Characterization and source apportionment of single particles from metalworking activities.

Industrial metalworking facilities emit a variety of air toxics including volatile organic compounds, polycyclic aromatic hydrocarbons (PAHs) and heavy metals. In order to investigate these emissions, a 1-month multi-instrument field campaign was undertaken at an industrial site in Grande-Synthe, Dunkirk (France), in May and June 2012. One of the main objectives of the study was to provide new information on the chemical composition of particulate matter with aerodynamic diameters smaller than 2.5 µm (PM2.5) in the vicinity of metalworking facilities. An aerosol time-of-flight mass spectrometer (ATOFMS) was deployed to provide size-resolved chemical mixing state measurements of ambient single particles at high temporal resolution. This mixing state information was then used to apportion PM2.5 to local metalworking facilities influencing the receptor site. Periods when the site was influenced by metalworking sources were characterised by a pronounced increase in particles containing toxic metals (manganese, iron, lead) and polycyclic aromatic hydrocarbons (PAHs) with a variety of chemical mixing states. The association of specific particle classes with a nearby ferromanganese alloy manufacturing plant was confirmed through comparison with previous analysis of raw materials (ores) and chimney filter particle samples collected at the facility. Particles associated with emissions from a nearby steelworks were also identified. The contribution of local metalworking activities to PM2.5 at the receptor site for the period when the ATOFMS was deployed ranged from 1 to 65% with an average contribution of 17%, while the remaining mass was attributed to other local and regional sources. These findings demonstrate the impact of metalworking facilities on air quality downwind and provide useful single particle signatures for future source apportionment studies in communities impacted by metalworking emissions.

Investigation on the near-field evolution of industrial plumes from metalworking activities.

In a context where a significant fraction of the population lives near industrial areas, the main objectives of this study are to provide (a) new data on PM2.5 chemical compositions, heavy-metal concentrations and trace gases released by metalworking activities and (b) new information on the near-field evolution (up to about a thousand meters) of such industrial plumes in terms of particle chemical composition and size distribution. For that purpose, a one-month field campaign was performed in an industrial area near the city of Dunkirk (Northern France), combining measurements of atmospheric dynamics and physico-chemical characterization of air masses. Comparisons between several elemental ratios (mainly Mn/Fe), particle size distributions and volatile organic compound (VOC) concentrations at the stacks and at a near-field site suggest that plumes of a ferromanganese alloy plant were quickly mixed with pollutants emitted by other sources (mainly other industries, possibly traffic and sea spray), in particular a neighboring steelworks, before reaching the sampling site. This led to the emergence of secondary particles related to condensation and/or aggregation phenomena inside the plumes. Metalworking emissions were also identified as a source of new particle formation, formed through the emission of gaseous precursors and their fast transformation and condensation, over a timescale of minutes before reaching the near-field site 800 m downwind. Ultrafine particles emitted at the stacks also quickly agglomerated to form larger particles before reaching the near-field site. These results show that, even over short distances, the chemical composition and size distribution of metalworking plumes may evolve rapidly and the characteristics of particles at the boundary of an industrial area (especially in contiguous urban areas) may differ from those emitted directly at the stacks.

Source characterization of urban particles from meat smoking activities in Chongqing, China using single particle aerosol mass spectrometry.

A Single Particle Aerosol Mass Spectrometer (SPAMS) was deployed in the urban area of Chongqing to characterize the particles present during a severe particulate pollution event that occurred in winter 2014-2015. The measurements were made at a time when residents engaged in traditional outdoor meat smoking activities to preserve meat before the Chinese Spring Festival. The measurement period was predominantly characterized by stagnant weather conditions, highly elevated levels of PM2.5, and low visibility. Eleven major single particle types were identified, with over 92.5% of the particles attributed to biomass burning emissions. Most of the particle types showed appreciable signs of aging in the stagnant air conditions. To simulate the meat smoking activities, a series of controlled smoldering experiments was conducted using freshly cut pine and cypress branches, both with and without wood logs. SPAMS data obtained from these experiments revealed a number of biomass burning particle types, including an elemental and organic carbon (ECOC) type that proved to be the most suitable marker for meat smoking activities. The traditional activity of making preserved meat in southwestern China is shown here to be a major source of particulate pollution. Improved measures to reduce emissions from the smoking of meat should be introduced to improve air quality in regions where smoking meat activity prevails.

