Generation of reactive oxygen species mediated by humic-like substances in atmospheric aerosols

Particulate matter (PM)-mediated reactive oxygen species (ROS) generation has been implicated in health effects posed by PM. Humic-like substances (HULIS) are an unresolved mixture of water-extracted organic compounds from atmospheric aerosol particles or isolated from fog/cloudwater samples. In this study, we use a cell-free dithiothreitol (DTT) assay to measure ROS production mediated by HULIS. The HULIS samples are isolated from aerosols collected at a rural location and a suburban location in the Pearl River Delta, China. In our experiments, ROS activities by residue metal ions in the HULIS fraction are suppressed by including a strong chelating agent in the DTT assay. Under conditions of DTT consumption not exceeding 90%, the HULIS-catalyzed oxidation of DTT follows the zero-order kinetics with respect to DTT concentration, and the rate of DTT oxidation is proportional to the dose of HULIS. The ROS activity of the aerosol HULIS, on a per unit mass basis is 2% of the ROS activity by a reference quinone compound, 1,4-naphthoquinone and exceeds that of two aquatic fulvic acids. The HULIS fraction in the ambient samples tested exhibits comparable ROS activities to the organic solvent extractable fraction, which would contain compounds such as quinones, a known organic compound class capable of catalyzing generation of ROS in cells. HULIS was found to be the major redox active constituent of the water-extractable organic fraction in PM. It is plausible that HULIS contains reversible redox sites, thereby serving as electron carriers to catalyze the formation of ROS. Our work suggests that HULIS could be an active PM component in generating ROS and further work is warranted to characterize its redox properties.     

O/C and OM/OC ratios of primary, secondary, and ambient organic aerosols with high-resolution time-of-flight aerosol mass spectrometry

A recently developed method to rapidly quantify the elemental composition of bulk organic aerosols (OA) using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) is improved and applied to ambient measurements. Atomic oxygen-to-carbon (O/C) ratios characterize the oxidation state of OA, and O/C from ambient urban OA ranges from 0.2 to 0.8 with a diurnal cycle that decreases with primary emissions and increases because of photochemical processing and secondary OA (SOA) production. Regional O/C approaches approximately 0.9. The hydrogen-to-carbon (H/C, 1.4--1.9) urban diurnal profile increases with primary OA (POA) as does the nitrogen-to-carbon (N/C, approximately 0.02). Ambient organic-mass-to-organic-carbon ratios (OM/OC) are directly quantified and correlate well with O/C (R2 = 0.997) for ambient OA because of low N/C. Ambient O/C and OM/OC have values consistent with those recently reported from other techniques. Positive matrix factorization applied to ambient OA identifies factors with distinct O/C and OM/OC trends. The highest O/C and OM/OC (1.0 and 2.5, respectively) are observed for aged ambient oxygenated OA, significantly exceeding values for traditional chamber SOA,while laboratory-produced primary biomass burning OA (BBOA) is similar to ambient BBOA, O/C of 0.3--0.4. Hydrocarbon-like OA (HOA), a surrogate for urban combustion POA, has the lowest O/C (0.06--0.10), similar to vehicle exhaust. An approximation for predicting O/C from unit mass resolution data is also presented.     

Size-resolved source apportionment of airborne particle mass in a roadside environment

