Mohammed S. Alam

Kinetics of stabilised Criegee intermediates derived from alkene ozonolysis: reactions with SO2, H2O and decomposition under boundary layer conditions

The removal of SO2 in the presence of alkene-ozone systems has been studied for ethene, cis-but-2-ene, trans-but-2-ene and 2,3-dimethyl-but-2-ene, as a function of humidity, under atmospheric boundary layer conditions. The SO2 removal displays a clear dependence on relative humidity for all four alkene-ozone systems confirming a significant reaction for stabilised Criegee intermediates (SCI) with H2O. The observed SO2 removal kinetics are consistent with relative rate constants, k(SCI + H2O)/k(SCI + SO2), of 3.3 (±1.1) × 10(-5) for CH2OO, 26 (±10) × 10(-5) for CH3CHOO derived from cis-but-2-ene, 33 (±10) × 10(-5) for CH3CHOO derived from trans-but-2-ene, and 8.7 (±2.5) × 10(-5) for (CH3)2COO derived from 2,3-dimethyl-but-2-ene. The relative rate constants for k(SCI decomposition)/k(SCI + SO2) are -2.3 (±3.5) × 10(11) cm(-3) for CH2OO, 13 (±43) × 10(11) cm(-3) for CH3CHOO derived from cis-but-2-ene, -14 (±31) × 10(11) cm(-3) for CH3CHOO derived from trans-but-2-ene and 63 (±14) × 10(11) cm(-3) for (CH3)2COO. Uncertainties are ±2σ and represent combined systematic and precision components. These values are derived following the approximation that a single SCI is present for each system; a more comprehensive interpretation, explicitly considering the differing reactivity for syn- and anti-SCI conformers, is also presented. This yields values of 3.5 (±3.1) × 10(-4) for k(SCI + H2O)/k(SCI + SO2) of anti-CH3CHOO and 1.2 (±1.1) × 10(13) for k(SCI decomposition)/k(SCI + SO2) of syn-CH3CHOO. The reaction of the water dimer with CH2OO is also considered, with a derived value for k(CH2OO + (H2O)2)/k(CH2OO + SO2) of 1.4 (±1.8) × 10(-2). The observed SO2 removal rate constants, which technically represent upper limits, are consistent with decomposition being a significant, structure dependent, sink in the atmosphere for syn-SCI.

Receptor modelling study of polycyclic aromatic hydrocarbons in Jeddah, Saudi Arabia

Measurements of 14 polycyclic aromatic hydrocarbons (PAH) have been made in Jeddah, Saudi Arabia, with a view to establishing the concentrations in this major city, and quantifying the contributions of major sources. Particulate and vapour forms have been sampled and analysed separately. The concentrations are compared to measurements from other sites in the Middle Eastern region and are towards the lower end of the range, being far lower than concentrations reported from Riyadh (Saudi Arabia), Assiut (Egypt) and Tehran (Iran) but broadly similar to those measured in Damascus (Syria) and higher than those measured in Kuwait. The partitioning between vapour and particle phases is similar to that in data from Egypt and China, but with many compounds showing a higher particle-associated percentage than in Birmingham (UK) possibly reflecting a higher concentration of airborne particulate matter in the former countries. Concentrations in Jeddah were significantly higher at a site close to the oil refinery and a site close to a major ring road than at a suburban site to the north of the city. Application of positive matrix factorisation to the pooled data elicited three factors accounting respectively for 17%, 33% and 50% of the measured sum of PAH and these are interpreted as arising from gasoline vehicles, industrial sources, particularly the oil refinery, and to diesel/fuel oil combustion.

Using Variable Ionization Energy Time-of-Flight Mass Spectrometry with Comprehensive GC×GC To Identify Isomeric Species

Although GC×GC-ToF-MS allows the separation of thousands of peaks, many of these peaks are not positively identified owing to the lack of mass spectral library data and/or standard materials, leading to a substantial amount of information being inaccessible. The fragmentation patterns of molecules in mass spectrometers using electron impact ionization at 70 eV can be useful for molecule identification, provided a match is available in a published EI MS library, but are indistinguishable for many isomeric organic compounds (for example, linear and branched alkanes). Lower ionization energies have been exploited leading to organic compounds being ionized with lower excess internal energy and less fragmentation, retaining the molecular ion and maximizing its relative signal. This has enabled the identification of a large number of isomeric organic compounds, both aliphatic and aromatic, between C12-C36, in the previously unresolved complex mixture (UCM) of two motor oil samples. This technique also demonstrates problems associated with separation of coeluting isomers, particularly for the n-alkanes, which are routinely measured by 1D GC/MS and may be overestimated, due to coelution. As a consequence retention times in 2 dimensions and mass spectra at variable ionization energies are shown to give unparalleled power to identify specific isomers.

