Evaluation of OC/EC Speciation by Thermal Manganese Dioxide Oxidation and the IMPROVE Method

Abstract

Ambient particulate samples are routinely analyzed for organic and elemental carbon (OC/EC) using either thermal manganese dioxide oxidation (TMO) or thermal volatil-ization-pyrolysis correction methods, such as the Inter-agency Monitoring of PROtected Visual Environments (IMPROVE) method with correction by reflectance, or a variation of the National Institute of Occupational Safety and Health (NIOSH) Method 5040 using thermal optical transmittance (TOT). With TMO, EC is modeled after the oxidation properties of submicron graphite and needle coke by MnO₂, and is the fraction of total carbon (TC) that is not oxidized at >525 °C. In thermal volatilization methods, EC is the fraction of TC that accounts for the light extinction properties of the sample at the start of analysis. Chow et al. (2001) compared IMPROVE and NIOSH methods implemented on the same instrument using 60 samples of various types and found that NIOSH EC was lower than IMPROVE. This study compares total, organic, and elemental carbon measurements from the TMO and IMPROVE thermal optical reflectance (TOR) methods using a sample set consisting of 60 IMPROVE nonurban, 16 Korean urban, 10 Hong Kong urban, and 14 synthetic carbon black samples.     

Comparison of PM2.5 carbon measurement methods in Hong Kong, China

Samples from Hong Kong, China, were analyzed for organic carbon (OC), elemental carbon (EC), and total carbon (TC) by three thermal protocols (low-temperature IMPROVE and high-temperature STN and NIOSH) and two optical monitoring methods: reflectance and transmittance. Good agreement (+/-10%) for TC among the three protocols was observed for sample loadings of 1-55 microg m(-3). The two protocols using a reflectance pyrolysis correction showed best agreement for EC, with <20% differences found for approximately 80% of the samples. Hong Kong has a large diesel fleet, and for some heavily loaded samples the light transmittance was too low for quantitative detection, resulting in large uncertainties in the OC/EC split based on transmittance. Hong Kong experienced OC levels similar to those at US sites, but has much higher EC concentrations. OC/EC ratios range from 2 to 5 at two US sites and from 0.2 to 1.2 at three Hong Kong sites.     

The IMPROVE_A temperature protocol for thermal/optical carbon analysis: maintaining consistency with a long-term database

Thermally derived carbon fractions including organic carbon (OC) and elemental carbon (EC) have been reported for the U.S. Interagency Monitoring of PROtected Visual Environments (IMPROVE) network since 1987 and have been found useful in source apportionment studies and to evaluate quartz-fiber filter adsorption of organic vapors. The IMPROVE_A temperature protocol defines temperature plateaus for thermally derived carbon fractions of 140 degrees C for OC1, 280 degrees C for OC2, 480 degrees C for OC3, and 580 degrees C for OC4 in a helium (He) carrier gas and 580 degrees C for EC1, 740 degrees C for EC2, and 840 degrees C for EC3 in a 98% He/2% oxygen (O2) carrier gas. These temperatures differ from those used previously because new hardware used for the IMPROVE thermal/optical reflectance (IMPROVE_TOR) protocol better represents the sample temperature than did the old hardware. A newly developed temperature calibration method demonstrates that these temperatures better represent sample temperatures in the older units used to quantify IMPROVE carbon fractions from 1987 through 2004. Only the thermal fractions are affected by changes in temperature. The OC and EC by TOR are insensitive to the change in temperature protocol, and therefore the long-term consistency of the IMPROVE database is conserved. A method to detect small quantities of O2 in the pure He carrier gas shows that O2 levels above 100 ppmv also affect the comparability of thermal carbon fractions but have little effect on the IMPROVE_TOR split between OC and EC.     

