Investigation of the radical product channel of the CH(3)OCH(2)O(2) + HO(2) reaction in the gas phase.

The reaction of CH(3)OCH(2)O(2) with HO(2) has been investigated at 296 K and 700 Torr using long path FTIR spectroscopy, during photolysis of Cl(2)/CH(3)OCH(3)/CH(3)OH/air mixtures. The branching ratio for the reaction channel forming CH(3)OCH(2)O, OH, and O(2) has been determined from experiments in which OH radicals were scavenged by addition of benzene to the system, with subsequent formation of phenol used as the primary diagnostic for OH radical formation. The dependence of the phenol yield on the initial peroxy radical precursor reagent concentration ratio, [CH(3)OH](0)/[CH(3)OCH(3)](0), is consistent with prompt OH formation resulting mainly from the reaction of CH(3)OCH(2)O(2) with HO(2), such that the inferred prompt yield of OH is well-correlated with that of CH(3)OCH(2)OOH, a well-established product of the CH(3)OCH(2)O(2) + HO(2) reaction. The system was fully characterized by simulation, using a detailed chemical mechanism which included other established sources of OH in the system. This allowed a branching ratio of k(2c)/k(2) = 0.19 +/- 0.08 to be determined. The results therefore provide strong indirect evidence for the participation of the radical-forming channel of the title reaction.

Detailed chemical analysis of regional-scale air pollution in western Portugal using an adapted version of MCM v3.1.

A version of the Master Chemical Mechanism (MCM) v3.1, refined on the basis of recent chamber evaluations, has been incorporated into a Photochemical Trajectory Model (PTM) and applied to the simulation of boundary layer photochemistry in the Portuguese west coast region. Comparison of modelled concentrations of ozone and a number of other species (NO(x) and selected hydrocarbons and organic oxygenates) was carried out, using data from three connected sites on two case study days when well-defined sea breeze conditions were established. The ozone concentrations obtained through the application of the PTM are a good approximation to the measured values, the average difference being ca. 15%, indicating that the model was acceptable for evaluation of the details of the chemical processing. The detailed chemistry is examined, allowing conclusions to be drawn concerning chemical interferences in the measurements of NO(2), and in relation to the sensitivity of ozone formation to changes in ambient temperature. Three important, and comparable, contributions to the temperature sensitivity are identified and quantified, namely (i) an effect of increasing biogenic emissions with temperature; (ii) an effect of increasing ambient water vapour concentration with temperature, and its influence on radical production; and (iii) an increase in VOC oxidation chain lengths resulting from the temperature-dependence of the kinetic parameters, particularly in relation to the stability of PAN and its higher analogues. The sensitivity of the simulations to the refinements implemented into MCM v3.1 are also presented and discussed.

Investigation of the radical product channel of the CH(3)C(O)CH(2)O(2) + HO(2) reaction in the gas phase.

The reaction of CH(3)C(O)CH(2)O(2) with HO(2) has been studied at 296 K and 700 Torr using long path FTIR spectroscopy, during photolysis of Cl(2)/acetone/methanol/air mixtures. The branching ratio for the reaction channel forming CH(3)C(O)CH(2)O, OH and O(2) () was investigated in experiments in which OH radicals were scavenged by addition of benzene to the system, with subsequent formation of phenol used as the primary diagnostic for OH radical formation. The observed prompt formation of phenol under conditions when CH(3)C(O)CH(2)O(2) reacts mainly with HO(2) indicates that this reaction proceeds partially by channel , which forms OH both directly and indirectly, by virtue of secondary generation of CH(3)C(O)O(2) (from CH(3)C(O)CH(2)O) and its reaction with HO(2) (). The secondary generation of OH radicals was confirmed by the observed formation of CH(3)C(O)OOH, a well-established product of the CH(3)C(O)O(2) + HO(2) reaction (via channel ). A number of delayed sources of OH also contribute to the observed phenol formation, such that full characterisation of the system required simulations using a detailed chemical mechanism. The dependence of the phenol and CH(3)C(O)OOH yields on the initial peroxy radical precursor reagent concentration ratio, [methanol](0)/[acetone](0), were well described by the mechanism, consistent with a small but significant fraction of the reaction of CH(3)C(O)CH(2)O(2) with HO(2) proceeding via channel . This allowed a branching ratio of k(3b)/k(3) = 0.15 +/- 0.08 to be determined. The results therefore provide strong indirect evidence for the participation of the radical-forming channel of the title reaction.

Investigation of the radical product channel of the CH3COO2 + HO2 reaction in the gas phase.

