Recent evidence has suggested that heterogeneous chemistry of oxygenated hydrocarbons, primarily carbonyls, plays a role in the formation of secondary organic aerosol (SOA); however, evidence is emerging that direct uptake of alkenes on acidic aerosols does occur and can contribute to SOA formation. In the present study, significant uptake of monoterpenes, oxygenated monoterpenes and sesquiterpenes to acidic sulfate aerosols is found under various conditions in a reaction chamber. Proton transfer mass spectrometry is used to quantify the organic gases, while an aerosol mass spectrometer is used to quantify the organic mass uptake and obtain structural information for heterogeneous products. Aerosol mass spectra are consistent with several mechanisms including acid catalyzed olefin hydration, cationic polymerization and organic ether formation, while measurable decreases in the sulfate mass on a per particle basis suggest that the formation of organosulfate compounds is also likely. A portion of the heterogeneous reactions appears to be reversible, consistent with reversible olefin hydration reactions. A slow increase in the organic mass after a fast initial uptake is attributed to irreversible reactions, consistent with polymerization and organosulfate formation. Uptake coefficients (gamma) were estimated for a fast initial uptake governed by the mass accommodation coefficient (alpha) and ranged from 1 x 10(-6)-2.5 x 10(-2). Uptake coefficients for a subsequent slower reactive uptake ranged from 1 x 10(-7)-1 x 10(-4). These processes may potentially lead to a considerable amount of SOA from the various biogenic hydrocarbons under acidic conditions, which can be highly significant for freshly nucleated aerosols, particularly given the large array of atmospheric olefins.

[1] Size-segregated atmospheric aerosols (11 stages separating particles from <0.04 to >14.2 mu m) collected in the Arctic during the polar sunrise at Alert were analyzed for aerosol mass, dicarboxylic acids, and major inorganic ions. Oxalic, malonic, succinic, and glutaric acids were detected in all size ranges, with oxalic acid being dominant. Their concentrations maximized in the accumulation mode either at 0.24-0.40 or 0.40-0.8 mu m aerodynamic diameters, suggesting that diacids were mainly formed by gas-to-particle conversion via photochemical oxidation of nonmethane hydrocarbons and oxygenated organics originated from continental pollution sources. The relative abundances of oxalic acid were higher in the 0.24- to 0.4-mu m size particles (73-78%) than in supermicrometer particles (40-60%), indicating that oxalic acid is produced by gas phase oxidation of precursors followed by accumulation on preexisting particles. Mass size distributions of NH4+ and SO42- peaked in the accumulation mode similar to those of small diacids. The sea-salt enrichment factor of K+ (biomass burning tracer) relative to Na+ maximized in 0.1- to 0.8-mu m sizes, whereas those of Mg2+ and Ca2+ (dust tracers) in 0.4- to 7.8-mu m particles. Maximized chlorine loss and bromine enrichment were found at 0.4-0.8 and 0.24-0.4 mu m sizes, respectively. Concentrations of Br-, which typically showed a submicrometer maximum, increased significantly during an O-3 depletion event having a shift of size distribution to a supermicrometer mode. During this event, oxalic acid concentration relative to succinic acid increased in submicrometer mode (0.24-0.4 mu m), adding to a growing body of evidence supporting the hypothesis that halogen chemistry is important in the production and loss of oxalic acid in the arctic atmosphere.

The direct polymerization of isoprene and alpha-pinene on acidic sulfate aerosols has been studied in a reaction chamber utilizing aerosol mass spectrometry. Results indicated that both species can be directly taken up into acidic aerosols to a significant extent, forming polymers that contain at least 4 isoprene or 2 alpha-pinene repeating units. Aerosol mass spectra indicate that double bonds in the polymers hydrate under acid catalysis, leading to partial oxygenation of the polymers. This reactive uptake depends highly upon relative humidity and particle acidity. This process is rapid and reaches equilibrium in less than 50 minutes, with effective partition coefficients (K-p,K-eff) between 1.2-14.1 x 10(-6) m(3) mu g(-1), from which it is estimated <0.5-5 ng m(-3) of polymers may be present from both species in acidic aerosols. The formation of biogenic polymers is an important mechanism for incorporating hydrophobic, unsaturated species into polar aerosols and enhanced SOA formation.