Identification of haze-creating sources from fine particulate matter in Dhaka aerosol using carbon fractions

Fine particulate matter (PM_2.5) samples were simultaneously collected on Teflon and quartz filters between February 2010 and February 2011 at an urban monitoring site (CAMS2) in Dhaka, Bangladesh. The samples were collected using AirMetrics MiniVol samplers. The samples on Teflon filters were analyzed for their elemental composition by PIXE and PESA. Particulate carbon on quartz filters was analyzed using the IMPROVE thermal optical reflectance (TOR) method that divides carbon into four organic carbons (OC), pyrolized organic carbon (OP), and three elemental carbon (EC) fractions. The data were analyzed by positive matrix factorization using the PMF2 program. Initially, only total OC and total EC were included in the analysis and five sources, including road dust, sea salt and Zn, soil dust, motor vehicles, and brick kilns, were obtained. In the second analysis, the eight carbon fractions (OC1, OC2, OC3, OC4, OP, EC1, EC2, EC3) were included in order to ascertain whether additional source information could be extracted from the data. In this case, it is possible to identify more sources than with only total OC and EC. The motor vehicle source was separated into gasoline and diesel emissions and a fugitive Pb source was identified. Brick kilns contribute 7.9 μg/m³ and 6.0 μg/m³ of OC and EC, respectively, to the fine particulate matter based on the two results. From the estimated mass extinction coefficients and the apportioned source contributions, soil dust, brick kiln, diesel, gasoline, and the Pb sources were found to contribute most strongly to visibility degradation, particularly in the winter.

Implications: Fine particle concentrations in Dhaka, Bangladesh, are very high and cause significant degradation of urban visibility. This work shows that using carbon fraction data from the IMPROVE OC/EC protocol provides improved source apportionment. Soil dust, brick kiln, diesel, gasoline, and the Pb sources contribute strongly to haze, particularly in the winter.     

The IMPROVE_A Temperature Protocol for Thermal/Optical Carbon Analysis: Maintaining Consistency with a Long-Term Database

Abstract

Thermally derived carbon fractions including organic carbon (OC) and elemental carbon (EC) have been reported for the U.S. Interagency Monitoring of PROtected Visual Environments (IMPROVE) network since 1987 and have been found useful in source apportionment studies and to evaluate quartz-fiber filter adsorption of organic vapors. The IMPROVE_A temperature protocol defines temperature plateaus for thermally derived carbon fractions of 140 °C for OC1, 280 °C for OC2, 480 °C for OC3, and 580 °C for OC4 in a helium (He) carrier gas and 580 °C for EC1, 740 °C for EC2, and 840 °C for EC3 in a 98% He/2% oxygen (O₂) carrier gas. These temperatures differ from those used previously because new hardware used for the IMPROVE thermal/optical reflectance (IMPROVE_TOR) protocol better represents the sample temperature than did the old hardware. A newly developed temperature calibration method demonstrates that these temperatures better represent sample temperatures in the older units used to quantify IMPROVE carbon fractions from 1987 through 2004. Only the thermal fractions are affected by changes in temperature. The OC and EC by TOR are insensitive to the change in temperature protocol, and therefore the long-term consistency of the IMPROVE database is conserved. A method to detect small quantities of O₂ in the pure He carrier gas shows that O₂ levels above 100 ppmv also affect the comparability of thermal carbon fractions but have little effect on the IMPROVE_TOR split between OC and EC.     

Quantification of elemental and total carbon in combustion particulate matter using thermal-oxidative analysis

