Grantee Research Project Results
Final Report: Integrating the Thermal Behavior and Optical Properties of Carbonaceous Particles: Theory, Laboratory Studies, and Application to Field Data
EPA Grant Number: R831085Title: Integrating the Thermal Behavior and Optical Properties of Carbonaceous Particles: Theory, Laboratory Studies, and Application to Field Data
Investigators: Bond, Tami C.
Institution: University of Illinois Urbana-Champaign
EPA Project Officer: Chung, Serena
Project Period: September 1, 2003 through August 31, 2006 (Extended to August 31, 2008)
Project Amount: $247,815
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:
Monitoring funded by EPA and other agencies relies on thermal/optical measurement techniques to assess the types of carbonaceous particles in ambient air and in emissions from combustion sources. This research sought to provide additional information from these existing data by examining the behavior of the particles during analysis, including changes in the optical transitions. Our objectives were: (1) Improving the confidence limits of combined thermal/optical transitions of specific fractions of primary carbonaceous aerosols; (2) Confirming to 95% confidence that elemental carbon can be treated as a conserved tracer; (3) Demonstrating enhanced interpretations of thermal/optical analyses of carbonaceous particles.
Summary/Accomplishments (Outputs/Outcomes):
The following abbreviations are used in this report: EC (Elemental carbon, as determined by thermal-optical analysis); OC (Organic carbon); PC (Pyrolyzed carbon, formed from OC during sample heating); TOA (Thermal-optical analysis).
A. Results from Sample Examination
We examined hundreds of samples including aerosol from controlled laboratory combustion, wood burning, and diesel engines, as well as analytical standards. Main findings are listed below; many of these confirm those of other investigators.
- Kinetic response. The kinetic response of these samples is not repeatable enough that they can serve as indicators of source types, even if no atmospheric processing has affected them.
- Absorptive properties. PC and EC have significantly different absorptive properties. For TOA monitored with a transmittance probe, this difference causes underestimation of EC. Absorptive properties of EC appear to depend on filter transmission, while those of PC do not.
- Heavy OC. Some “heavy” OC does not evolve even at the highest temperatures in the inert atmosphere, producing an overestimate of EC. This problem is exacerbated by low peak temperatures in the inert portion of the analysis. The amount of OC remaining could not be predicted from TOA-recorded data.
- Early release of EC. The timing of elemental carbon release can be affected by inorganic compounds and by oxygen in the sample, so release times cannot be assumed to remain consistent. EC is not a conserved tracer.
- Artifact due to PC. The amount of EC reported by the analysis is affected by the amount of PC formed. For a transmittance probe, mixing EC with pyrolyzable OC would change (decrease) the amount of apparent EC. Sources of pyrolyzable OC may be both location-dependent and seasonally-dependent.
- Artifact due to heavy OC. The amount of EC reported by the analysis is affected by the amount of “heavy” OC in the sample. For either transmittance or reflectance, mixing EC with heavy OC would change (probably increase) the amount of apparent EC. Our proxies for heavy OC were humic and fulvic acids. The presence and quantity of humic-like substances would dependent on location and season, and therefore the artifacts would also.
B. Theoretical evaluation and laboratory experiments
- Some variation in light-absorbing properties of carbon particles has been explained. Light-absorbing carbon was previously thought to have wildly varying absorptive properties. Upon re-examination of the literature reporting this variation, however, we found several errors and misinterpretations. In fact, both refractive index and mass absorption cross-section coalesced around a set of recommended values (Bond and Bergstrom, 2006).
- Large variation in optical response. We developed an optical model of the particle-filter system. However, the model didn’t predict the behavior of real particle-laden filters, especially the absorption enhancement and the wide variability. Instead of using this theoretical model to understand TOA, we created a “best representation” of optical response based on about 100 samples of laboratory and source aerosol (Boparai et al., 2008).
- Organic carbon on filters is collected as liquid fiber coatings. Using scanning electron microscopy, we showed that organic carbon is initially present as liquid coatings on the filter’s fibers. When this material chars, it is not particulate matter but dark fiber coatings, and this morphological difference may explain its vastly different optical properties (Subramanian et al., 2007).
- Complex, semi-aromatic, non-volatile materials form pyrolytic carbon. We hoped to use the formation of pyrolytic carbon as an indicator for source apportionment studies. Several model organic compounds did not result in PC. Only complex organic mixtures such as wood smoke and humic acid pyrolyzed, and large, complex molecules pyrolyzed at lower temperatures (Subramanian et al., 2008).
- Thermal-optical analysis is presently underdetermined Organic carbon on filters is collected as liquid fiber coatings. We developed an optical model of the particle-filter system. However, the model didn’t predict the behavior of real particle-laden filters, especially the absorption enhancement and the wide variability. Instead of using this theoretical model to understand TOA, we created a “best representation” of optical response based on about 100 samples of laboratory and source aerosol (Boparai et al., 2008).
5. Given the confirmed sins of thermal-optical analysis, can it be improved, or is it doomed to remain full of unquantifiable artifacts?
We attempted to quantify some problems, such as the rapid oxidation of EC in the presence of catalysts and the retention of heavy OC, by developing automated analysis for thermal and optical traces. However, a fundamental problem remains. The current system has two outputs (carbon release and optical monitoring) and three unknowns (evolving PC, LAC and EC).All three evolve together during portions of the analysis: at high temperatures in helium environment and at low temperatures under oxygen/helium environment. Thus, the system is underdetermined and has no unique solution.