Motor vehicle (MV) emissions and ambient particle concentrations under a variety of situations were studied in Toronto and Vancouver, Canada. Petroleum biomarkers (i.e., hopanes and steranes) were used to determine the fraction of fine particle organic carbon (OC) attributed to primary particles in MV exhaust. Source profiles obtained from a tunnel and from direct tailpipe emissions were applied to ambient measurements at locations ranging from rush hour traffic to a regional background site. The greatest amount of MV OC, 4.0 mu gC m(-3) out of 9.1 mu gC m(-3) or 43%, was observed 75 m south of a commuter highway during a period that included morning rush hour. Monthly estimates of MV-OC were determined for a downtown Toronto monitoring site for 2 years. Total OC concentrations were greater in the summer, due to secondary OC, but the amount of MV-OC did not exhibit a strong seasonal pattern. However, on a per cent basis, MV contributions from primary OC emissions were greatest in the winter (15-20%) and smallest in the summer (10-15%) with a two-year average of 14% of the OC or about 5% of the PM2.5. (c) 2006 Elsevier Ltd. All rights reserved.

Some nitrogen-containing organic compounds (NOCs) in PM2.5 aerosols in forest, tunnel, urban, rural, and mixed forest/urban areas in the Lower Fraser Valley (LFV), British Columbia, Canada, were measured to assess their chemical characteristics, temporal and spatial distributions, and origins. The levels of is an element of-caprolactam, isoindole-1,3-dione, benzothiazolone, and N-butyl-benzensulfonamide showed significant differences among the sites, with the highest level at the mixed forest/urban site, indicating that aerosols at this site were impacted by chemical manufacturing activities. N, N-diethyl-m-toluamide (deet) was detected at all locations but was highest in the forest area, demonstrating a widespread usage as an insect repellent in the LFV and at camps at the forest site. Alkyl amides, tracers from wood burning and cooking, ranging from C-6 to C-20 including two unsaturated amides, hexadecenamide, and 9-octadecenamide, were detected at all sites. Three patterns of carbon number distributions of alkyl amides varied with location and time, and were mainly impacted by biomass burning or cooking compared to levoglucosan and cholesterol in the LFV. Ratio of oleamide to stearamide (C-18:1/C-18:0) was discussed as a potential indicator for determining `' age `' or transport range of biomass combustion plumes.

Sources of sulphate and oxidation pathways important to the formation of PM 2.5 during the Pacific 2001 field campaign in Canada, were studied in diurnal samples from an urban coastal and a rural inland site in the Fraser Valley, B.C. Ion and elemental characteristics of aerosols as well as the sulphur and oxygen isotopes in sulphate were compared and related using multiple linear regressions. Sources of sulphate in PM2.5 were distinguished at each site and included: 1. sulphate from DMS oxidation during the day-time in Vancouver that coincided with high sodium and oxalate concentrations inland, 2. well-mixed pollutant SO, from the Strait of Georgia that was oxidized to sulphate as it moved into the Vancouver area, 3. vehicle exhaust or a mixture of biomass burning and vehicle exhaust which has a larger fraction of primary sulphate than other sulphate sources and is possibly associated with Cu, and 4. isotopically light SO2 from a refinery in the US that was strongly associated with higher Zn concentrations in aerosols. Primary and secondary sulphate were quantified using an oxygen isotope apportionment model. On average 19-42% and 58-81% of the non-sea salt sulphate in PM2.5 was primary and secondary, respectively. More primary sulphate was present in Vancouver (0.21-0.47 mu gm(-3)) PM2.5 aerosols than at the site further inland (0.13-0.33 mu g m(-3)) but there was no difference between the amount of primary sulphate present during the day and night at each site. (c) 2006 Elsevier Ltd. All rights reserved.