The use of a two-step thermal-oxidative analysis (TOA) technique for quantification of the mass of total carbon (TC) and elemental carbon (EC) of turbine engine-borne particulate matter (PM) has been evaluated. This approach could be used in lieu of analysis methods which were developed to characterize diluted PM. This effort is of particular interest as turbine engine PM emissions typically have a higher EC content than ambient aerosols, and filter sample mass loadings can be significantly greater than recommended for existing analysis techniques. Analyses were performed under a pure oxygen environment using a two-step temperature profile; reference carbon and actual PM samples were used to identify appropriate analysis conditions. Thermal gravimetric analysis (TGA) methods were used to provide guidance on the nature of the carbon in several of the materials. This was necessary as a standard reference material does not exist for determination of the EC fraction in PM. The TGA also assisted in identifying an appropriate temperature range for the first-stage of the TOA method. Quantification of TC and EC for turbine engine PM samples using TOA was compared to results obtained using the NIOSH 5040 thermal-optical method. For first-stage TOA temperatures of 350°C and 400°C, excellent agreement between the techniques was observed in both the quantified TC and EC, supporting the viability for using TOA for analysis of turbine engine PM samples. A primary benefit of using TOA for these types of PM samples is that filters with relatively high PM mass loadings (sampled at the emission source) can be readily analyzed. In addition, an entire filter sample can be evaluated, as compared to the use of a filter punch sample for the NIOSH technique. While the feasibility of using a TOA method for engine PM samples has been demonstrated, future studies to estimate potential OC charring and oxidation of EC-type material may provide additional data to assess its impact on the OC/EC fractions for other carbon-type measurements.

Implications: This work presents results and procedures of an analytical method for the determination of total and elemental carbon, i.e., TC and EC present in combustion source particulate matter samples. In general, it is shown that the LECO TOA methodology is as reliable and comprehensive as NIOSH 5040 for determining TC and EC carbon types in particulate matter present in turbine emission sources, and should be considered as an alternative. Principles of the methodology, differences, and corresponding agreement with the standard NIOSH 5040 method and TGA analysis are discussed.     

Evaluation of the thermal/optical reflectance method for quantification of elemental carbon in sediments

The IMPROVE thermal/optical reflectance (TOR) method, commonly used for EC quantification in atmospheric aerosols, is applied to soils and sediments and compared with a thermochemical method commonly applied to these non-atmospheric samples. TOR determines elemental carbon (EC) by an optical method, but it also yields thermally defined EC fractions in a 2% O2/98% He oxidizing atmosphere at 550 degrees C (EC1), 700 degrees C (EC2), and 800 degrees C (EC3). Replicate TOR TC, OC, and EC values exhibited precisions of approximately +/-10% as determined from multiple analyses of the same samples. EC abundances relative to total mass concentrations were within the ranges reported by other methods for diesel exhaust soot, n-hexane soot, wood and rice chars, and coals, as well as for environmental matrices. A direct comparison with the chemothermal (CTO) method of Gustafson et al. for ten soil and sediment samples demonstrated that almost all of the OC and EC1 are eliminated, as is part of the EC2. The CTO soot carbon is bounded by the EC3 and EC2+EC3 fractions of the IMPROVE TOR analysis. It might be possible to adjust these fractions to obtain better agreement between atmospheric aerosol and soil/sediment analysis methods. Given its linking the EC measurement in the atmosphere to sediments, the TOR method will not only provide useful information on the explanation and comparison between different environmental matrices, but also can be used to derive information on global cycling of EC.     

Evaluation of the thermal/optical reflectance method for discrimination between char- and soot-EC

Many optical, thermal and chemical methods exist for the measurement of elemental carbon (EC) but are unable or neglect to differentiate between the different forms of EC such as char- or soot-EC. The thermal/optical reflectance (TOR) method applies different temperatures for measuring EC and organic carbon (OC) contents through programmed, progressive heating in a controlled atmosphere, making available eight separate carbon fractions - four OC, one pyrolyzed organic carbon, and three EC. These fractions were defined by temperature protocol, oxidation atmosphere, and laser-light reflectance/transmittance. Stepwise thermal evolutional oxidation of the TOR method makes it possible to distinguish char- from soot-EC. In this study, different EC reference materials, including char and soot, were used for testing it. The thermograms of EC reference materials showed that activation energy is lower for char- than soot-EC. Low-temperature EC1 (550 degrees C in a 98% He/2% O2 atmosphere) is more abundant for char samples. Diesel and n-hexane soot samples exhibit similar EC2 (700 degrees C in a 98% He/2% O2 atmosphere) peaks, while carbon black samples peaks at both EC2 and EC3 (800 degrees C in a 98% He/2% O2 atmosphere). These results supported the use of the TOR method to discriminate between char- and soot-EC.     