Airborne particulate hopanes, steranes, and polycyclic aromatic hydrocarbons (PAHs) were measured in six size fractions < 1.8 microm particle diameter at one site upwind and two sites downwind of the Interstate 5 freeway in San Diego, CA. The smallest size fraction collected was exclusively in the ultrafine size range (D(p) < 0.1 microm; PM0.1). Size distributions of hopanes, steranes, and PAHs peaked between 0.10-0.18 microm particle aerodynamic diameter with a tail extending into the PM0.1 size range. This pattern is similar to previous dynamometer studies of hopane, sterane, and PAH size distributions emitted from gasoline- and diesel-powered vehicles. Size-resolved source profiles were combined to form an "on-road" profile for motor oil, diesel, and gasoline contributions to EC and OC. The resulting equations were used to predict source contributions to the size distributions of EC and OC in the roadside environment. The method successfully accounted for the majority of the carbonaceous material in particles with diameter < 0.18 microm, with significant residual material in larger size fractions. The peak in both the measured and predicted EC size distribution occurred between 0.1-0.18 microm particle aerodynamic diameter. The predicted OC size distribution peaked between 0.1-0.18 microm particle diameter, butthe measured OC size distribution peaked between 0.56-1.0 microm particle diameter, possibly because of secondary organic aerosol formation. Predicted OC concentrations in particles with diameter < 0.18 microm were greater than measured values 18 m downwind of the roadway but showed good agreement 37 m downwind. The largest source contributions to the PM0.1 and PM0.18 size fractions were different. PM0.18 was dominated by diesel fuel and motor oil combustion products while PM0.1 was dominated by diesel fuel and gasoline fuel combustion products. Total source contributions to ultrafine (PM0.1) EC concentrations 37 m downwind of the roadway were 44 +/- 6% diesel fuel, 21 +/- 1% gasoline, 5 +/- 6% motor oil, and 30% unknown. Total source contributions to ultrafine (PM0.1) OC concentrations 37 m downwind of the roadway were 46 +/- 5% diesel fuel, 44 +/- 5% gasoline, 20 +/- 15% motor oil with a slight overprediction (11%). Diesel fuel appears to make the single largest contribution to ultrafine (PM0.1) particle mass given the fleet distribution during the current experiment.     

Temporal trends in atmospheric PM₂.₅, PM₁₀, elemental carbon, organic carbon, water-soluble organic carbon, and optical properties: impact of biomass burning emissions in the Indo-Gangetic Plain

The first simultaneous measurements and analytical data on atmospheric concentrations of PM(2.5), PM(10), inorganic constituents, carbonaceous species, and their optical properties (aerosol optical depth, AOD; absorption coefficient, b(abs); mass absorption efficiency, σ(abs); and single scattering albedo, SSA) from an urban site (Kanpur) in the Indo-Gangetic Plain are reported here. Significantly high aerosol mass concentration (>100 μg m(-3)) and AOD (> 0.3) are seen as a characteristic feature throughout the sampling period, from October 2008 to April 2009. The temporal variability in the mass fractions of carbonaceous species (EC, OC, and WSOC) is pronounced during October-January when emissions from biomass burning are dominant and OC is a major constituent (～30%) of PM(2.5) mass. The WSOC/OC ratio varies from 0.21 to 0.65, suggesting significant contribution from secondary organic aerosols (SOAs). The mass fraction of SO(4)(2-) in PM(2.5) (Av: 12.5%) exceeds that of NO(3)(-) and NH(4)(+). Aerosol absorption coefficient (@ 678 nm) decreases from 90 Mm(-1) (in December) to 20 Mm(-1) (in April), and a linear regression analysis of the data for b(abs) and EC (n = 54) provides a measure of the mass absorption efficiency of EC (9.6 m(2) g(-1)). In contrast, scattering coefficient (@ 678 nm) increases from 98 Mm(-1) (in January) to 1056 Mm(-1) (in April) and an average mass scattering efficiency of 3.0 ± 0.9 m(2) g(-1) is obtained for PM(10) samples. The highest b(scat) was associated with the dust storm event (April 17, 2009) over northern Iraq, eastern Syria, and southern Turkey; thus, resulting in high SSA (0.93 ± 0.02) during March-April compared to 0.82 ± 0.04 in October-February. These results have implications to large temporal variability in the atmospheric radiative forcing due to aerosols over northern India.     

Development and evaluation of a personal particulate organic and mass sampler

Accurate measurement of personal exposure to particulate matter and its constituents requires samplers that are accurate, compact, lightweight, inexpensive, and convenient to use. The personal particulate organic and mass sampler (PPOMS) has been developed to meet these criteria. The PPOMS uses activated carbon-impregnated foam as a combined 2.5-microm size-selective inlet and denuder for assessment of fine particle mass and organic carbon. Proof of the PPOMS concept has been established by comparing mass and organic carbon in particles collected with collocated samplers in Seattle, at a central outdoor site, and in residences. Daily particulate mass concentrations averaged 10.0 +/- 5.2, 12.0 +/- 5.3, and 11.2 +/- 5.1 microg m(-3) for the Federal Reference Method, the Harvard Personal Exposure Monitor, and the PPOMS, respectively, for 10 24-h sampling periods. During a series of PM2.5 indoor organic carbon (OC) measurements from single quartz filters, the apparent indoor OC averaged 7.7 +/- 0.8 microg of C m(-3), which was close to the indoor PM2.5 mass from collocated Teflon filters (7.3 +/- 2.3 microg of C m(-3)), indicating the presence of a large positive OC artifact. In collocated measurements, the PPOMS eliminated this artifact just as well as the integrated gas and particle sampler that incorporated a macroreticular polystyrene-divinylbenzene (XAD-4) resin-coated denuder, yielding OC concentrations of 2.5 +/- 0.4 and 2.4 +/- 1.0 microg of C m(-3), respectively. Thermal analysis for OC indicated that the indoor positive artifact was due to adsorption of gas-phase semivolatile organic compounds (SVOC). This study shows that the PPOMS design provides a 2.5-microm size-selective inlet that also prevents the adsorption of gas-phase SVOC onto quartz filters, thus eliminating the filter positive artifact The PPOMS meets a significant current challenge for indoor and personal sampling of particulate organic carbon. The PPOMS design can also simplify accurate ambient sampling for PM2.5.     