The data management and archiving activities of the Pacific 2001 Air Quality Study were handled by the Pacific 2001 Data Centre which was run by the National Atmospheric Chemistry (NAtChem) Database and Analysis Facility of Environment Canada. To ensure that the Pacific 2001 Air Quality Study data were archived in a common way, the NARSTO Data Exchange Standard (DES) was used as the mandatory format for the data files, partially because it allowed for the inclusion of metadata within the data files and partially because it provided the necessary flexibility for handling the many measurement types used in the study. Described in detail in the paper, the DES is now readily available to the scientific community. After each DES data file was submitted to the Data Centre, a read-and-verify program was run to check its conformity to the DES and to detect incorrect and problematic data. The errors detected by the read-and-verify program were automatically documented and an error report was sent to the data originators for data correction and resubmission. Statistical summaries and data plots were created for all data files and subsequently sent to the data originators for review and further error detection. Of the 125 data files submitted to the Data Centre, only 5 were error-free upon first submission. A test of 17 randomly selected files determined that all but two required at least four iterations of the submission-error checking-resubmission cycle in order to produce final error-free files. It was therefore concluded that both data originators and data centres alike should assume that errors exist in all submitted data files until proven differently by a set of automated error-checking programs. It was also concluded that data visualization plots and statistical summaries are highly effective tools for detecting errors in data files. Metadata associated with the measurement data were documented in Quality Assurance Project Plans that were archived in the Data Centre with the DES data files. (c) 2006 Elsevier Ltd. All rights reserved.

The size distribution of aerosol chemical compositions was studied over a 2-week period in August 2001 at three locations (one urban and two semi-rural) in the Lower Fraser Valley (LFV) of British Columbia, Canada. The size distributions varied diurnally and were bimodal with a pronounced peak at about 0.3-0.55 mu m (accumulation mode) and SO42- 2-7 mu m (coarse mode). Generally, on an equivalent basis, the aerosol was dominated by SO42- (balanced by NH4+) in the accumulation mode range; SO42- often exhibited a diurnal variation that was primarily the result of fog deposition overnight and photochemical formation during the day. SO42- in the 3.1-6.2 mu m particles was mostly of marine origin. Most of the smaller particle SO42- was from anthropogenic sources and it is estimated that about 75% of the SO42- in the smaller particles were due to secondary processes. Oxalate, C2O42-, although at low concentrations (generally < 0.1 SO42- displayed a bi-modal size distribution, the accumulation mode being similar to SO42- and the coarse mode peaking at 4 1-1.8 mu m. Frequently at night, there was also a significant increase in NO3- in the accumulation mode that is attributed to the co-condensation of gas phase NH3 + HNO3 to form aerosol NO3- and the reactive uptake of N2O5 onto aerosols. In the coarse mode, the sea salt Cl- was frequently deficient relative to Na+ yet accompanied by a corresponding increase in NO3-; this was attributed to reaction of NO3- precursors (e.g., HNO3 and N2O5) with sea salt aerosol Cl-. (c) 2006 Elsevier Ltd. All rights reserved.

As part of the Pacific 2001 Air Quality Study in August 2001, aerosol samples were collected at three sites in the Lower Fraser Valley, BC twice daily over a 2-week period. In this paper, the results for two compounds in the samples, levoglucosan and dehydroabietic acid (DHAA), both tracers for biomass burning plumes, are presented. Concentrations of the compounds were generally low throughout the study. Average values for levoglucosan were 14.4, 14.7, and 26.0 ng m(-3) for the urban, mixed urban/forest, and rural sites respectively. Elevations in the concentrations during two periods indicate evidence of biomass burning. Satellite images from 15 to 18 August show the transport of smoke plumes from forest fires in northern Washington and southern British Columbia into the Lower Fraser Valley. The residues of the smoke plumes in the river valleys probably contributed to the elevated ground-level levoglucosan levels. Emission ratios from reported source studies, 4.5-10% for levoglucosan/organic carbon and 4% for DHAA/organic carbon, were used to estimate the contributions of biomass burning to aerosol organic carbon. Average contributions of biomass burning to organic carbon varied significantly throughout the region. Contributions were between 5 and 10%, 12 and 27% and 5 and 12% for the urban, rural and mixed urban/forest sites respectively, but contributions up to 75% were found. Crown Copyright (c) 2006 Published by Elsevier Ltd. All rights reserved.