Molecular composition of organic aerosols at urban background and road tunnel sites using ultra-high resolution mass spectrometry.

Organic aerosol composition in the urban atmosphere is highly complex and strongly influenced by vehicular emissions which vary according to the make-up of the vehicle fleet. Normalized test measurements do not necessarily reflect real-world emission profiles and road tunnels are therefore ideal locations to characterise realistic traffic particle emissions with minimal interference from other particle sources and from atmospheric aging processes affecting their composition. In the current study, the composition of fine particles (diameter ≤2.5 µm) at an urban background site (Elms Road Observatory Site) and a road tunnel (Queensway) in Birmingham, UK, were analysed with direct infusion, nano-electrospray ionisation ultrahigh resolution mass spectrometry (UHRMS). The overall particle composition at these two sites is compared with an industrial harbour site in Cork, Ireland, with special emphasis on oxidised mono-aromatics, polycyclic aromatic hydrocarbons (PAHs) and nitro-aromatics. Different classification criteria, such as double bond equivalents, aromaticity index and aromaticity equivalent are used and compared to assess the fraction of aromatic components in the approximately one thousand oxidized organic compounds at the different sampling locations.

Simulation of particle diversity and mixing state over Greater Paris: a model-measurement inter-comparison.

Air quality models are used to simulate and forecast pollutant concentrations, from continental scales to regional and urban scales. These models usually assume that particles are internally mixed, i.e. particles of the same size have the same chemical composition, which may vary in space and time. Although this assumption may be realistic for continental-scale simulations, where particles originating from different sources have undergone sufficient mixing to achieve a common chemical composition for a given model grid cell and time, it may not be valid for urban-scale simulations, where particles from different sources interact on shorter time scales. To investigate the role of the mixing state assumption on the formation of particles, a size-composition resolved aerosol model (SCRAM) was developed and coupled to the Polyphemus air quality platform. Two simulations, one with the internal mixing hypothesis and another with the external mixing hypothesis, have been carried out for the period 15 January to 11 February 2010, when the MEGAPOLI winter field measurement campaign took place in Paris. The simulated bulk concentrations of chemical species and the concentrations of individual particle classes are compared with the observations of Healy et al. (Atmos. Chem. Phys., 2013, 13, 9479-9496) for the same period. The single particle diversity and the mixing-state index are computed based on the approach developed by Riemer et al. (Atmos. Chem. Phys., 2013, 13, 11423-11439), and they are compared to the measurement-based analyses of Healy et al. (Atmos. Chem. Phys., 2014, 14, 6289-6299). The average value of the single particle diversity, which represents the average number of species within each particle, is consistent between simulation and measurement (2.91 and 2.79 respectively). Furthermore, the average value of the mixing-state index is also well represented in the simulation (69% against 59% from the measurements). The spatial distribution of the mixing-state index shows that the particles are not mixed in urban areas, while they are well mixed in rural areas. This indicates that the assumption of internal mixing traditionally used in transport chemistry models is well suited to rural areas, but this assumption is less realistic for urban areas close to emission sources.

Gas- and Particle-Phase Products from the Chlorine-Initiated Oxidation of Polycyclic Aromatic Hydrocarbons.

The chlorine atom (Cl)-initiated oxidation of three polycyclic aromatic hydrocarbons (PAHs; namely, naphthalene, acenaphthylene, and acenaphthene) was investigated. Experiments were performed in an atmospheric simulation chamber using a proton transfer reaction time-of-flight mass spectrometer (TOF-MS) and an aerosol TOF-MS to characterize the oxidation products in the gas and particle phases, respectively. The major products identified from the reaction of Cl atoms with naphthalene were phthalic anhydride and chloronaphthalene, indicating that H atom abstraction and Cl addition reaction pathways are both important. Acenaphthenone was the principal product arising from reaction of Cl with acenaphthene, while 1,8-naphthalic anhydride, acenaphthenone, acenaphthenequinone, and chloroacenaphthenone were all identified as products of acenaphthylene oxidation, confirming that the cylcopenta-fused ring controls the reactivity of these PAHs toward Cl atoms. Possible reaction mechanisms are proposed for the formation of these products, and favored pathways have been suggested. Large yields of secondary organic aerosol (SOA) were also observed in all experiments, and the major products were found to undergo significant partitioning to the particle-phase. This work suggests that Cl-initiated oxidation could play an important role in SOA formation from PAHs under specific atmospheric conditions where the Cl atom concentration is high, such as the marine boundary layer.