Source attribution of submicron organic aerosols during wintertime inversions by advanced factor analysis of aerosol mass spectra

Real-time measurements of submicrometer aerosol were performed using an Aerodyne aerosol mass spectrometer (AMS) during three weeks at an urban background site in Zurich (Switzerland) in January 2006. A hybrid receptor model which incorporates a priori known source composition was applied to the AMS highly time-resolved organic aerosol mass spectra. Three sources and components of submicrometer organic aerosols were identified: the major component was oxygenated organic aerosol (OOA), mostly representing secondary organic aerosol and accounting on average for 52-57% of the particulate organic mass. Radiocarbon (14C) measurements of organic carbon (OC) indicated that approximately 31 and approximately 69% of OOA originated from fossil and nonfossil sources, respectively. OOA estimates were strongly correlated with measured particulate ammonium. Particles from wood combustion (35-40%) and 3-13% traffic-related hydrocarbon-like organic aerosol (HOA) accounted for the other half of measured organic matter (OM). Emission ratios of modeled HOA to measured nitrogen oxides (NOx) and OM from wood burning to levoglucosan from filter analyses were found to be consistent with literature values.     

Aircraft measurement of organic aerosols over China

Lower to middle (0.5-3.0 km altitude) tropospheric aerosols (PM2.5) collected by aircraft over inland and east coastal China were, for the first time, characterized for organic molecular compositions to understand anthropogenic, natural, and photochemical contribution to the air quality. n-Alkanes, fatty acids, sugars, polyacids are detected as major compound classes, whereas lignin and resin products, sterols, polycyclic aromatic hydrocarbons, and phthalic acids are minor species. Average concentrations of all the identified compounds excluding malic acid correspond to 40-50% of those reported on the ground sites. Relative abundances of secondary organic aerosol (SOA) components such as malic acid are much higher in the aircraft samples, suggesting an enhanced photochemical production over China. Organic carbon (OC) concentrations in summer (average, 24.3 microg m(-3)) were equivalent to those reported on the ground sites. Higher OC/EC (elemental carbon) ratios in the summer aircraft samples also support a significant production of SOA over China. High loadings of organic aerosols in the Chinese troposphere may be responsible to an intercontinental transport of the pollutants and potential impact on the regional and global climate changes.     

Determination of methoxyphenols in ambient atmospheric particulate matter: tracers for wood combustion

Combustion of wood and other biomass fuels produces source-specific organic compounds arising from pyrolysis of lignin, including substantial amounts of 4-substituted methoxylated phenolic compounds (methoxyphenols). These compounds have been used as atmospheric markers to determine the contribution of wood smoke to ambient atmospheric fine particulate matter (PM). However, reliable quantification of methoxyphenols represents an analytical challenge because these compounds are polar, semi-volatile, and somewhat reactive. We reportherein an improved gas chromatographic-mass spectrometric (GC/MS) method for the sensitive and reliable determination of methoxyphenols in low-volume ambient PM samples. Deuterated standard compounds are added to the environmental samples prior to extraction to determine analyte recoveries in each sample. Analytical figures of merit for the assay, as applied to ambient PM2.5 and PM10 samples are as follows: recovery = 63-100%; precision = 2-6%; analytical limit of detection (S/N 2) = 0.002 microg/mL; limit of quantitation = 0.07-0.45 ng/m3 (assuming a 14 m3 sample). The improved method was applied to ambient PM samples collected between 1999 and 2000 in Seattle, WA. Particle-bound methoxyphenol concentrations in the range <0.1 to 22 ng/m3 were observed and the methoxyphenols were present almost exclusively in the fine (PM2.5) size fraction. We also demonstrated that XRF analysis of samples of atmospheric PM collected on Teflon filters significantly reduced the levels of methoxyphenols measured in the PM samples in subsequent assay of the same filters. Therefore, XRF analysis of filters, commonly undertaken to obtain trace element concentrations for use in source apportionment analyses, would preclude the subsequent analysis of those filters for methoxyphenols and other similarly semivolatile or reactive organic chemicals.     