Variations in speciated emissions from spark-ignition and compression-ignition motor vehicles in California's south coast air basin

The U.S. Department of Energy Gasoline/Diesel PM Split Study examined the sources of uncertainties in using an organic compound-based chemical mass balance receptor model to quantify the contributions of spark-ignition (SI) and compression-ignition (CI) engine exhaust to ambient fine particulate matter (PM2.5). This paper presents the chemical composition profiles of SI and CI engine exhaust from the vehicle-testing portion of the study. Chemical analysis of source samples consisted of gravimetric mass, elements, ions, organic carbon (OC), and elemental carbon (EC) by the Interagency Monitoring of Protected Visual Environments (IMPROVE) and Speciation Trends Network (STN) thermal/optical methods, polycyclic aromatic hydrocarbons (PAHs), hopanes, steranes, alkanes, and polar organic compounds. More than half of the mass of carbonaceous particles emitted by heavy-duty diesel trucks was EC (IMPROVE) and emissions from SI vehicles contained predominantly OC. Although total carbon (TC) by the IMPROVE and STN protocols agreed well for all of the samples, the STN/IMPROVE ratios for EC from SI exhaust decreased with decreasing sample loading. SI vehicles, whether low or high emitters, emitted greater amounts of high-molecular-weight particulate PAHs (benzo[ghi]perylene, indeno[1,2,3-cd]pyrene, and coronene) than did CI vehicles. Diesel emissions contained higher abundances of two- to four-ring semivolatile PAHs. Diacids were emitted by CI vehicles but are also prevalent in secondary organic aerosols, so they cannot be considered unique tracers. Hopanes and steranes were present in lubricating oil with similar composition for both gasoline and diesel vehicles and were negligible in gasoline or diesel fuels. CI vehicles emitted greater total amounts of hopanes and steranes on a mass per mile basis, but abundances were comparable to SI exhaust normalized to TC emissions within measurement uncertainty. The combustion-produced high-molecular-weight PAHs were found in used gasoline motor oil but not in fresh oil and are negligible in used diesel engine oil. The contributions of lubrication oils to abundances of these PAHs in the exhaust were large in some cases and were variable with the age and consumption rate of the oil. These factors contributed to the observed variations in their abundances to total carbon or PM2.5 among the SI composition profiles.     

Separation of brown carbon from black carbon for IMPROVE and Chemical Speciation Network PM2.5 samples

The replacement of the Desert Research Institute (DRI) model 2001 with model 2015 thermal/optical analyzers (TOAs) results in continuity of the long-term organic carbon (OC) and elemental carbon (EC) database, and it adds optical information with no additional carbon analysis effort. The value of multiwavelength light attenuation is that light absorption due to black carbon (BC) can be separated from that of brown carbon (BrC), with subsequent attribution to known sources such as biomass burning and secondary organic aerosols. There is evidence of filter loading effects for the 25% of all samples with the highest EC concentrations based on the ratio of light attenuation to EC. Loading corrections similar to those used for the seven-wavelength aethalometer need to be investigated. On average, nonurban Interagency Monitoring of PROtected Visual Environments (IMPROVE) samples show higher BrC fractions of short-wavelength absorption than urban Chemical Speciation Network (CSN) samples, owing to greater influence from biomass burning and aged aerosols, as well as to higher primary BC contributions from engine exhaust at urban sites. Sequential samples taken during an Everglades National Park wildfire demonstrate the evolution from flaming to smoldering combustion, with the BrC fraction increasing as smoldering begins to dominate the fire event.