Single-particle speciation of alkylamines in ambient aerosol at five European sites.

Alkylamines are associated with both natural and anthropogenic sources and have been detected in ambient aerosol in a variety of environments. However, little is known about the ubiquity or relative abundance of these species in Europe. In this work, ambient single-particle mass spectra collected at five sampling sites across Europe have been analysed for their alkylamine content. The aerosol time-of-flight mass spectrometer (ATOFMS) data used were collected in Ireland (Cork), France (Paris, Dunkirk and Corsica) and Switzerland (Zurich) between 2008 and 2013. Each dataset was queried for mass spectral marker ions associated with the following ambient alkylamines: dimethylamine (DMA), trimethylamine (TMA), diethylamine (DEA), triethylamine (TEA), dipropylamine (DPA) and tripropylamine (TPA). The fraction of ambient particles that contained detectable alkylamines ranged from 1 to 17 % depending on location, with the highest fractions observed in Paris and Zurich in the winter months. The lowest fractions were observed at coastal sites, where the influence of animal husbandry-related alkylamine emissions is also expected to be lowest. TMA was the most ubiquitous particle phase alkylamine detected and was observed at all locations. Alkylamines were found to be internally mixed with both sulphate and nitrate for each dataset, suggesting that aminium salt formation may be important at all sites investigated. Interestingly, in Corsica, all alkylamine particles detected were also found to be internally mixed with methanesulphonic acid (MSA), indicating that aminium methanesulphonate salts may represent a component of marine ambient aerosol in the summer months. Internal mixing of alkylamines with sea salt was not observed, however. Alkylamine-containing particle composition was found to be reasonably homogeneous at each location, with the exception of the Corsica and Dunkirk sites, where two and four distinct mixing states were observed, respectively.

Apportionment of urban aerosol sources in Cork (Ireland) by synergistic measurement techniques.

The sources of ambient fine particulate matter (PM2.5) during wintertime at a background urban location in Cork city (Ireland) have been determined. Aerosol chemical analyses were performed by multiple techniques including on-line high resolution aerosol time-of-flight mass spectrometry (Aerodyne HR-ToF-AMS), on-line single particle aerosol time-of-flight mass spectrometry (TSI ATOFMS), on-line elemental carbon-organic carbon analysis (Sunset_EC-OC), and off-line gas chromatography/mass spectrometry and ion chromatography analysis of filter samples collected at 6-h resolution. Positive matrix factorization (PMF) has been carried out to better elucidate aerosol sources not clearly identified when analyzing results from individual aerosol techniques on their own. Two datasets have been considered: on-line measurements averaged over 2-h periods, and both on-line and off-line measurements averaged over 6-h periods. Five aerosol sources were identified by PMF in both datasets, with excellent agreement between the two solutions: (1) regional domestic solid fuel burning--"DSF_Regional," 24-27%; (2) local urban domestic solid fuel burning--"DSF_Urban," 22-23%; (3) road vehicle emissions--"Traffic," 15-20%; (4) secondary aerosols from regional anthropogenic sources--"SA_Regional" 9-13%; and (5) secondary aged/processed aerosols related to urban anthropogenic sources--"SA_Urban," 21-26%. The results indicate that, despite regulations for restricting the use of smoky fuels, solid fuel burning is the major source (46-50%) of PM2.5 in wintertime in Cork, and also likely other areas of Ireland. Whilst wood combustion is strongly associated with OC and EC, it was found that peat and coal combustion is linked mainly with OC and the aerosol from these latter sources appears to be more volatile than that produced by wood combustion. Ship emissions from the nearby port were found to be mixed with the SA_Regional factor. The PMF analysis allowed us to link the AMS cooking organic aerosol factor (AMS_PMF_COA) to oxidized organic aerosol, chloride and locally produced nitrate, indicating that AMS_PMF_COA cannot be attributed to primary cooking emissions only. Overall, there are clear benefits from factor analysis applied to results obtained from multiple techniques, which allows better association of aerosols with sources and atmospheric processes.