Radical Product Yields from the Ozonolysis of Short Chain Alkenes under Atmospheric Boundary Layer Conditions

The gas-phase reaction of ozone with unsaturated volatile organic compounds (VOCs), alkenes, is an important source of the critical atmospheric oxidant OH, especially at night when other photolytic radical initiation routes cannot occur. Alkene ozonolysis is also known to directly form HO2 radicals, which may be readily converted to OH through reaction with NO, but whose formation is poorly understood. We report a study of the radical (OH, HO2, and RO2) production from a series of small alkenes (propene, 1-butene, cis-2-butene, trans-2-butene, 2-methylpropene, 2,3-dimethyl-2-butene (tetramethyl ethene, TME), and isoprene). Experiments were performed in the European Photoreactor (EUPHORE) atmospheric simulation chamber, with OH and HO2 levels directly measured by laser-induced fluorescence (LIF) and HO2 + ΣRO2 levels measured by peroxy-radical chemical amplification (PERCA). OH yields were found to be in good agreement with the majority of previous studies performed under comparable conditions (atmospheric pressure, long time scales) using tracer and scavenger approaches. HO2 yields ranged from 4% (trans-2-butene) to 34% (2-methylpropene), lower than previous experimental determinations. Increasing humidity further reduced the HO2 yields obtained, by typically 50% for an RH increase from 0.5 to 30%, suggesting that HOx production from alkene ozonolysis may be lower than current models suggest under (humid) ambient atmospheric boundary layer conditions. The mechanistic origin of the OH and HO2 production observed is discussed in the context of previous experimental and theoretical studies.

Polycyclic aromatic hydrocarbons, brachial artery distensibility and blood pressure among children residing near an oil refinery.

BACKGROUND Polycyclic aromatic hydrocarbons (PAH) are produced by the burning and processing of fuel oils, and have been associated with oxidant stress, insulin resistance and hypertension in adults. Few studies have examined whether adolescents are susceptible to cardiovascular effects of PAHs. OBJECTIVE To study associations of PAH exposure with blood pressure (BP) and brachial artery distensibility (BAD), an early marker of arterial wall stiffness, in young boys attending three schools in Jeddah, Saudi Arabia in varying proximity to an oil refinery. METHODS Air samples collected from the three schools were analyzed for PAHs. PAH metabolites (total hydroxyphenanthrenes and 1-hydroxypyrene) were measured in urine samples from 184 adolescent males, in whom anthropometrics, heart rate, pulse pressure, brachial artery distensibility and blood pressure were measured. Descriptive, bivariate and multivariable analyses were performed to assess relationships of school location and urinary PAH metabolites with cardiovascular measures. RESULTS Total suspended matter was significantly higher (444 ± 143 μg/m(3)) at the school near the refinery compared to a school located near a ring road (395 ± 65 μg/m(3)) and a school located away from vehicle traffic (232 ± 137 μg/m(3)), as were PAHs. Systolic (0.47 S D units, p = 0.006) and diastolic (0.53 SD units, p < 0.001) BP Z-scores were highest at the school near the refinery, with a 4.36-fold increase in prehypertension (p = 0.001), controlling for confounders. No differences in pulse pressure, BAD and heart rate were noted in relationship to school location. Urinary total hydroxyphenanthrenes and 1-hydroxypyrene were not associated with cardiovascular outcomes. CONCLUSIONS Proximity to an oil refinery in Saudi Arabia is associated with prehypertension and increases in PAH and particulate matter exposures. Further study including insulin resistance measurements, better control for confounding, and longitudinal measurement is indicated.

Urinary metabolites of polycyclic aromatic hydrocarbons in Saudi Arabian schoolchildren in relation to sources of exposure

Polycyclic aromatic hydrocarbons contain a number of known carcinogenic compounds, and urinary biomarkers have been widely used as a measure of exposure but quantitative relationships with exposure variables have proved elusive. This study aimed to quantify the relationship between exposures to phenanthrene and pyrene from atmospheric and dietary sources with the excretion of 1-hydroxypyrene and hydroxyphenanthrenes in urine as biomarkers of exposure. The study population consisted of 204 male schoolchildren attending three schools in different parts of Jeddah, Saudi Arabia who provided urine samples on each of three consecutive days. Outdoor air measurements of polycyclic aromatic hydrocarbons were made at the schools and the children provided information on diet, exposure to environmental tobacco smoke and incense, and various lifestyle factors through a questionnaire. Mixed models with random effects for subjects nested within site were fitted in order to examine the relationship between exposure variables and urinary PAH metabolites. A unit increase (1 ng m(-3)) in ambient pyrene (particulate plus gaseous phase) was associated with a 3.5% (95% CI: 1.01%, 5.13%) increase in urinary 1-hydroxypyrene concentration. A unit increase in ambient phenanthrene was associated with a 1.01% (95% CI: 0.03%, 2.02%) increase in total hydroxyphenanthrene concentrations. Consumption of chargrilled food increased the 1-hydroxypyrene and hydroxyphenanthrene concentrations by 24% (95% CI: 11%, 37%) and 17% (95% CI: 8%, 26%) respectively. We did not find evidence of association for environmental tobacco smoke exposure or incense burning. It is concluded that both respiratory exposure and consumption of chargrilled food are considerable sources of PAH exposure in this population as reflected by concentrations of urinary biomarkers.