Implications: The inclusion of seven wavelengths in thermal/optical carbon analysis of speciated PM_2.5 (particulate matter with an aerodynamic diameter ≤2.5 μm) samples allows contributions from biomass burning and secondary organic aerosols to be estimated. This separation is useful for evaluating control strategy effectiveness, identifying exceptional events, and determining natural visibility conditions.     

Comparison of continuous and filter-based carbon measurements at the Fresno supersite

Results from six continuous and semicontinuous black carbon (BC) and elemental carbon (EC) measurement methods are compared for ambient samples collected from December 2003 through November 2004 at the Fresno Supersite in California. Instruments included a multi-angle absorption photometer (MAAP; lambda = 670 nm); a dual-wavelength (lambda = 370 and 880 nm) aethalometer; seven-color (lambda = 370, 470, 520, 590, 660, 880, and 950 nm) aethalometers; the Sunset Laboratory carbon aerosol analysis field instrument; a photoacoustic light absorption analyzer (lambda = 1047 nm); and the R&P 5400 ambient carbon particulate monitor. All of these acquired BC or EC measurements over periods of 1 min to 1 hr. Twenty-four-hour integrated filter samples were also acquired and analyzed by the Interagency Monitoring of Protected Visual Environments (IMPROVE) thermal/optical reflectance carbon analysis protocol. Site-specific mass absorption efficiencies estimated by comparing light absorption with IMPROVE EC concentrations were 5.5 m2/g for the MAAP, 10 m2/g for the aethalometer at a wavelength of 880 nm, and 2.3 m2/g for the photoacoustic analyzer; these differed from the default efficiencies of 6.5, 16.6, and 5 m2/g, respectively. Scaling absorption by inverse wavelength did not provide equivalent light absorption coefficients among the instruments for the Fresno aerosol measurements. Ratios of light absorption at 370 nm to those at 880 nm from the aethalometer were nearly twice as high in winter as in summer. This is consistent with wintertime contributions from vehicle exhaust and from residential wood combustion, which is believed to absorb more shorter-wavelength light. To reconcile BC and EC measurements obtained by different methods, a better understanding is needed of the wavelength dependence of light-absorption and mass-absorption efficiencies and how they vary with different aerosol composition.     

Results of the “carbon conference” international aerosol carbon round robin test stage I

An international round robin test on the analysis of carbonaceous aerosols on quartz fiber filters sampled at an urban site was organized by the Vienna University of Technology. Seventeen laboratories participated using nine different thermal and optical methods. For the analysis of total carbon (TC), a good agreement of the values obtained by all laboratories was found (7 and 9% r.s.d.) with only two outliers in the complete data set. In contrast the results of the determination of elemental carbon (EC) in two not pre-extracted samples were highly variable ranging over more than one order of magnitude and the relative standard deviations (r.s.d.) of the means were 36.6 and 45.5%. The laboratories that obtained similar results by using methods which reduce the charring artifact were put together to a new data set in order to approach a “real EC” value. The new data set consisting of the results of 10 laboratories using seven different methods showed 16 and 24% lower averages and r.s.d. of 14 and 24% for the two not pre-extracted samples. Taking the current filters as “equivalents” for urban aerosol samples we conclude that the following methods can be used for the analysis of EC in carbonaceous aerosols: thermal methods with an optical feature to correct for charring during pyrolysis, two-step thermal procedures reducing charring during pyrolysis, the VDI 2465/1 method (removal of OC by solvent extraction and thermodesorption in nitrogen) and the VDI 2465/2 method (combustion of OC and EC at different temperatures) with an additional pre-extraction with a dimethyl formamide (DMF)/toluene mixture. Only thermal methods without any correction for charring during pyrolysis and the VDI 2465/2 method were outside the range of twice the standard deviation of the new data set. For a filter sample pre-extracted with the DMF/toluene mixture the average and r.s.d. from all laboratories (20.7 μgC; 24.4% r.s.d.) was very similar as for the laboratory set reduced to 10 laboratories (20.6 μgC; 19% r.s.d.). Thus DMF pre-extraction appears to improve the performance of the thermal methods without charring during pyrolysis control, e.g. the VDI 2465/2 methods.     