Kinetics of the gas-phase reactions of chlorine atoms with naphthalene, acenaphthene, and acenaphthylene.

Reactions of polycyclic aromatic hydrocarbons (PAHs) with chlorine atoms may occur in specific areas such as coastal regions and the marine boundary layer. In this work, rate constants for the gas-phase reactions of naphthalene, acenaphthene, and acenaphthylene with chlorine atoms have been measured using the relative rate technique. Experiments were performed at room temperature (293 ± 2 K) and atmospheric pressure in an atmospheric simulation chamber using a proton-transfer reaction mass spectrometer (PTR-MS) to monitor the concentrations of PAHs and the reference compounds (acetone, methanol, 1,3,5-trimethylbenzene, and isoprene) as a function of time. The rate constants obtained in this work were (in units of cm(3) molecule(-1) s(-1)) (4.22 ± 0.46) × 10(-12), (3.01 ± 0.82) × 10(-10), and (4.69 ± 0.82) × 10(-10) for naphthalene, acenaphthene, and acenaphthylene, respectively. These are the first measurements of the rate constants for gas-phase reactions of Cl atoms with acenaphthene and acenaphthylene. The rate constant determined in this study for the reaction of naphthalene with Cl atoms is not in agreement with the only other previously reported value in the literature. The results are used to assess the potential role of chlorine atom reactions in the atmospheric oxidation of PAHs.

Rapid formation of secondary organic aerosol from the photolysis of 1-nitronaphthalene: role of naphthoxy radical self-reaction.

The chemical composition of secondary organic aerosol (SOA) formed from the photolysis of 1-nitronaphthalene in a series of simulation chamber experiments has been investigated using an aerosol time-of-flight mass spectrometer (ATOFMS). The resulting SOA is characterized by the presence of a dimer (286 Da) proposed to be formed through the self-reaction of naphthoxy radicals along with the expected product nitronaphthol. The molecular formulas of the SOA products were confirmed by collecting filter samples and analyzing the extracts using ultrahigh resolution mass spectrometry. Further evidence for the radical self-reaction mechanism was obtained by photolyzing 1-nitronaphthalene in the presence of excess nitrobenzene, where it was shown that the resulting SOA contained a product consistent with the cross-reaction of phenoxy and naphthoxy radicals. The naphthoxy dimer was formed from the photolysis of 1-nitronaphthalene under a variety of different experimental conditions including the presence of excess butyl ether as an OH scavenger and the presence of ambient air and particles. However, formation of the dimer was suppressed when 1-nitronaphthalene was photolyzed in the presence of excess NO and nitronaphthol was instead found to be the dominant particle-phase product indicating that the yield of the dimer is dependent upon the concentration of pre-existing NO(x). The results of this work suggest that photolysis of 1-nitronaphthalene represents a previously unidentified pathway to SOA formation in the troposphere. Analogous mechanisms may also be important for other nitrated polycyclic aromatic hydrocarbons.

Near-ultraviolet absorption cross sections of nitrophenols and their potential influence on tropospheric oxidation capacity.

Nitrophenols and methylnitrophenols have been identified as photolytic precursors of nitrous acid, HONO, but their gas-phase absorption has not previously been reported. In this study, the absorption cross sections of 2-nitrophenol, 3-methyl-2-nitrophenol, and 4-methyl-2-nitrophenol were measured from 320 to 450 nm using incoherent broad-band cavity-enhanced absorption spectroscopy (IBBCEAS). The benzaldehyde absorption spectrum was measured to validate the approach and was in good agreement with literature spectra. The nitrophenol absorption cross sections are large (ca. 10(-17) cm(2) molecule(-1)) and blue-shifted about 20 nm compared to previously measured solution spectra. Besides forming HONO, nitrophenol absorption influences other photochemistry by reducing the available actinic flux. The magnitudes of both effects are evaluated as a function of solar zenith angle, and nitrophenol absorption is shown to lower the photolysis rates of O(3) and NO(2).

The atmospheric photolysis of o-tolualdehyde.