The reaction of CH(3)C(O)O(2) with HO(2) has been investigated at 296 K and 700 Torr using long path FTIR spectroscopy, during photolysis of Cl(2)/CH(3)CHO/CH(3)OH/air mixtures. The branching ratio for the reaction channel forming CH(3)C(O)O, OH and O(2) (reaction ) has been determined from experiments in which OH radicals were scavenged by addition of benzene to the system, with subsequent formation of phenol used as the primary diagnostic for OH radical formation. The dependence of the phenol yield on benzene concentration was found to be consistent with its formation from the OH-initiated oxidation of benzene, thereby confirming the presence of OH radicals in the system. The dependence of the phenol yield on the initial peroxy radical precursor reagent concentration ratio, [CH(3)OH](0)/[CH(3)CHO](0), is consistent with OH formation resulting mainly from the reaction of CH(3)C(O)O(2) with HO(2) in the early stages of the experiments, such that the limiting yield of phenol at high benzene concentrations is well-correlated with that of CH(3)C(O)OOH, a well-established product of the CH(3)C(O)O(2) + HO(2) reaction (via channel (3a)). However, a delayed source of phenol was also identified, which is attributed mainly to an analogous OH-forming channel of the reaction of HO(2) with HOCH(2)O(2) (reaction ), formed from the reaction of HO(2) with product HCHO. This was investigated in additional series of experiments in which Cl(2)/CH(3)OH/benzene/air and Cl(2)/HCHO/benzene/air mixtures were photolysed. The various reaction systems were fully characterised by simulations using a detailed chemical mechanism. This allowed the following branching ratios to be determined: CH(3)C(O)O(2) + HO(2)--> CH(3)C(O)OOH + O(2), k(3a)/k(3) = 0.38 +/- 0.13; --> CH(3)C(O)OH + O(3), k(3b)/k(3) = 0.12 +/- 0.04; --> CH(3)C(O)O + OH + O(2), k(3c)/k(3) = 0.43 +/- 0.10: HOCH(2)O(2) + HO(2)--> HCOOH + H(2)O + O(2), k(17b)/k(17) = 0.30 +/- 0.06; --> HOCH(2)O + OH + O(2), k(17c)/k(17) = 0.20 +/- 0.05. The results therefore provide strong evidence for significant participation of the radical-forming channels of these reactions, with the branching ratio for the title reaction being in good agreement with the value reported in one previous study. As part of this work, the kinetics of the reaction of Cl atoms with phenol (reaction (14)) have also been investigated. The rate coefficient was determined relative to the rate coefficient for the reaction of Cl with CH(3)OH, during the photolysis of mixtures of Cl(2), phenol and CH(3)OH, in either N(2) or air at 296 K and 760 Torr. A value of k(14) = (1.92 +/- 0.17) x 10(-10) cm(3) molecule(-1) s(-1) was determined from the experiments in N(2), in agreement with the literature. In air, the apparent rate coefficient was about a factor of two lower, which is interpreted in terms of regeneration of phenol from the product phenoxy radical, C(6)H(5)O, possibly via its reaction with HO(2).

A kinetics and mechanistic study of the OH and NO2 initiated oxidation of cyclohexa-1,3-diene in the gas phase.

The kinetics and products of the OH and NO2-initiated oxidation of cyclohexa-1,3-diene have been investigated at 296 K and 700 Torr using long path FTIR spectroscopy. Relative rate methods were employed using the photolysis of cyclohexa-1,3-diene/CH3ONO/NO/air mixtures to measure kappa(OH + cyclohexa-1,3-diene) = (1.68 +/- 0.43) x 10(-10) cm3 molecule(-1) s(-1). From the pseudo-first order decay of cyclohexa-1,3-diene in the presence of excess NO2, a value of kappa(NO2 + cyclohexa-1,3-diene) = (1.75 +/- 0.15) x 10(-18) cm3 molecule)-1) s(-1) was derived. An upper limit of kappa < or = 7 x 10(-21) cm3 molecule(-1) s(-1) was established for the reaction of NO with cyclohexa-1,3-diene. Benzene was observed as a product of both the OH and NO2 initiated oxidation, providing evidence of H atom abstraction in both reactions. Assuming the reaction of cyclohexadienyl radicals (C6H7) with O2 produces benzene as the sole organic product, the results are consistent with abstraction channel branching ratios of (8.1 +/- 0.2)% and (1.5 +/- 0.4)%, respectively. The results also indicate that C6H7 reacts with NO2, with a relative rate coefficient kappa(C6H7 + NO2)/kappa(C6H7 +O2) = (1.8 +/- 0.5) x 10(5), and that this partially forms benzene, with a branching ratio of (27 +/- 7)%. The stoichiometry and products of the NO2 reaction were investigated in the absence of O2, in the presence of O2, and in the presence of O2 and NO. Reaction mechanisms consistent with the observations are presented. In the presence of NO and O2, the NO2-initiated chemistry leads to NO-to-NO2 conversion, and the formation of HOx radicals in significant yield, (0.79 +/- 0.05), such that cyclohexa-1,3-diene removal occurs by reaction with both NO2 and OH. HCOOH was detected as a product in this system, providing evidence for significant formation of stabilised C6 alpha-hydroxyperoxy radicals from the OH-initiated chemistry, and their subsequent reaction with NO. An estimate of ca. 500-1000 s(-1) is made for their decomposition rate, based on the [NO]-dependence of the HCOOH yields. The implications of the results are discussed within the context of the atmospheric chemistry of conjugated dienes.