Characterization of carbon fractions for atmospheric fine particles and nanoparticles in a highway tunnel

Fine particles (PM_2.5) and nanoparticles (PM_0.1) were sampled using Dichotomous sampler and MOUDI, respectively, in Xueshan Tunnel, Taiwan. Eight carbon fractions were analyzed using IMPROVE thermal-optical reflectance (TOR) method. The concentrations of different temperature carbon fractions (OC1–OC4, EC1–EC3) in both PM_2.5 and PM_0.1 were measured and the correlations between OC and EC were discussed. Results showed that the ratios of OC/EC were 1.26 and 0.67 for PM_2.5 and PM_0.1, respectively. The concentration of EC1 was found to be more abundant than other elemental carbon fractions in PM_2.5, while the most abundant EC fraction in PM_0.1 was found to be EC2. The variation of contributions for elemental carbon fractions was different among PM_2.5 and PM_0.1 samples, which was partly owing to the metal catalysts for soot oxidation. The correlations between char-EC and soot-EC showed that char-EC dominated EC in PM_2.5 while soot-EC dominated EC in PM_0.1. Using eight individual carbon fractions, the gasoline and diesel source profiles of PM_0.1 and PM_2.5 were extracted and analyzed with the positive matrix factorization (PMF) method.     

An evaluation of measurement methods for organic, elemental and black carbon in ambient air monitoring sites

The carbonaceous components of Particulate Matter samples form a substantial fraction of their total mass, but their quantification depends strongly on the instruments and methods used. United Kingdom monitoring networks have provided many relevant data sets that are already in the public domain. Specifically, hourly organic carbon (OC) and elemental carbon (EC) were determined at four sites between 2003 and 2007 using Rupprecht and Pattashnik (R & P) 5400 automatic instruments. Since 2007, daily OC/EC measurements have been made by manual thermo-optical analysis of filter samples using a Sunset Laboratory Carbon Aerosol Analysis instrument. In parallel, long term daily measurements of Black Smoke, a quantity directly linked to black carbon (measured by aethalometers) and indirectly related to elemental carbon, have been made at many sites. The measurement issues associated with these techniques are evaluated in the context of UK measurements, making use of several sets of parallel data, with the aim of aiding the interpretation of network results. From the results available, the main conclusions are that the R & P 5400 instruments greatly under-read EC and total carbon (TC = OC + EC) at kerbside sites, probably due to the fact that the smaller particles are not sampled by the instrument; the R & P 5400 instrument is inherently difficult to characterise, so that all quantitative results need to be treated with caution; both aethalometer and Black Smoke (converted to black carbon) measurements can show reasonable agreement with elemental carbon results; and manual thermo-optical OC/EC results may under-read EC (and hence over-read OC), whether either transmittance or reflectance is used for the pyrolysis correction, and this effect is significant at rural sites.     

Evaluations of the chemical mass balance method for determining contributions of gasoline and diesel exhaust to ambient carbonaceous aerosols