Chemical characterization of ambient particulate matter near the World Trade Center: elemental carbon, organic carbon, and mass reconstruction

Concentrations of elemental carbon (EC), organic carbon matter (OM), particulate matter less than 2.5 microm (PM2.5), reconstructed soil, trace element oxides, and sulfate are reported from four locations near the World Trade Center (WTC) complex for airborne particulate matter (PM) samples collected from September 2001 through January 2002. Across the four sampling sites, daily mean concentrations ranged from 1.5 to 6.8 microg/m3 for EC, from 10.2 to 31.4 microg/m3 for OM, and from 22.6 to 66.2 microg/m3 for PM2.5. Highest concentrations of PM species were generally measured north and west of the WTC complex. Total carbon matter and sulfate constituted the largest fraction of reconstructed PM2.5 concentrations. Concentrations of PM species across all sites decreased from the period when fires were present at the WTC complex (before December 19, 2001) to the period after the fires. Averaged over all sites, concentrations decreased by 25.6 microg/m3 for PM2.5, 2.7 microg/m3 for EC, and 9.2 microg/m3 for OM from the fire period to after fire period.     

Molecular characteristics of urban organic aerosols from Nanjing: a case study of A mega-city in China

Over 90 organic species have been determined in fine aerosols (PM2.5) collected during the summer and winter in Nanjing, a typical mega-city in China, using gas chromatography-mass spectrometry. The organic compounds detected were apportioned to four emission sources (i.e., plant emission, fossil fuel combustion, biomass burning, and soil resuspension) and secondary oxidation products. The most abundant classes of compounds are fatty acids, followed by sugars, dicarboxylic acids excluding oxalic and malonic acids, and n-alkanes, while alcohols, polyols/polyacids and lignin/sterols are less abundant. Total amounts of the seven classes of compounds were on average 938 ng m(-3) in the summer and 1301 ng m(-3) in the winter, respectively, contributing 0.26-1.96% of particle mass (PM2.5). In the summer, n-alkanes were heavily enhanced by vegetation emissions with a maximum carbon number (Cmax) at C29, whereas they were dominated by emissions from fossil fuels combustion with a Cmax at C22/ C23 in the winter. Concentrations of unsaturated fatty acids were lower in the summer than in the winter, being consistent with enhanced photooxidation of unsaturated fatty acids in the summer. Concentrations of dicarboxylic acids for the summer aerosols were much higher in the daytime than in the nighttime, indicating increased photochemical production in the daytime. In the summer, plant emissions were the most significant source of organic aerosols, contributing more than 33% of total compound mass (TCM), followed by fossil fuel combustion or secondary oxidation. In contrast, fossil fuel combustion was the dominant source of winter organic aerosols, contributing more than 51% of TCM, followed by plant emissions and secondary oxidation products. The quantitative results on sugars and lignin pyrolysis products further suggested that biomass burning and soil resuspension are also significant sources of urban organic aerosols.     

Diagnosis of aged prescribed burning plumes impacting an urban area

An unanticipated wind shift led to the advection of plumes from two prescribed burning sites that impacted Atlanta, GA, producing a heavy smoke event late in the afternoon on February 28, 2007. Observed PM2.5 concentrations increased to over 140 microg/m3 and O3 concentrations up to 30 ppb in a couple of hours, despite the late hour in February when photochemistry is less vigorous. A detailed investigation of PM2.5 chemical composition and source apportionment analysis showed that the increase in PM2.5 mass was driven mainly by organic carbon (OC). However, both results from source apportionment and an observed nonlinear relationship between OC and PM2.5 potassium (K) indicate that the increased OC was not due solely to primary emissions. Most of the OC was water-soluble organic carbon (WSOC) and was dominated by hydrophobic compounds. The data are consistent with large enhancements in isoprenoid (isoprene and monoterpenes) and other volatile organic compounds emitted from prescribed burning that led to both significant O3 and secondary organic aerosol (SOA) production. Formation of oligomers from oxidation products of isoprenoid compounds or condensation of volatile organic compounds (VOCs) with multiple functional groups emitted during prescribed burning appears to be a major component of the secondary organic contributor of the SOA. The results from this study imply that enhanced emissions due to the fire itself and elevated temperature in the burning region should be considered in air quality models (e.g., receptor and emission-based models) to assess impacts of prescribed burning emissions on ambient air quality.     