The photolysis of o-tolualdehyde by natural sunlight has been investigated at the large outdoor European Photoreactor (EUPHORE) in Valencia, Spain. The photolysis rate coefficient was measured directly under different solar flux levels, with values in the range j(o-tolualdehyde) = (1.62-2.15) × 10(-4) s(-1) observed, yielding an average value of j(o-tolualdehyde)/j(NO(2)) = (2.53 ± 0.25) × 10(-2). The estimated photolysis lifetime is 1-2 h, confirming that direct photolysis by sunlight is the major atmospheric degradation pathway for o-tolualdehyde. Published UV absorption cross-section data were used to derive an effective quantum yield (290-400 nm) close to unity, within experimental error. Possible reaction pathways for the formation of the major photolysis products, benzocyclobutenol (tentatively identified) and o-phthalaldehyde, are proposed. Appreciable yields (5-13%) of secondary organic aerosol (SOA) were observed at EUPHORE and also during supplementary experiments performed in an indoor chamber using an artificial light source. Off-line analysis by gas chromatography-mass spectrometry allowed identification of o-phthalaldehyde, phthalide, phthalic anhydride, o-toluic acid, and phthalaldehydic acid in the particle phase.

The use of polar organic compounds to estimate the contribution of domestic solid fuel combustion and biogenic sources to ambient levels of organic carbon and PM2.5 in Cork Harbour, Ireland.

PM(2.5) samples collected at Cork Harbour, Ireland during summer, autumn, late autumn and winter, 2008-2009 were analyzed for polar organic compounds that are useful markers for aerosol source characterization. The determined compounds include tracers for biomass burning primary particles, fungal spores, markers for secondary organic aerosol (SOA) from isoprene, α-/ß-pinene, and d-limonene. Seasonal and temporal variations and other characteristic features of the detected tracers are discussed in terms of aerosol sources and processes. The biogenic species were detected only during the summer period where the contributions of isoprene SOA and fungal spores to the PM(2.5) organic carbon (OC) were estimated to be 1.6% and 1% respectively. The biomass burning markers, and in particular levoglucosan, were present in all samples and attributed to the combustion of cellulose-containing fuels including wood, peat, bituminous and smokeless coal. The contribution of domestic solid fuel (DSF) burning to the measured OC mass concentration was estimated at 10.8, 50, 66.4 and 74.9% for summer, autumn, late autumn and winter periods, respectively, based on factors derived from a series of burning experiments on locally available fuels. Application of an alternative approach, namely principal component analysis-multiple linear regression (PCA-MLR), to the measured concentrations of the polar organic marker compounds used in conjunction with real-time air quality data provided similar trends and estimates for DSF combustion during all seasons except summer. This study clearly demonstrates that, despite the ban on the sale of bituminous coal in Cork and other large urban areas in Ireland, DSF combustion is still the major source of OC during autumn and winter periods and also makes a significant contribution to PM(2.5) levels. The developed marker approach for estimating the contribution of DSF combustion to ambient OC concentrations can, in principle, also be applied to other locations.

Structure-activity relationship (SAR) for the prediction of gas-phase ozonolysis rate coefficients: an extension towards heteroatomic unsaturated species.

Heteroatomic unsaturated volatile organic compounds (HUVOCs) are common trace components of the atmosphere, yet their diverse chemical behaviour presents difficulties for predicting their oxidation kinetics using structure-activity relationships (SARs). An existing SAR is adapted to help meet this challenge, enabling the prediction of ozonolysis rates with unprecedented accuracy. The new SAR index, x(H), correlates strongly with available literature measurements of ozonolysis rate coefficients (R(2) = 0.87), a database representing 110 species. It was found that capturing the inductive effect rather than the steric effect is of primary importance in predicting the reactivity of these species, which is to be anticipated since HUVOCs can possess a variety of functional groups with a range of electron-withdrawing and donating tendencies. New experimental measurements of ozonolysis rate coefficients were conducted for 1-penten-3-ol, 3-methyl; ethene, 1,1-dimethoxy; E-2-pentenoic acid; E-1,2-dichloroethene; Z-1,2-dichloroethene; trichloroethene; tetrachloroethene; 1-butene, 3-chloro and 2-chloropropene, and were determined to be 5.15 × 10(-18), 4.82 × 10(-16), 3.07 × 10(-18), 8.05 × 10(-20), 4.88 × 10(-21), 6.04 × 10(-22), 1.56 × 10(-24), 2.26 × 10(-18) and 1.13 × 10(-19) cm(3) molecule(-1) s(-1), respectively. The index of the inductive effect, i(H), is compared with other indices of the electron-withdrawing capacity of a substitution, notably the Taft σ* constants and the rate of reaction of a given species with the hydroxyl radical, both of which are expected to be unaffected by steric factors. i(H) correlates strongly in both cases and suggests a universal response by olefinic species towards electrophilic addition.

