Grantee Research Project Results
2006 Progress Report: Measurement, Modeling and Analysis Methods for Airborne Carbonaceous Fine Particulate Matter (PM2.5)
EPA Grant Number: R831086Title: Measurement, Modeling and Analysis Methods for Airborne Carbonaceous Fine Particulate Matter (PM2.5)
Investigators: Chow, Judith C. , Chen, Lung-Wen Antony , Chakrabarty, Rajan K. , Watson, John L. , Barber, Peter W. , Moosmuller, Hans , Arnott, Pat
Current Investigators: Chow, Judith C. , Chen, Lung-Wen Antony , Watson, John L. , Arnott, William P. , Barber, Peter W. , Paredes-Miranda, Guadalupe , Moosmuller, Hans
Institution: Desert Research Institute
EPA Project Officer: Chung, Serena
Project Period: September 1, 2003 through August 31, 2006 (Extended to August 31, 2008)
Project Period Covered by this Report: September 1, 2005 through August 31, 2006
Project Amount: $449,456
RFA: Measurement, Modeling, and Analysis Methods for Airborne Carbonaceous Fine Particulate Matter (PM2.5) (2003) RFA Text | Recipients Lists
Research Category: Air , Air Quality and Air Toxics , Particulate Matter
Objective:
The overall objectives for this project are to: (1) reconcile different thermal/optical methods by determining factors that influence organic carbon (OC) and elemental carbon (EC) split; (2) specify how optical properties differ and change between particles in the air, particles on a filter, and particles undergoing changes owing to thermal analysis; (3) quantify differences in thermal carbon fractions determined by commonly used thermal and optical analysis methods; and (4) optimize thermal and optical monitoring methods to meet multiple needs of health, visibility, global climate, and source apportionment.
Progress Summary:
Results from the previous year indicated the need for a more general system for synthesizing standard samples of diesel soot, wood smoke, and carbon black particles on quartz-fiber filters to evaluate common thermal/optical methods (Thermal Optical Reflectance [TOR] and Thermal Optical Transmittance [TOT]), including the IMPROVE_A protocol developed last year, the Speciation Trend Network (STN)-TOT method, and the French two-step method that is widely used in Europe. A major achievement in this period was the development of an aerosol generation and collection system. This system generates carbon aerosols from various combustion sources, simulates atmospheric aging through a dilution sampling tunnel, and permits simultaneous continuous measurements and integrated sample collection. Combustion sources include: (1) an Onan Cummins diesel generator; (2) acetylene flame; (3) an electric arc generator (PALAS GFG-1000 with graphite rods); and (4) wood smoke (white oak) from a wood-burning stove. Inorganic salts (e.g., sodium chloride) can be generated through a nebulizer and mixed with the combustion aerosol in the dilution tunnel. The setup is configured to produce variable, but reproducible, carbon loadings. This allows for examination of thermochemical properties for the same source aerosol with different concentration levels, and for single aerosol types as well as mixtures.
Tests performed to date have acquired samples of diesel exhaust (35 pure and 9 mixed with NaCl), acetylene flame (10 pure and 9 mixed with NaCl), electric arc (13 pure and 9 mixed with NaCl) and wood smoke (23 pure and 14 mixed with NaCl) on quartz-fiber filters. NaCl was added to examine the catalytic effects of a mixed aerosol on light absorption (babs) and EC measurements. Three carbon black samples were also nebulized using a monomodal aerosol generator. Portions of the quartz-fiber filters were analyzed by three commonly used thermal evolution protocols: IMPROVE_A (TOR/TOT), STN-TOT, and the French two-step protocols. EC/total carbon (TC) ratios measured by thermal/optical methods showed consistency within each source type, as well as diversity between source types. The STN and French two-step protocols yielded EC/TC ratios similar to (within ±5%) those of the IMPROVE_A protocol for diesel soot (EC/TC ~60%), acetylene flame soot (~96%), and electric arc soot (~50%). The French two-step and STN protocols yielded lower EC (86% and 46%, respectively) for wood smoke compared to the IMPROVE_A protocol. The presence of NaCl caused EC to be released at lower temperatures, and was limited by the presence of oxygen (O2) and the charring correction. While it affects the abundance in the EC fractions, it does not affect the OC/EC split in the IMPROVE_A analysis (Figure 1) and STN protocols. The French two-step protocol that operates in pure O2, without charring corrections, reported 60 to 90% lower EC than IMPROVE_A (TOR) for all 19 samples. When comparing the IMPROVE_A EC to photoacoustic (1047 nm) babs, the EC σabs (1047 nm) varied by approximately 50 percent in the range of 2.7 to 5.3 m2/g among the different source types. Only the absorption efficiency of diesel soot is in good agreement (within ±15%) with the model predictions based on Mie scattering theory and volumetric mixing. There appears to be no universal conversion factor that can be applied to convert babs to EC concentrations. These results will be presented in several publications currently under preparation.