Origins of primary and secondary organic aerosol in Atlanta: results of time-resolved measurements during the Atlanta Supersite Experiment

Time-resolved ambient particulate organic (OC) and elemental carbon (EC) data measured in Atlanta, GA, during the Atlanta Supersite Experiment (August3-September 1, 1999) were investigated to determine the temporal trends of atmospheric carbonaceous aerosol and to examine the relative contributions of primary and secondary OC to measured particulate OC. Mean 1-h average concentrations (ranges in parentheses) of PM2.5 OC, EC, and total carbon were 8.3 (3.6-15.8), 2.3 (0.3-9.6), and 10.6 (4.6-24.6) microg of C m(-3), respectively, based on Rutgers University/Oregon Graduate Institute in situ thermal-optical carbon analyzer measurements. Carbonaceous matter (organic material 40%; EC 8%) comprised approximately 48% of PM2.5 mass in Atlanta. Primary and secondary OC concentrations were estimated using an EC tracer method. Secondary OC contributed approximately 46% of measured particulate OC, and 1-h average contributions ranged up to 88%. Vehicle emissions appear to be the dominant contributors to measured EC and primary OC concentrations based on temporal patterns of EC, primary OC, and CO. This research suggests that secondary OC concentrations in Atlanta were influenced by (1) "fresh" secondary organic aerosol formed by photochemical reactions locally in the early afternoons as seen in the Los Angeles air basin and (2) "aged" secondary organic aerosol transported from upwind regions or formed on previous days. Nocturnal peaks in secondary OC and ozone concentrations were observed on several days. The most probable explanation for this is the favorable partitioning of semivolatile organic compounds to the particulate phase driven by temperature decreases and relative humidity increases at night and vertical transport of regional pollutants from above to ground level.     

Atmospheric pressure mass spectrometry: a new analytical chemical characterization method for dissolved organic matter in rainwater

The complex mixture of organic compounds in the atmosphere influences climate, air quality, and ecosystem processes. Atmospheric pressure electrospray ionization mass spectrometry (APESI-MS) was evaluated as a potential tool for direct measurement of the total suite of individual dissolved organic matter (DOM) compounds in rainwater. The APESI-MS response was linear to all DOM compounds of atmospheric significance examined as standard solutions. Urban precipitation samples from New Brunswick, NJ (USA) were analyzed by APESI-MS over the mass-to-charge (m/z) range 50-3,000. Over 95% of the m/z ions detected were in the low m/z range (50-500). Over 300 unique m/z ions were detected across the 11 rainwater samples indicating the complexity of the mixture of DOM in rainwater. Forty percent of the organic bases (positive mode detection) and 22% of the organic acids (negative mode) occurred in at least 6 of the 11 rainwater samples. Ions corresponding to the m/z of carboxylic acids standards (nonanedioic acid; 1,4-butanedicarboxylic acid; pentanedioic acid; hydroxybutanedioic acid; and butanedioic acid) and to reduced N standards (allylurea; caffeine; imidazole; and N-2-propenylurea) occurred in at least one of the 11 rainwater samples. Total dissolved organic carbon (DOC) estimated from the APESI-MS analysis and measured by standard DOC methods were not statistically different.     