Product study of the OH radical and Cl atom initiated oxidation of 1,3-dioxane.

The products of the hydroxyl (OH) radical and chlorine (Cl) atom initiated oxidation of 1,3-dioxane are determined under various reaction conditions in a 50 L teflon reaction chamber using FTIR spectroscopy for analysis. The major products detected in all experiments are (2-oxoethoxy)methyl formate, formic acid and methylene glycol diformate with average molar yields of 0.50±0.05, 0.41±0.02 and 0.03±0.01 respectively for the OH initiated oxidation in the presence of NO(x). The yields of these products do not vary significantly with O(2) partial pressure or oxidising agent (OH or Cl). However, the yield of formic acid decreased by at least a factor of two in the absence of NO(x). The results of these experiments are used to elucidate a simplified gas-phase atmospheric degradation scheme for 1,3-dioxane and also provide valuable information on the atmospheric fate of the cyclic and linear alkoxy radicals produced in these and similar reactions. The available experimental data suggests that the relative importance of the competing pathways (reaction with O(2) and ring opening by C-C or C-O bond fission) is a strong function of the ring strain in the cycloalkoxy radicals.

Rate coefficients for the gas-phase reaction of hydroxyl radicals with 2-methoxyphenol (guaiacol) and related compounds.

2-Methoxyphenol (guaiacol) and its derivatives are potential marker compounds for wood smoke emissions in the atmosphere. To investigate the atmospheric reactivity of this type of compounds, rate coefficients for their reactions with hydroxyl (OH) radicals have been determined at 294 ± 2 K and 1 atm using the relative rate method with gas chromatography for chemical analysis. The rate coefficients (in units of cm³ molecule⁻¹ s⁻¹) are: 2-methoxyphenol, (7.53 ± 0.41) × 10⁻¹¹; 3-methoxyphenol, (9.80 ± 0.46) × 10⁻¹¹; 4-methoxyphenol, (9.50 ± 0.55) × 10⁻¹¹; 2-methoxy-4-methylphenol, (9.45 ± 0.59) × 10⁻¹¹; and methoxybenzene, (2.20 ± 0.15) × 10⁻¹¹. The estimated atmospheric lifetime for 2-methoxyphenol is ~2 h, indicating that it is too reactive to be used as a tracer for wood smoke emissions. The reactivity of the methoxyphenols is compared with other substituted aromatics and interpreted in relation to the type, number, and positions of the different substituents on the aromatic ring. The atmospheric implications of the reactions are also discussed.

Porous silica spheres as indoor air pollutant scavengers.

Porous silica spheres were investigated for their effectiveness in removing typical indoor air pollutants, such as aromatic and carbonyl-containing volatile organic compounds (VOCs), and compared to the commercially available polymer styrene-divinylbenzene (XAD-4). The silica spheres and the XAD-4 resin were coated on denuder sampling devices and their adsorption efficiencies for VOCs evaluated using an indoor air simulation chamber. Real indoor sampling was also undertaken to evaluate the affinity of the silica adsorbents for a variety of indoor VOCs. The silica sphere adsorbents were found to have a high affinity for polar carbonyls and found to be more efficient than the XAD-4 resin at adsorbing carbonyls in an indoor environment.

Effect of relative humidity on gas/particle partitioning and aerosol mass yield in the photooxidation of p-xylene.