Ambient concentrations of 1,3-butadiene in the UK.

This paper assesses the current knowledge of 1,3-butadiene as an atmospheric pollutant, considers measurement techniques and reviews available data on 1,3-butadiene monitoring and emissions estimates. Atmospheric chemistry, sources of emission, current legislation, measurement techniques and monitoring programmes for 1,3-butadiene are reviewed. There have been comparatively few studies of the products of oxidation of 1,3-butadiene in the atmosphere. However, on the basis of the available information, and by analogy with the oxidation mechanism for the widely-studied and structurally similar natural hydrocarbon isoprene (2-methyl-1,3-butadiene), it is possible to define some features of the likely oxidation pathways for 1,3-butadiene. The total UK 1,3-butadiene emission to the atmosphere for 1996 has been estimated at 10.60 kTonnes. 1,3-Butadiene is a product of petrol and diesel combustion; consequently this total is dominated by road transport exhaust emissions (accounting for some 68% of the total). Off-road vehicles and machinery are responsible for 14% of the total UK emission. 1,3-Butadiene is used in the manufacture of numerous rubber compounds, and consequently emissions arise from both the manufacture and use of 1,3-butadiene in industrial processes. Emissions from the chemical industry account for 18% of the UK total emission- 8% from 1,3-butadiene manufacture and 10% from 1,3-butadiene use. The United Kingdom Expert Panel on Air Quality Standards (EPAQS) has published a report on 1,3-butadiene, and recommended a national air quality standard of 1.0 ppb (expressed as an annual rolling mean). This was adopted by the Government as part of the National Air Quality Strategy (NAQS) in 1997, and a target of compliance by 2005 was set. Work conducted for the review of the NAQS (1999) indicated that it was likely that all locations would be compliant with the national standard by the end of 2003. As a result, the review updated the air quality objective for 1,3-butadiene, with the deadline for compliance being brought forward to 31/12/2003. The UK Hydrocarbon Monitoring Network provides continuous hourly measurements of 1,3-butadiene at 13 sites, and has been operational since 1993. The dataset that is available allows spatial and temporal trends to be evaluated, and has proved to be invaluable in characterising the current ambient levels of 1,3-butadiene in the UK. Hourly maximum concentrations of 1,3-butadiene of up to 10 ppb (1 ppb=1 ppb, i.e. 1 vol. of 1,3-butadiene in 1,000,000,000 vol. of air. 1 ppb of 1,3-butadiene is ca. equal to 2.25 microg m(-3) at 20 degrees C) may be measured for several hours at the sites. Monthly mean concentrations are typically 0.1-0.4 ppbv. At most sites, these levels are driven by emissions from motor vehicles. Occasionally emissions of 1,3-butadiene from industrial sources may elevate 1,3-butadiene concentrations to several tens of ppb. Trend analysis of the data suggests that ambient concentrations of 1,3-butadiene in the UK are declining at about 10% per year.

Characterization of the reactivities of volatile organic compounds using a master chemical mechanism.

A comprehensive description of the ozone-forming potentials of 101 organic compounds has been constructed under North American urban "averaged conditions" using a detailed master chemical mechanism and a simple air parcel trajectory model. This chemical mechanism describes the reactions of 3603 chemical species taking part in more than 10,500 chemical reactions. An index value has been calculated for each organic compound, which describes the increment in ozone concentrations found downwind of an urban area following the emission of a fixed increment in the mass emission of each organic compound. These indices, termed photochemical ozone creation potentials (POCPs), have been expressed on a scale relative to ethylene (ethene) = 100, and, a reactivity scale has been generated for alkanes, alkenes, and oxygenated and halogenated organic compounds. A high degree of correlation (R2 = 0.9) was found between these POCP values and the most widely accepted urban reactivity scale. While the reactivities of most of the 86 organic compounds compared fell within a consistent range, significant discrepancies were found for only 5 compounds. Single-day or multiday conditions appear to be important in establishing quantitative reactivity scales for the less reactive organic compounds.