The Pacific 2001 Air Quality Study was undertaken to characterize the physical and chemical properties of particulate matter in the Lower Fraser Valley (LFV) airshed of British Columbia. Aerosol transport in the LFV was observed to occur through several regional scale mechanisms, including seabreeze-landbreeze flows, upslope-downslope flows and north-south valley flows. Enhanced aerosol concentrations were observed in the eastern portion of the LFV, the north-south tributary valleys and the Gulf/San Juan islands convergence zone. Particle composition was found to vary according to proximity to emission sources relative to surface flows. Approximately half of the particle mass was comprised of organic carbon, with the rest being inorganic species. Sodium nitrate was an important component of the coarse particle fraction, while ammonium sulphate was concentrated in the fine fraction. A broad suite of organic substances were detected, and both anthropogenic and biogenic sources were found to contribute to secondary organic aerosol fort-nation. Particle formation and growth was observed to occur via nucleation, condensation and coagulation. In general, the western part of the LFV airshed was dominated by combustion-related compounds, sea salt chemistry and organics (both anthropogenic and biogenic), while the eastern part of the airshed was dominated by NH3 chemistry and biogenic organics. The extent of processing of air masses was found to increase in a west to east direction, although the occasional accumulation of photochemically aged pollutants in the Gulf/San Juan islands convergence zone was observed to reverse this gradient. NH3 was found not to be a limiting species in the formation of fine mode inorganic aerosols, suggesting that NH3 emission reductions would have to be substantial in order to confer a significant improvement in PM and visibility. This coupled with the non-linear relationships between NOx, VOC and secondary particulates, suggests that PM and visibility improvement in the LFV would most likely require concurrent reductions in NH3, SOx, NOx and VOC emissions. Crown Copyright (c) 2006 Published by Elsevier Ltd. All rights reserved.

Ninety daytime/nighttime PM2.5 aerosol samples were collected at 5 sites in forest, tunnel, urban, rural, and mixed forest/urban areas in the Lower Fraser Valley during the Pacific 2001 Air Quality Study. Solvent-extractable organic matter, such as n-alkanes (C-14-C-33), n-alkan-2-ones (C-10-C-31), and 6, 10, 14- trimethylpentadecan-2-one on the fine aerosols, were quantified. The concentrations of total n-alkanes from primary sources were 45.5-112 ng m(-3) at the tunnel site, 3.3-34.6 ng m(-3) at the urban site, 0.6-18.1 ng m-3 at the rural site, and 1.7-16.9 ng m(-3) at the forest and the mixed areas. The homologue distributions of the n-alkanes displayed different patterns at the 5 sites, showing day-night differences and reflecting their primary source types and impacts of episodes. The carbon preference index (CPI) values of the n-alkanes showed highest value (average of 2.39 +/- 0.47) at the forest, lowest (1.15 +/- 0.11) at the tunnel. The CPI showed higher values in night samples at all sites except the urban site which was impacted by specific episodes such as biomass burning and/or fuel burning occurring during the nighttimes, the higher nighttime values of CPI, with consistent lower n-alkane concentrations, suggested that weaker anthropogenic emissions during night were a more likely cause. The total n-alkan-2-ones on the aerosols were 1.8-12.6ng m(-3) at the tunnel site and 0.2-7.2ng m-3 at the other 4 sites. Low-molecular weight n-alkan-2-ones (< C-22) were observed at all sites with the highest level at the tunnel. High molecular weight n-alkan-2-one (> C-23) were consistently higher at the tunnel but varied with date at the forest site. The n-alkan-2ones, both low and high molecular weight, could have multiple sources including vehicular emissions and oxidation processes. The branched ketone, 6, 10, 14- trimethylpentadecan-2-one, an oxidative product of phytol on the fine aerosols, 3 was enriched in the forest with the average of 7.6 +/- 7.1 ng mg(-3) (c) 2006 Elsevier Ltd. All rights reserved.