The formation of secondary organic aerosol and gas/particle partitioning of carbonyl products from the photooxidation of p-xylene has been investigated as a function of relative humidity. Experiments were performed in an atmospheric simulation chamber at atmospheric pressure and ambient temperature in the presence of NOx. Aerosol yields increased by a factor of approximately two over the relative humidity range 5-75% and were found to correlate with initial water vapor concentration and hydroxyl radical (OH) concentration. The results indicate that an increase in relative humidity results in higher levels of HONO formation in the chamber which leads to increased OH concentration, a faster p-xylene decay rate, and higher aerosol mass yields. A recently developed denuder-filter sampling technique was used to investigate the gas/ particle partitioning behavior of the carbonyl photooxidation products. The identified products accounted for up to 18% of the aerosol mass formed. Dicarbonyls with at least one aldehyde functionality (e.g., glyoxal and methylglyoxal) exhibited gas/ particle partitioning coefficients several orders of magnitude higher than expected from absorptive partitioning theory, suggesting that reactive uptake and particle phase reactions are important processes for aerosol formation from these species. Experimental gas/particle partitioning coefficients were also found to be dependent on relative humidity, with every dicarbonyl exhibiting much lower values when the relative humidity was increased from 50% to 75%.

Structure-activity relationship (SAR) for the gas-phase ozonolysis of aliphatic alkenes and dialkenes.

The configuration of alkyl substituents about carbon-carbon unsaturated bonds exerts a controlling influence on the rate of the ozonolysis reaction. Alkyl substituents can increase (via the inductive effect) and decrease (via the steric effect) the activity of unsaturated bonds, and an accurate description of this information ought to correlate with the ozonolysis rate coefficient. A strong linear relationship is observed (R2 = 0.94), providing the basis of our SAR method. SAR estimates were tested against literature measurements of ozonolysis rate coefficients for 48 aliphatic alkenes and dialkenes, and were found to be accurate to within a factor of 2.3 of the measured value for the entire dataset. This represents a significant improvement over methods reported in the literature, where quoted predictions are at best accurate to within a factor of 6.5. Rates of gas-phase ozonolysis of alkenes and dialkenes can now be predicted with unprecedented accuracy using a simple SAR. The SAR was then validated against new experimental data. Absolute rate coefficients for the gas-phase reaction of ozone with a series of alkenes were determined in a simulation chamber at 295 +/- 2 K and atmospheric pressure by monitoring the loss of ozone in the presence of excess alkene. The rate coefficients (in units of 1 x 10(-18) cm3 molecule(-1) s(-1)) are: 5.12 +/- 0.93 for 1-pentene, 2,3-dimethyl; 406 +/- 49 for 2-pentene, 2-methyl; 151 +/- 5 for (E)-2-hexene, 14.5 +/- 1.0 for 1,5-hexadiene and 20.7 +/- 3.1 for 1,5-hexadiene, 2-methyl. There is good agreement between the experimental and predicted values and the adjustable parameters of the SAR are shown to be insensitive to the inclusion of the new data. The use of the SAR in atmospheric chemical modelling is investigated. Ozonolysis and OH radical rate coefficients are estimated for each alkene and dialkene present in the MCM v3.1. The effects of error within predicted rate coefficients upon modelled concentrations of a number of key species, including O3, OH, HO2, NO and NO2 were rather small and is not in itself a major cause of uncertainty in modelled concentrations.

Characterization of polar compounds and oligomers in secondary organic aerosol using liquid chromatography coupled to mass spectrometry.

A generic method has been developed for the analysis of polar compounds and oligomers in secondary organic aerosol (SOA) formed during atmospheric simulation chamber experiments. The technique has been successfully applied to SOA formed in a variety of systems, ranging from ozonolysis of biogenic volatile organic compounds to aromatic photooxidation. An example application of the method is described for the SOA produced from the reaction of ozone with cis-3-hexenyl acetate, an important biogenic precursor. A range of solvents were tested as extraction media, and water was found to yield the highest recovery. Extracts were analyzed using reversed-phase liquid chromatography coupled to ion trap mass spectrometry. In order to determine correct molecular weight assignments and increase sensitivity for less polar species, a series of low-concentration mobile-phase additives were used (NaCl, LiBr, NH4OH). Lithium bromide produced better fragmentation patterns, with more structural information than in the other cases with no reduction in sensitivity. The main reaction products identified in the particle-phase were 3-acetoxypropanal, 3-acetoxypropanoic acid, and 3-acetoxypropane peroxoic acid and a series of dimers and trimers up to 500 Da. Structural identification of oligomers indicates the presence of linear polyesters possibly formed via esterfication reactions or decomposition of peroxyhemiacetals.