IMPROVE_A Protocol (Front Filter)
Figure 1. Mass Percentage of Thermally Separated Carbon Fractions in PM
The optical modeling for thermal/optical analysis based on Chen, et al. (2004) was improved. The previous algorithm used an empirical approach to estimate the multiple-scattering effects, which was replaced with a more general algorithm based on Monte Carlo simulations. This method describes local rules for photon propagation that are expressed, in the simplest case, as probability distributions describing the step size of photon movement between sites of photon-medium interaction, and the angles of deflection in a photon’s trajectory when a scattering event occurs (Wang, et al., 1995; Chen and Bai, 1998). The Monte Carlo method is probably the only way to handle radiative transfer with complex geometry (such as that encountered in a conventional thermal/optical analyzer) and requires very high accuracy. However, since the method is statistical in nature, it relies on calculating the propagation of a large number of photons and requires a large amount of computation time. We have applied this method to calculate the filter reflectance and transmittance as measured by a photoelectric sensor during thermal analysis. A filter sample is described as a two-layer turbid medium. The top layer contains pyrolyzed organics, charring (optical pyrolysis [OP] and EC), while the second layer contains exclusively OP.
The calculation demonstrates that continuous filter-based absorption measurements, such as the aethalometer and multi-angle absorption photometer (MAAP), still do not fully address the multiple scattering and loading effects. The reflectance is insensitive to absorption in the second layer, and therefore absorption by the top and bottom layer can be retrieved separately from the reflectance and transmittance measurements. This model of the filters, combined with the Monte-Carlo simulation, explains qualitatively the observations during thermal analysis (i.e., the darkening and whitening of the filters, and the observed changes in filter reflectance and transmittance). The simulation also suggests that an accurate charring (for OC/EC split) correction should be made when the absorption, rather than reflectance or transmittance, returns to its initial value. This absorption could be estimated from simultaneous measurements of reflectance and transmittance. More extensive comparisons with experimental data are needed. Particularly, the optical depth of the blank filter and particle-layer thickness should be quantified more accurately. An integrating sphere and a polar nephelometer (Figure 2) were developed to determine the hemispheric and angular transmittance to confine the model predictions.
Figure 2. Integrating Sphere (Upper) and Polar Nephelometer (Lower) Setup to Determine the Hemispheric and Angular Transmittance, Respectively, of Blank Filters and Particle-Loaded Filters Prior to or Retrieved From Thermal Analysis.
Summer and winter intensive observation periods (IOP) were carried out at the Fresno Supersite during 2005 to: (1) acquire filter samples representing typical urban atmospheres, in contrast to standard samples from the aerosol generator; (2) inter-compare the continuous and integrated babs and EC measurements at Fresno; and (3) evaluate the range of uncertainty involved in the EC absorption efficiency, i.e., σabs (m2/g), estimates due to the different measurement and analysis methods. A total of 18 and 25 samples collected by Hi-Vol and Federal Reference Method (FRM) Reference Ambient Aerosol Sampler (RAAS) , respectively, during the summer IOP, and a total of 14 filters acquired by Hi-Vol and RAAS (7 and 7, respectively) during the winter IOP, were analyzed by the IMPROVE_A and STN protocols, and a subset of the Hi-Vol samples (8 during summer and 7 during winter) were analyzed by the French two-step protocol. Continuous babs was acquired by two aethalometers, a Particle Soot Absorption Photometer PSAP (summer only), a MAAP, and photoacoustic sensors at 532 nm (summer only) and at 1047 nm.