A method to measure C-13/ C-12 ratios of individual carbon fractions of airborne particular matter (PM) from filter samples using a stepwise thermal desorption/combustion OC/EC analyzer (via thermal optical transmission, (TOT) coupled with gas chromatography separation, followed by isotopic ratio mass spectrometer (GC-IRMS) analysis has been developed. In the TOT instrument, carbon fractions are released at different temperature ranges and different redox conditions. Organic carbon fraction (OC) was released at a relatively low temperature (T = 550 degrees C), whereas, elemental carbon or black carbon fraction (EC or BC was released at a high temperature (T > 800 degrees C) via combustion. A temperature step of 870 degrees C without oxygen was chosen to remove the impact of carbonate carbon (CC) and possible crossimpact from OC and EC. All the fractions were collected cryogenically and subject to carbon isotope measurements via GC-IRMS. To evaluate the precision, accuracy and linearity range of the measurements, the different types of blanks and standards were investigated, including OC (i.e. glucose, sucrose, n-Alkanes and polycyclic aromatic hydrocarbons (PAHs), CC (i.e. carbonates) and EC (i.e. carbon black and graphite). The overall precision and the accuracy of the method is similar to 0.3 parts per thousand. The method was applied to Pacific2001 aerosol samples from the Greater Vancouver area in Canada. The results show that good baseline separations in thermographs can be achieved for individual carbon fractions (i.e. OC and EC) using optimized temperature plateau and retention times; relative small difference in carbon isotopic composition between OC and EC (Delta(13) C-OC-(EC) = delta C-13(OC)delta C-13(EC)) were found in tunnel samples, whereas, the largest Delta C-13(OC-EC) were obtained in forest air samples; the Delta C-13(OC-EC) in ambient PM is likely dependant upon the dominant sources present in the vicinity of the sampling sites; the distribution of 13C/ 12 C ratios of OC/EC can provide useful information for source characterization and apportionment of ambient particulate matter. (c) 2006 Elsevier Ltd. All rights reserved.

Measurements of cloud condensation nuclei (CCN) were made in downtown Toronto during August and September, 2003. CCN measurements were performed at 0.58% supersaturation using a thermal-gradient diffusion chamber, whereas the aerosol size distribution and composition were simultaneously measured with a TSI SMPS and APS system and an Aerodyne Aerosol Mass Spectrometer (AMS), respectively. Aerosol composition data shows that the particles were predominately organic in nature, in particular for those with a vacuum aerodynamic diameter of <0.25 mu m. In this study, the largest contribution to CCN concentrations came from this size range, suggesting that the CCN are also organic-rich. Using the size and composition information, detailed CCN closure analyses were performed. In the first analysis, the particles were assumed to be internally mixed, the organic fraction was assumed to be insoluble, and the inorganic fraction was assumed to be ammonium sulfate. The AMS time-of-flight data were used for Koohler theory predictions for each particle size and composition to obtain the dry diameter required for activation. By so doing, this closure analysis yielded an average value of CCNpredicted/CCNobserved = 1.12 +/- 0.05. However, several sample days showed distinct bimodal distributions, and a closure analysis was performed after decoupling the two particle modes. This analysis yielded an average value of CCNpredicted/CCNobserved = 1.03 +/- 0.05. A sensitivity analysis was also performed to determine the aerosol/CCN closure if the organic solubility, droplet surface tension, or chamber supersaturation were varied.