The US. Department of Energy Gasoline/Diesel PM Split Study was conducted to assess the sources of uncertainties in using an organic compound-based chemical mass balance receptor model to quantify the relative contributions of emissions from gasoline (or spark ignition [SI]) and diesel (or compression ignition [CI]) engines to ambient concentrations of fine particulate matter (PM2.5) in California's South Coast Air Basin (SOCAB). In this study, several groups worked cooperatively on source and ambient sample collection and quality assurance aspects of the study but worked independently to perform chemical analysis and source apportionment. Ambient sampling included daily 24-hr PM2.5 samples at two air quality-monitoring stations, several regional urban locations, and along freeway routes and surface streets with varying proportions of automobile and truck traffic. Diesel exhaust was the dominant source of total carbon (TC) and elemental carbon (EC) at the Azusa and downtown Los Angeles, CA, monitoring sites, but samples from the central part of the air basin showed nearly equal apportionments of CI and SI. CI apportionments to TC were mainly dependent on EC, which was sensitive to the analytical method used. Weekday contributions of CI exhaust were higher for Interagency Monitoring of Protected Visual Environments (IMPROVE; 41+/-3.7%) than Speciation Trends Network (32+/-2.4%). EC had little effect on SI apportionment. SI apportionments were most sensitive to higher molecular weight polycyclic aromatic hydrocarbons (indeno[123-cd]pyrene, benzo(ghi)perylene, and coronene) and several steranes and hopanes, which were associated mainly with high emitters. Apportionments were also sensitive to choice of source profiles. CI contributions varied from 30% to 60% of TC when using individual source profiles rather than the composites used in the final apportionments. The apportionment of SI vehicles varied from 1% to 12% of TC depending on the specific profile that was used. Up to 70% of organic carbon (OC) in the ambient samples collected at the two fixed monitoring sites could not be apportioned to directly emitted PM emissions.     

PM2.5 carbon measurements in two urban areas: Seoul and Kwangju,Korea

24-h PM_2.5 carbonaceous samples were collected between 27 November and 9 December 1999 in Seoul, and between 7 and 20 June 2000 in Kwangju to investigate characteristics of carbonaceous species, and the relationship between elemental carbon (EC) and Aethalometer-based black carbon (BC) measurements. 5-min PM_2.5 BC and criteria air pollutant data were also measured using the Aethalometer and ambient air monitoring system. The PM_2.5 samples were analyzed for EC and OC using a selective thermal manganese dioxide oxidation (TMO) method. The daily average EC and OC concentrations in Seoul were higher in the winter than in the summer (Atmos. Environ. 35 (2001a) 657). It was found that difference between ambient BC levels in the two cities was not directly proportional to the population ratio (∼8) or diesel traffic ratio (∼5.9) since particulate matter or BC concentration is strongly influenced by a result of varying traffic and meteorological conditions at the site. Using the primary OC/EC ratio approach, the results suggest that most of the measured OC in Kwangju is of primary origin during the summer. In Seoul, the observed OC includes additional secondary organic aerosol during the wintertime conditions. The relationship between the 24-h TMO-EC and Aethalometer BC measurements in PM_2.5 reflected very good agreement for the two urban sites, with correlation coefficients of R²=0.99 and 0.92, and BC/EC slopes of 0.93 and 1.07, respectively. It was found that comparing TMO-EC to BC at a different location in Korea, a different scaling factor was needed.     

Source apportionment of fine particles in Washington, DC, utilizing temperature-resolved carbon fractions

Integrated ambient particulate matter < or =2.5 microm in aerodynamic diameter (PM2.5) samples were collected at a centrally located urban monitoring site in Washington, DC, on Wednesdays and Saturdays using Interagency Monitoring of Protected Visual Environments samplers. Particulate carbon was analyzed using the thermal optical reflectance method that divides carbon into four organic carbon fractions, pyrolyzed organic carbon, and three elemental carbon fractions. A total of 35 variables measured in 718 samples collected between August 1988 and December 1997 were analyzed. The data were analyzed using Positive Matrix Factorization and 10 sources were identified: sulfate (SO4(2-))-rich secondary aerosol I (43%), gasoline vehicle (21%), SO4(2-)-rich secondary aerosol II (11%), nitrate-rich secondary aerosol (9%), SO4(2-)-rich secondary aerosol III (6%), incinerator (4%), aged sea salt (2%), airborne soil (2%), diesel emissions (2%), and oil combustion (2%). In contrast to a previous study that included only total organic carbon and elemental carbon fractions, motor vehicles were separated into fractions identified as gasoline vehicle and diesel emissions containing carbon fractions whose abundances were different between the two sources. This study indicates that the temperature-resolved carbon fraction data can be utilized to enhance source apportionment, especially with respect to the separation of diesel emissions from gasoline vehicle sources. Conditional probability functions using surface wind data and deduced source contributions aid in the identifications of local sources.     