Interannual variations in PM2.5 due to wildfires in the Western United States

In this study we have evaluated the role of wildfires on concentrations of fine particle (d < 2.5 microm) organic carbon (OC) and particulate mass (PM2.5) in the Western United States for the period 1988-2004. To do this, we examined the relationship between mean summer PM2.5 and OC concentrations at 39 IMPROVE sites with a database of fires developed from federal fire reports. The gridded database of area burned was used to generate a database of biomass fuel burned using ecosystem-specific fuel loads. The OC, PM2.5, and fire data were evaluated for five regions: Northern Rocky Mountains (Region 1), Central Rocky Mountains (Region 2), Southwest (Region 3), California (Region 4), and Pacific Northwest (Region 5). In Regions 1, 2, and 5, we found good correlations of seasonal mean PM2.5 concentrations among the sites within each region. This indicates that a common influence was important in determining the PM concentration at all sites across each region. In Regions 1 and 2, we found a significant correlation between PM2.5 and both the area burned and biomassfuel burned in each region. This relationship is statistically significant using either the area burned or fuel burned, but the correlations are stronger using the biomass fuel burned. In all five regions we found a statistically significant relationship between biomass burned and organic carbon. Using these relationships, we can estimate the amount of PM2.5 due to fires in each region during summer. For the Regions 1 through 5, the average summer-long enhancement of PM2.5 due to fires is 1.84, 1.09, 0.61, 0.81, and 1.21 microg/m3, respectively, and approximately twice these values during large fire years.     

Real-time gaseous, PM and ultrafine particle emissions from a modern marine engine operating on biodiesel

Emissions from harbor-craft significantly affect air quality in populated regions near ports and inland waterways. This research measured regulated and unregulated emissions from an in-use EPA Tier 2 marine propulsion engine on a ferry operating in a bay following standard methods. A special effort was made to monitor continuously both the total Particulate Mass (PM) mass emissions and the real-time Particle Size Distribution (PSD). The engine was operated following the loads in ISO 8178-4 E3 cycle for comparison with the certification standards and across biodiesel blends. Real-time measurements were also made during a typical cruise in the bay. Results showed the in-use nitrogen oxide (NOx) and PM(2.5) emission factors were within the not to exceed standard for Tier 2 marine engines. Comparing across fuels we observed the following: a) no statistically significant change in NO(x) emissions with biodiesel blends (B20, B50); b) ～ 16% and ～ 25% reduction of PM(2.5) mass emissions with B20 and B50 respectively; c) a larger organic carbon (OC) to elemental carbon (EC) ratio and organic mass (OM) to OC ratio with B50 compared to B20 and B0; d) a significant number of ultrafine nuclei and a smaller mass mean diameter with increasing blend-levels of biodiesel. The real-time monitoring of gaseous and particulate emissions during a typical cruise in the San Francisco Bay (in-use cycle) revealed important effects of ocean/bay currents on emissions: NO(x) and CO(2) increased 3-fold; PM(2.5) mass increased 6-fold; and ultrafine particles disappeared due to the effect of bay currents. This finding has implications on the use of certification values instead of actual in-use emission values when developing inventories. Emission factors for some volatile organic compounds (VOCs), carbonyls, and poly aromatic hydrocarbons (PAHs) are reported as supplemental data.     

Particulate-phase and gaseous elemental mercury emissions during biomass combustion: controlling factors and correlation with particulate matter emissions

Mercury emissions from wildfires are significant natural sources of atmospheric mercury, but little is known about what controls speciation of emissions important to mercury deposition processes. The goal of this study was to quantify gaseous elemental mercury (GEM) and particulate-phase mercury (PHg) emissions from biomass combustion to identify key factors controlling the speciation. Emissions were characterized in an exhaust stack 17 m above fires using a gaseous mercury analyzer and quartz-fiber filters. Fuels included fresh and air-dried leaves, needles, and branches with different fuel moistures (9-95% of dry weight) and combustion properties (e.g., from < 10 to 90% of fire durations characterized by flaming phases). Fuel moisture was the overall driving factor defining emissions, with GEM being the dominant fraction (> or = 95%) in low moisture fuels and substantial PHg contributions--up to 50% of total mercury emissions--in fresh fuels. High PHg emissions were observed during smoldering combustion whereas flaming-dominated fires showed insignificant PHg emissions. PHg mass emissions were correlated with particulate matter (PM; r2 = 0.67), organic carbon (OC; r2 = 0.63) and sulfur (S; r2 = 0.46) mass emissions, but not with elemental carbon (EC) nor with the total mercury emissions. These data suggest that the formation of PHg involves similar processes as the formation of particulate OC, for example condensation of volatile species onto preexisting smoke particles during cooling and dilution. Based on the observed relationship between PM and OC mass concentrations and published emission inventories, we estimate global PHg emissions by wildfires of 4-5 Mg yr(-1).     