Organosulfates are observed in studies of pinonaldehyde reactions with acidic sulfate aerosols using aerosol mass spectrometry, during which a significant fraction of the pinonaldehyde reaction products were found to consist of organosulfate compounds that account for 6 - 51% of the initial SO4= mass. Resultant aerosol mass spectra were consistent with proposed sulfate ester mechanisms, which likely form stable products. The existence of organosulfates was also confirmed in studies of the reaction system in bulk solution. The formation of organosulfates suggests that conventional inorganic SO4= chemical analysis may underestimate total SO4= mass in ambient aerosols.

[ 1] The reactive uptake of pinonaldehyde, a monoterpene oxidation product, on aerosols has been studied in a reaction chamber. Monodisperse inorganic seed aerosols, consisting of acidic mixtures of (NH4)(2)SO4 and H2SO4, were exposed to gaseous pinonaldehyde for several hours within the chamber under relative humidity conditions of 3-65%. The aerosol inorganic and organic mass were quantitatively monitored in real time with an aerosol mass spectrometer (AMS) which also measured the mass spectra of the aerosols. Numerous fragments in the mass spectra were observed with masses greater than what can be accounted for by pinonaldehyde alone and have arisen from oligomerization in reactive uptake processes. The evolution of the mass spectra also revealed a progression toward larger oligomers over time. Significant organic mass was added to the aerosols in most experiments immediately upon exposure, resulting in maximum organic mass loadings from 3.5-110 mu gm(-3) depending on the experiments. Organic mass to seed aerosol SO4= ratios were also highly variable (0.06-4.75), resulting in particles ranging in composition from primarily inorganic to mostly organic. This reactive uptake was highly dependent upon the aerosol water activity, and hence acidity and did not occur on neutral (NH4)(2)SO4 aerosols suggesting that acidity is necessary. Reactive uptake coefficients (g) of pinonaldehyde were calculated by fitting a model of organic mass growth to the data. The coefficients spanned two orders of magnitude (1.2 x 10(-5)-1.3 x 10(-3)) and were primarily dependent upon aerosol water activity and acidity but independent of gas phase pinonaldehyde concentrations. These coefficients indicated that the heterogeneous reactions of pinonaldehyde are of little importance as a gas phase loss mechanism but potentially of major importance as a source of secondary organic aerosols (SOA). Estimates of SOA production via pinonaldehyde, using the derived gamma, suggest that 1-750 ng m(-3) of organic material can be formed in a short time, consistent with ambient measurements.

The uptake of gaseous glyoxal onto particulate matter has been studied in laboratory experiments under conditions relevant to the ambient atmosphere using an aerosol mass spectrometer. The growth rates and reactive uptake coefficients, g, were derived by fitting a model of particle growth to the experimental data. Organic growth rates varied from 1.05 x 10(-1)1 to 23.1 x 10(-11) mu g particle(-1) min(-1) in the presence of similar to 5 ppb glyoxal. Uptake coefficients (gamma) of glyoxal varied from 8.0 x 10(-4) to 7.3 x 10(-3) with a median gamma = 2.9 x 10(-3), observed for (NH4)(2)SO4 seed aerosols at 55% relative humidity. Increased g values were related to increased particle acidity, indicating that acid catalysis played a role in the heterogeneous mechanism. Experiments conducted at very low relative humidity, with the potential to be highly acidic, resulted in very low reactive uptake. These uptake coefficients indicated that the heterogeneous loss of glyoxal in the atmosphere is at least as important as gas phase loss mechanisms, including photolysis and reaction with hydroxyl radicals. Glyoxal lifetime due to heterogeneous reactions under typical ambient conditions was estimated to be tau(het) = 5-287 min. In rural and remote areas the glyoxal uptake can lead to 5-257 ng m(-3) of secondary organic aerosols in 8 hours, consistent with recent ambient measurements.