Using the IMPROVE_A protocol, the EC/TC ratios at the Fresno Supersite were 0.22 ± 0.04 and 0.26 ± 0.05 for summer and winter IOPs, respectively. The EC/TC ratio and carbon fractions during winter were all close to those in wood smoke. The σabs (1047 nm) of EC during the winter IOP (2.5 m2/g) was also similar to that of wood smoke EC (2.7 m2/g). This implies that the IMPROVE_A protocol does reveal a consistent pattern of thermal carbon fractions for a specific source, and that may be used for source apportionment. The value of spectral absorption exponent, α, in the Angstrom Power Law, determined by the aethalometer during the summer IOP (0.95 ± 0.04) was 10 percent to 20 percent higher than that observed for diesel and acetylene flame soot (0.79 ± 0.09 to 0.86 ± 0.12), from both pure source aerosol and when mixed with NaCl. This indicates that the summer aerosol at Fresno, while being influenced by diesel emissions, might be mixed with aged or secondary aerosols. The α during the winter period (1.2 ± 0.11) was closer to that observed for emissions from wood combustion (1.2 ± 0.51). Despite the potential bias in the aethalometer, this study confirms a higher α for wood smoke than for diesel soot.
References:
Wang LH, Jacques SL. Analysis of diffusion theory and similarity relations. In: Chance B, Alfano RR, eds. Photon Migration and Imaging in Random Media and Tissues. Proceedings SPIE, 1993, pp. 107-116.
Wang LH, Jacques SL, Zheng LQ. MCML - Monte Carlo modeling of photon transport in multi-layered tissues. Computer Methods and Programs in Biomedicine 1995;47:131-146.
Wang LH, Jacques SL, Zheng LQ. MCML - Monte Carlo modeling of photon transport in multi-layered tissues. Computer Methods and Programs in Biomedicine 1995;47:131-146.
Chen NG, Bai J. Monte Carlo approach to modeling of boundary conditions for the diffusion equation. Physical Reviews Letters 1998;80(24):5321-5325.
Kopp C, Petzold A, Niessner R. Investigation of the specific attenuation cross-section of aerosols deposited on fiber filters with a polar photometer to determine black carbon. Journal of Aerosol Science 1999;30(9):1153-1163.
Chen L-WA, Chow JC, Watson JG, Moosmüller H, Arnott WP. Modeling reflectance and transmittance of quartz-fiber filter samples containing elemental carbon particles: implications for thermal/optical analysis. Journal of Aerosol Science 2004;35(6):765-780 doi:10.1016/j.jaerosci.2003.12.005.
Chow JC, Watson JG, Chen L-WA, Arnott WP, Moosmüller H, Fung KK. Equivalence of elemental carbon by thermal/optical reflectance and transmittance with different temperature protocols. Environmental Science & Technology 2004;38(16):4414-4422.
Arnott WP, Hamasha K, Moosmüller H, Sheridan PJ, Ogren JA. Towards aerosol light-absorption measurements with a 7-wavelength Aethalometer: evaluation with a photoacoustic instrument and 3-wavelength nephelometer. Aerosol Science Technology 2005;39(1):17-29, ISI:000225877400002.
Sheridan PJ, Arnott WP, Ogren JA, Andrews E, Atkinson DB, Covert DS, Moosmüller H, Petzold A, Schmid B, Strawa AW, Varma R, Virkkula A. The Reno aerosol optics study: an evaluation of aerosol absorption measurement methods. Aerosol Science Technology 2005;39(1):1-16, ISI:000225877400001.