Insights into the Formation and Evolution of Individual Compounds in the Particulate Phase during Aromatic Photo-Oxidation

Secondary organic aerosol (SOA) is well-known to have adverse effects on air quality and human health. However, the dynamic mechanisms occurring during SOA formation and evolution are poorly understood. The time-resolved SOA composition formed during the photo-oxidation of three aromatic compounds, methyl chavicol, toluene and 4-methyl catechol, were investigated at the European Photoreactor. SOA was collected using a particle into liquid sampler and analyzed offline using state-of-the-art mass spectrometry to produce temporal profiles of individual photo-oxidation products. In the photo-oxidation of methyl chavicol, 70 individual compounds were characterized and three distinctive temporal profile shapes were observed. The calculated mass fraction (Ci,aer/COA) of the individual SOA compounds showed either a linear trend (increasing/decreasing) or exponential decay with time. Substituted nitrophenols showed an exponential decay, with the nitro-group on the aromatic ring found to control the formation and loss of these species in the aerosol phase. Nitrophenols from both methyl chavicol and toluene photo-oxidation experiments showed a strong relationship with the NO2/NO (ppbv/ppbv) ratio and were observed during initial SOA growth. The location of the nitrophenol aromatic substitutions was found to be critically important, with the nitrophenol in the photo-oxidation of 4-methyl catechol not partitioning into the aerosol phase until irradiation had stopped; highlighting the importance of studying SOA formation and evolution at a molecular level.

Production of the Atmospheric Oxidant Radicals OH and HO2 from the Ozonolysis of Alkenes

The reactions of ozone with alkenes are of importance within atmospheric chemistry as a non-photolytic source of the oxidant radicals OH, HO2 and RO2. While OH yields are relatively well constrained, few data exist for production of HO2 or RO2. We report direct measurements of total radical yields from a range of small (C2–C5) alkenes, using LIF and PERCA techniques within large simulation chamber experiments. OH yields are found to be consistent with established understanding, while HO2 yields are substantially smaller than previous measurements suggest, but in good agreement with those assumed within current atmospheric chemical mechanisms.

The influence of particle composition uponthe evolution of urban ultrafine dieselparticles on the neighbourhood scale

Abstract. A recent study demonstrated that diesel particles in urban air undergo evaporative shrinkage when advected to a cleaner atmosphere (Harrison et al., 2016). We explore, in a structured and systematic way, the sensitivity of nucleation-mode diesel particles to changes in particle composition and saturation vapour pressure. We use a multi-component aerosol microphysics model based on surrogate molecule (C 16 -C 32 n-alkane) volatilities. For standard atmospheric conditions (298 K, 1013.25 hPa), and over timescales (ca. 100 s) relevant for dispersion on the neighbourhood scale (up to 1 km), the choice of a particular vapour pressure dataset changes the range of compounds that are appreciably volatile by 2–6 carbon numbers. The nucleation-mode peak diameter, after 100 s of model runtime, is sensitive to the vapour pressure parameterisations for particles with compositions centred on surrogate molecules between C 22 H 46 and C 24 H 50 . The vapour pressures of components in this range are therefore critical for the modelling of nucleation-mode aerosol dynamics on the neighbourhood scale and need to be better constrained. Laboratory studies have shown this carbon number fraction to derive predominantly from engine lubricating oil. The accuracy of vapour pressure data for other (more and less volatile) components from laboratory experiments, is less critical. The influence of a core of involatile material is also considered. The new findings of this study may also be used to identify the Semi-Volatile Organic Compound (SVOC) compositions that play dominating roles in the evaporative shrinkage of the nucleation mode observed in field measurements (e.g. Dall’Osto et al., 2011). As well as reconciling model and observations, identifying the most significant vapour pressure regime for nucleation-mode dynamics offers a way to improve the computing efficiency of urban aerosol models by adopting simplified schemes for those less important components: e.g., an equilibrium scheme for low-carbon-number components and a linear scheme for high-carbon-number components.