Reactive uptake of glyoxal onto particulate matter has been studied in laboratory experiments in a 2 m(3) Teflon reaction chamber. Inorganic seed particles of different composition were utilized, including (NH4)(2)SO4, (NH4)(2)SO4/ H2SO4, NaNO3, and simulated sea salt, while the relative humidity and acid concentration were varied. The organic composition of the growing particles was measured in situ with an aerosol mass spectrometer, providing particle mass spectra as a means of product identification. Aerosol physical characteristics were also measured with a differential mobility analyzer and condensation nucleus counter. Regardless of seed composition, particle growth was rapid and continuous over the course of several hours. Identification of several mass fragments greater than the glyoxal monomer suggested that heterogeneous reactions to form glyoxal adducts of low volatility had occurred. Temporal analysis of the mass fragments was consistent with a proposed acid-catalyzed mechanism whereby glyoxal is first hydrated, followed by self-reaction to form cyclic acetal structures. Increased relative humidity slowed the formation of higher order oligomers, also consistent with the proposed mechanism. The relative contribution of various oligomers to the overall organic composition was strongly dependent on the relative humidity and hence the particulate water concentration. A mild acid catalysis was also observed upon increasing the acidity of the seed particles. Specific mass fragments were found that could only arise from sulfate esters and were not present on the non-sulfur-containing seed particles. This first evidence of the formation of organic sulfates in particles is presented together with a proposed mechanism and molecular structure. These results suggest that the formation of these products of glyoxal uptake can contribute significantly to secondary organic aerosol.

High-volume air samples collected over the period Aug. 14-30, 2001, in the Lower Fraser Valley, BC, Canada, were used to assess urban/rural differences of organochlorine pesticides (OCPs) for ground level samples and to attempt to directly measure events of trans-Pacific inputs through the mid-troposphere. Hexachlorocyclohexanes (alpha- and gamma-isomers; 2-25 pg m(-3)) and endosulfan] and -2 (5-150 pg m(-3)) were detected in all ground level samples. Seven air samples were collected during mid-troposphere flights (similar to4400 m altitude) over the Lower Fraser Valley. These flights occurred concurrently with ground level sampling. Trajectory analysis identified three events of substantial mid-troposphere, trans-Pacific flow where 10-day back trajectories stemmed from potential source regions in Asia. These events were also characterized by higher air concentrations of alpha-HCH at 4400 m as compared to the ground level stations. This represents the first event-based, aircraft measurement of advection inputs of OCPs in the mid-troposphere of the west coast of North America.

Atmospheric sampling was conducted at a rural site near Egbert, about 70 km north of Toronto, Ontario, Canada from March 27 to May 8, 2003 to characterize the physical and chemical properties of the ambient aerosol in near real-time. The instrumentation included a tapered element oscillating microbalance (TEOM), an ultrafine condensation particle counter (UCPC), a scanning mobility particle sizer (SMPS), an aerodynamic particle sizer (APS), an aerosol mass spectrometer (AMS), and a particulate nitrate monitor (R&P 8400N) for aerosol measurements. Gas-phase non-methane hydrocarbon compounds (NMHCs) were measured by gas chromatograph-flame ionization detection (GCFID). Filter samples were also collected for analysis of inorganic ions by ion chromatography (IC). Aerosol properties varied considerably depending upon meteorological conditions and airmass histories. For example, urban and industrial emissions advected from the south strongly influenced the site occasionally, resulting in higher particulate mass with the higher fractions of nitrate and organics. Cleaner northwesterly winds carried aerosols with relatively higher fractions of organics and sulfate. The AMS derived mass size distributions showed that the inorganic species in the particles with vacuum aerodynamic diameters between about 60 nm and 600 nm had mass modal vacuum aerodynamic diameters around 400 - 500 nm. The particulate organics often exhibited two modes at about 100 nm and 425 nm, more noticeable during fresh pollution events. The small organic mode was well correlated with gas-phase nonmethane hydrocarbons such as ethylbenzene, toluene, and propene, suggesting that the likely sources of small organic particles were combustion related emissions. The particulate nitrate exhibited a diurnal variation with higher concentrations during dark hours and minima in the afternoon. Particulate sulfate and organics showed evidence of photochemical processing with higher levels of sulfate and oxygenated organics in the afternoon. Reasonable agreement among all of the co-located measurements is found, provided the upper size limit of the AMS is considered.