Virkkula A, Ahlquist NC, Covert DS, Arnott WP, Sheridan PJ, Quinn PK, Coffman DJ. Modification, calibration and a field test of an instrument for measuring light absorption by particles. Aerosol Science Technology 2005;39(1):68-83, ISI:000225877400006.
Watson JG, Chow JC, Chen L-WA. Summary of organic and elemental carbon/black carbon analysis methods and intercomparisons. Aerosol and Air Quality Research 2005;5(1):65-102.
Future Activities:
Future activities for this research project include the following goals:
- Better determine the necessary parameters required in the Monte Carlo Multi-Layer MCML radiative transfer model using integrating sphere and polar nephelometer measurements. Evaluate the improved optical model with source and ambient samples. Determine how well reflectance and transmittance can be predicted by the model and how the OC/EC split may be improved.
- Evaluate the influence of organic sampling artifacts on OC/EC split. Develop methods to mitigate this influence during thermal/optical analysis (e.g., using different types of sampling substrates).
- Improve the current optical model to consider the morphology, internal mixing and fractal dimension of carbonaceous aerosols and examine the relative importance of these factors with respect to the quantification of EC.
Journal Articles on this Report : 5 Displayed | Download in RIS Format
Other project views: | All 108 publications | 32 publications in selected types | All 29 journal articles |
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Chen L-WA, Moosmuller H, Arnott WP, Chow JC, Watson JG, Susott RA, Babbitt RE, Wold CE, Lincoln EN, Hao WM. Particle emissions from laboratory combustion of wildland fuels:in situ optical and mass measurements. Geophysical Research Letters 2006;33(4):L04803 (4 pp.). |
R831086 (2005) R831086 (2006) R831086 (Final) |
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Chow JC, Watson JG, Park K, Lowenthal DH, Robinson NF, Park K, Magliano KA. Comparison of particle light scattering and fine particulate matter mass in central California. Journal of the Air & Waste Management Association 2006;56(4):398-410. |
R831086 (2006) R831086 (Final) |
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Chow JC, Watson JG, Lowenthal DH, Chen L-WA, Magliano KL. Particulate carbon measurements in California's San Joaquin Valley. Chemosphere 2006;62(3):337-348. |
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Chow JC, Watson JG, Lowenthal DH, Chen LWA, Zielinska B, Mazzoleni LR, Magliano KL. Evaluation of organic markers for chemical mass balance source apportionment at the Fresno Supersite. Atmospheric Chemistry and Physics 2007;7(7):1741-1754. |
R831086 (2006) R831086 (2007) R831086 (Final) |
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Park K, Chow JC, Watson JG, Trimble DL, Doraiswamy P, Park K, Arnott WP, Stroud KR, Bowers K, Bode R, Petzold A, Hansen ADA. Comparison of continuous and filter-based carbon measurements at the Fresno Supersite. Journal of the Air & Waste Management Association 2006;56(4):474-491. |
R831086 (2005) R831086 (2006) R831086 (Final) |
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Supplemental Keywords:
thermal protocols, optical monitoring methods, visibility, light-absorbing carbon, black carbon, elemental carbon, organic carbon, sampling artifact, radiative forcing,, RFA, Scientific Discipline, PHYSICAL ASPECTS, Air, Ecosystem Protection/Environmental Exposure & Risk, Air Quality, particulate matter, air toxics, Environmental Chemistry, Monitoring/Modeling, Analytical Chemistry, Physical Processes, Engineering, Chemistry, & Physics, Environmental Engineering, carbon aerosols, air quality modeling, particle size, environmental monitoring, atmospheric particulate matter, atmospheric measurements, atmospheric dispersion models, particulate organic carbon, aerosol particles, atmospheric particles, mass spectrometry, analysis of organic particulate matter, chemical characteristics, PM 2.5, air modeling, air quality models, exposure, airborne particulate matter, air sampling, gas chromatography, thermal desorption, carbon particles, air quality model, emissions, particulate matter mass, ultrafine particulate matter, particle phase molecular markers, aersol particles, modeling studies, aerosol analyzers, measurement methods, exposure assessment, carbonaceous particulate matter, chemical speciation samplingProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.