REACTO was our answer to this dilemma. The system can be represented and solved as a matrix equation. During portions of the analysis when the equations are fully determined, there is one unique solution. When the system is underdetermined, the range of solutions can be represented by a line rather than a point, and the selected solutions are limited by physical considerations (i.e., carbon does not accumulate on the filter; the net amount of PC formed plus lost is zero). Uncertainties in optical properties can also be propagated through this equation to obtain uncertainties in PC, EC and OC at each time step. Iteration is required (Boparai et al., 2008). Results for one of the samples with the highest uncertainty, that of the humic acid sample shown earlier, are shown below.
C. Implementation of Findings
The findings listed above are implemented in two analysis tools developed during this project. (1) The “thermabsgram” is a take-off on the standard thermal-optical result called the “thermogram.” It embodies the practice of adding the change in laser signal to thermal-optical results. (2) “REACTO” (REAnalyzing Carbon Traces Optically) is the program we developed to account for the peculiarities of thermal-optical analysis. It solves the matrix equation representing the TOA system. We believe that REACTO accounts for many of the analysis imperfections which are suspected in TOA. While the aerosol community is aware of such flaws, it has lacked a framework to account for them.
Both the thermabsgram and REACTO are written in MatLab and operate by reading the analysis files provided by the OC/EC analyzer from Sunset Laboratories (Tigard, Oregon). The data reading program could be altered to accept the output of any analyzer, and analysis module would use these data directly. These programs are being made available to the environmental community under an appropriate copyright (or copyleft) agreement. We have optimized REACTO based on samples from several sources. However, additional considerations may arise when the tool is used to interpret a wider variety of samples.
Conclusions:
We embarked on this study with the hope that combining thermal and optical data would yield enough additional information to interpret TOA results in a unified fashion, regardless of the temperature protocol used. Because the reactor system is underdetermined during critical portions of the analysis, we were not able to manipulate the analysis to provide decisive answers about the amount of OC and EC in each sample. However, we did develop a tool that works on existing analyses, as well as providing guidelines for sample behavior based on examining hundreds of samples. Our analysis tool, REACTO, provides quantitative data about uncertainty in each sample by determining the range of solutions that is consistent with the carbon analysis. Assumptions are now explicit, and the best thermal protocol can now be defined as the one which results in the least uncertainty.
We recommend the following further research: (1) Implement quality control procedures for the laser signal, which plays a critical role in the determination of OC and EC. (2) Confirm values of absorptive properties for pyrolytic carbon. (3) Confirm and explain variation of absorptive properties of EC from different sources. (4) Explore reflectance to provide constraints—if sufficiently different from the transmission response, it could provide the missing constraint on the underdetermined system.
Journal Articles on this Report : 5 Displayed | Download in RIS Format
Other project views: | All 19 publications | 5 publications in selected types | All 5 journal articles |
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Type | Citation | ||
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Bond TC, Bergstrom RW. Light absorption by carbonaceous particles: an investigative review. Aerosol Science and Technology 2006;40(1):27-67. |
R831085 (2005) R831085 (2006) R831085 (Final) |
Exit Exit Exit |
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Boparai P, Lee JM, Bond TC. Revisiting thermal-optical analyses of carbonaceous aerosol using a physical model. Aerosol Science and Technology 2008;42(11):930-948. |
R831085 (Final) |
Exit Exit Exit |
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Oanh NTK, Thiansathit W, Bond TC, Subramanian R, Winijkul E, Paw-armart I. Compositional characterization of PM2.5 emitted from in-use diesel vehicles. Atmospheric Environment 2010;44(1):15-22. |
R831085 (Final) |
Exit Exit Exit |
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Subramanian R, Roden CA, Boparai P, Bond TC. Yellow beads and missing particles:trouble ahead for filter-based absorption measurements. Aerosol Science and Technology 2007;41(6):630-637. |
R831085 (2006) R831085 (Final) |
Exit Exit Exit |
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Subramanian R, Winijkul E, Bond TC, Thiansathit W, Oanh NTK, Paw-armart I, Duleep KG. Climate-relevant properties of diesel particulate emissions: results from a piggyback study in Bangkok, Thailand. Environmental Science & Technology 2009;43(11):4213-4218. |
R831085 (Final) |
Exit Exit Exit |
Supplemental Keywords:
RFA, Scientific Discipline, Air, Ecosystem Protection/Environmental Exposure & Risk, particulate matter, Air Quality, air toxics, Environmental Chemistry, Air Pollution Effects, Monitoring/Modeling, Analytical Chemistry, Environmental Monitoring, Atmospheric Sciences, Engineering, Chemistry, & Physics, Environmental Engineering, air quality modeling, health effects, particle size, carbon aerosols, particulate organic carbon, atmospheric particulate matter, chemical characteristics, PM 2.5, atmospheric particles, aerosol particles, air quality models, air modeling, airborne particulate matter, emissions, thermal desorption, air sampling, carbon particles, air quality model, ultrafine particulate matter, particulate matter mass, particle phase molecular markers, modeling studies, thermal properties, aersol particles, aerosol analyzers, chemical speciation sampling, measurement methods, particle dispersionProgress 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.