Determination of the organic aerosol mass to organic carbon ratio in IMPROVE samples

The ratio of organic mass (OM) to organic carbon (OC) in PM(2.5) aerosols at US national parks in the IMPROVE network was estimated experimentally from solvent extraction of sample filters and from the difference between PM(2.5) mass and chemical constituents other than OC (mass balance) in IMPROVE samples from 1988 to 2003. Archived IMPROVE filters from five IMPROVE sites were extracted with dichloromethane (DCM), acetone and water. The extract residues were weighed to determine OM and analyzed for OC by thermal optical reflectance (TOR). On average, successive extracts of DCM, acetone, and water contained 64%, 21%, and 15%, respectively, of the extractable OC, respectively. On average, the non-blank-corrected recovery of the OC initially measured in these samples by TOR was 115+/-42%. OM/OC ratios from the combined DCM and acetone extracts averaged 1.92 and ranged from 1.58 at Indian Gardens, AZ in the Grand Canyon to 2.58 at Mount Rainier, WA. The average OM/OC ratio determined by mass balance was 2.07 across the IMPROVE network. The sensitivity of this ratio to assumptions concerning sulfate neutralization, water uptake by hygroscopic species, soil mass, and nitrate volatilization were evaluated. These results suggest that the value of 1.4 for the OM/OC ratio commonly used for mass and light extinction reconstruction in IMPROVE is too low.     

Characterization of the Sunset Semi-Continuous Carbon Aerosol Analyzer

Abstract

The field-deployable Sunset Semi-Continuous Organic Carbon/Elemental Carbon (Sunset OCEC) aerosol analyzer utilizes the modified National Institute for Occupational Safety and Health thermal-optical method to determine total carbon (TC), organic carbon (OC), and elemental carbon (EC) at near real-time. Two sets of OC and EC are available: thermal OC and EC, and optical OC and EC. The former is obtained by the thermal-optical approach, and the latter is obtained by directly determining EC optically and deriving optical OC from TC. However, the performance of the Sunset OCEC is not yet fully characterized. Two collocated Sunset OCEC analyzers, Unit A and Unit B, were used to determine the pooled relative standard deviation (RSD) and limit of detection (LOD) between September 18 and November 6, 2007 in Richland, WA. The LOD of Unit A was approximately 0.2 μgC/m³ (0.1 μgC/cm²) for TC, optical OC, and thermal OC, and 0.01 μgC/m³ (0.01 μgC/cm²) for optical EC. Similarly, Unit B had an LOD of approximately 0.3 μgC/m³ (0.2 μgC/cm²) for TC, optical OC, and thermal OC, and 0.02 μgC/m³ (0.01 μgC/cm²) for optical EC. The LOD for thermal EC is estimated to be 0.2 μgC/m³ (0.1 μgC/cm²) for both units. The pooled RSDs were 4.9% for TC (carbon mass loadings 0.6–6.0 μgC/cm²), 5.6% for optical OC (carbon mass loadings 0.6–5.4 μgC/cm²), 5.3% for thermal OC (carbon mass loadings 0.6–5.3 μgC/cm²), and 9.6% for optical EC (carbon mass loadings 0–1.4 μgC/cm²), which indicates good precision between the instruments. The RSD for thermal EC is higher at 24.3% (carbon mass loadings 0–1.2 μgC/cm²). Low EC mass loadings in Richland contributed to the poor RSD of EC. The authors found that excessive noise from the nondispersive infrared (NDIR) laser in the Sunset OCEC analyzer could result in a worsened determination of OC and EC. It is recommended that a “quieter” NDIR laser and detector be used in the Sunset OCEC analyzer to improve quantification. Future work should re-evaluate the precision of the EC parameters in an environment favorable for EC collection. Investigation among quantification differences using various thermal-optical protocols to determine OC and EC is also in need.