Use of synthetic data to evaluate positive matrix factorization as a source apportionment tool for PM2.5 exposure data

Positive matrix factorization (PMF) was applied to synthetic datasets that simulate personal exposures to airborne PM2.5 from 12 sources. Three differentfilter analysis scenarios using different analytical chemistry techniques were considered. The full suite scenario quantified elemental carbon, organic carbon, inorganic ions, trace elements, and trace organic species including carboxylic acids and organic compounds with -OH functionality. A second scenario excluded trace elements and a third assumed that derivatization steps to quantify polar organic compounds were not performed. Similar errors in source apportionment were seen with all three scenarios. In most cases, PMF failed to separate out factors corresponding to road dust and vegetative debris, two sources that made relatively uniform contributions to the synthetic exposures. Factors representing wood smoke, natural gas combustion, and meat cooking sources were difficult to identify due to a lack of unique tracers with concentrations reliably above the detection limits assumed in the study. Factors representing cigarette smoke, candle smoke, gasoline exhaust, and secondary aerosols were comparatively easy to identify. When contributions from a pair of sources, such as diesel and gasoline exhaust, were highly correlated in the synthetic datasets, a single factor corresponding to both sources was usually found.     

Analysis of sugars and sugar polyols in atmospheric aerosols by chloride attachment in liquid chromatography/negative ion electrospray mass spectrometry

Sugars and sugar polyols are relatively abundant groups of water-soluble constituents in atmospheric aerosols. This paper describes a method that uses liquid chromatography-mass spectrometry (LC-MS) to analyze sugars and sugar polyols in atmospheric aerosols, ranging from C3 sugar alcohols to trisaccharides. Postcolumn addition of chloroform in acetonitrile was found to greatly enhance ionization of these compounds by forming chloride adduct ions in the negative-ion mode using electrospray ionization. A gradient elution program starting at 5%:95% H20/acetonitrile and ending at 30%:70% H2O/acetonitrile provides baseline separations of the sugars and sugar polyols on an amino-based carbohydrate column. The detection limits based on quantification of [M + 35Cl]- adduct ions were in the order of 0.1 microM. By eliminating the need for derivatization, this LC-MS based method provides a simpler alternative method to the commonly used and more laborious gas-chromatography based methods. It also has an additional advantage of being able to quantify trisaccharide sugars. The method was applied to analyze 30 ambient samples of fine particulate matter collected at a site away from urban centers in Hong Kong. The sugar compounds positively identified and detected in the ambient samples included four sugar alcohols (glycerol, erythritol, xylitol, and mannitol), three monosacchride sugars (xylose, fructose, and glucose), two disaccharides (sucrose, trehalose), two trisaccharides (melezitose, raffinose), and one anhydrosugar (levoglucosan). The sum of these sugar and sugar polyol compounds ranged from 38 to 1316 ng m(-3), accounting for an average of 1.3% organic carbon mass. Through the use of a principal component analysis of the ambient measurements, the mono- to trisactharide sugars and C3-C5 sugar polyols were identified to be mainly associated with soil/soil microbiota while the anhydrosugar (levoglucosan) was associated with biomass burning.     

Simulating the degree of oxidation in atmospheric organic particles

Modeled ratios of organic mass to organic carbon (OM/OC) and oxygen to carbon (n(O)/n(C)) in organic particulate matter are presented across the US for the first time and evaluated extensively against ambient measurements. The base model configuration systematically underestimates OM/OC ratios during winter and summer months. Model performance is greatly improved by applying source-specific OM/OC ratios to the primary organic aerosol (POA) emissions and incorporating a new parametrization to simulate oxidative aging of POA in the atmosphere. These model improvements enable simulation of urban-scale gradients in OM/OC with values in urban areas as much as 0.4 lower than in the surrounding regions. Modeled OM/OC and n(O)/n(C) ratios in January range from 1.4 to 2.0 and 0.2 to 0.6, respectively. In July, modeled OM/OC and n(O)/n(C) ratios range from 1.4 to 2.2 and 0.2 to 0.8, respectively. Improved model performance during winter is attributed entirely to our application of source-specific OM/OC ratios to the inventory. During summer, our treatment of oxidative aging also contributes to improved performance. Advancements described in this paper are codified in the latest public release of the Community Multiscale Air Quality model, CMAQv5.0.