1-propanol (i-butanol), using the recently developed Master Chemical Mechanism (MCM) as the base case. The POCP values are determined from the calculated formation of ozone in the boundary layer over a period of approximately five days along an idealised straight line trajectory, using a photochemical trajectory model and methodology described in detail previously. The results allow the relative impacts on calculated ozone formation of various classes of chemical reaction within the degradation chemistry to be assessed. The calculated POCP is found to be very insensitive to many of the changes investigated. However, it is found to be sensitive to variations in the rate coefficient for the initiating reaction with OH (kOH), although the sensitivity decreases with increasing kOH. The POCP appears to vary approximately linearly with kOH at low values (i.e. kOH less than ca. 4×10-13 cm3 molecule-1 s-1), whereas at high reactivities (i.e. kOH greater than ca. 4×10-11 cm3 molecule-1 s-1), the calculated POCP value is comparatively insensitive to the precise value of kOH. The POCP is also very sensitive to mechanistic changes which influence the yields of unreactive oxygenated products (i.e. those with OH reactivities below ca. 10-12 cm3 molecule-1 s-1), for example acetone. The propensity of the organic compound to produce organic nitrates (which act as comparatively unreactive reservoirs for free radicals and NOx) also appears to have a notable influence on the calculated POCP. Recently reported information relevant to the degradation of oxygenated VOCs is then used to update the chemical schemes for the 17 alcohols and glycols, 10 ethers and glycol ethers, and 8 esters included in the MCM, and new schemes are incorporated for dimethoxy methane (CH3OCH2OCH3) and dimethyl carbonate (CH3OC(O)OCH3), which are proposed fuel additives. New or updated POCP values are calculated for all 37 oxygenated VOCs and, where applicable, these are compared with the previous POCP values and reported Maximum Incremental Reactivity (MIR) values.

Development and application of a possible mechanism for the generation of cis-pinic acid from the ozonolysis of α- and β-pinene

Abstract Recent experimental studies have identified cis- pinic acid (a C 9 dicarboxylic acid) as a condensed-phase product of the ozonolysis of both α - and β -pinene, and it is currently believed to be the most likely degradation product leading to the prompt formation of new aerosols by nucleation. The observed timescale of aerosol formation appears to require that cis- pinic acid is a first-generation product, and a possible mechanism for its formation has therefore been developed. The key step in the proposed mechanism requires that the isomerisation of a complex C 9 acyl-oxy radical by a 1,7 H atom shift is able to compete with the alternative decomposition to CO 2 and a C 8 organic radical: Thermodynamic and kinetic arguments are presented, on the basis of semi-empirical electronic structure calculations, which support this proposed mechanism, and thereby the competition between the two pathways. The transfer of the labile aldehydic H atom is shown to be especially facile in this case because it occurs though an unstrained transition state; this feature can in turn be attributed to the cis -substitution of the four-membered ring, which enforces the steric proximity of the acyl-oxy and aldehyde groups. The mechanism can explain the formation of cis -pinic acid from both α - and β -pinene, because the acyl-oxy radical is likely to be formed following the decomposition of excited Criegee biradicals formed in both systems. It is also possible that a similar isomerisation reaction of a complex C 10 α -carbonyl oxy radical by a 1,8 H atom shift might explain the very recently observed formation of cis -10-hydroxy-pinonic acid from α -pinene ozonolysis, and this possibility is also explored. An existing detailed scheme describing the degradation of α -pinene (part of the Master Chemical Mechanism, MCM) is updated to include the proposed cis- pinic acid and cis -10-hydroxy-pinonic acid formation mechanisms, and the values of several uncertain parameters are adjusted on the basis of reported yields of a series of organic products from the ozonolysis of α -pinene. The updated degradation scheme is incorporated into a boundary layer box model, and representative ambient concentrations of the organic acids and other oxygenated products are calculated for a range of representative conditions appropriate to the boundary layer over central Europe. The simulated concentrations of the organic acids in general, and cis -pinic acid in particular, are strongly dependent on the level of NO X , and suggest that new aerosol formation from the oxidation of α -pinene is likely to be more favoured at lower NO X levels.