Grantee Research Project Results
Final Report: Development and Validation of a Novel Technique to Measure Ambient Particle Properties: Bound Water, Mass Density, and Mean Diameter
EPA Grant Number: R825336Title: Development and Validation of a Novel Technique to Measure Ambient Particle Properties: Bound Water, Mass Density, and Mean Diameter
Investigators: Koutrakis, Petros
Institution: Harvard University
EPA Project Officer: Packard, Benjamin H
Project Period: December 1, 1996 through November 30, 1999 (Extended to March 31, 2001)
Project Amount: $380,111
RFA: Analytical and Monitoring Methods (1996) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Environmental Statistics , Water , Land and Waste Management , Air , Ecological Indicators/Assessment/Restoration
Objective:
The main objectives of this research project were to:
(1) develop a novel
particle monitoring technique that measures particle mass, density, mean
diameter, and bound water. This technique is based on a Continuous Particle Mass
Monitor (CPMM) that was developed recently; and (2) conduct laboratory and field
studies to validate the performance of the new technique and develop a better
understanding of the hygroscopic properties of ambient particles. Several
existing techniques for measurement of particle mass properties were used in
these laboratory and field studies; this allowed a high level of confidence in
the expected results in data from the NCPMM system validation studies.
Several existing techniques for measurement of particle properties in laboratory and field studies were used to allow a high level of confidence in the system validation studies. In the first year of the study, the original design of the Continuous Ambient Mass Monitor (CAMM) required extensive modification to be used as a suitable platform for the new CAMM, which was to be used to measure the target parameters ( particle mass, density, mean diameter, and bound water). During the first and second years of the study, laboratory and field evaluation and validation tests were conducted of the modified CAMM.
A key feature of the original proposal was to measure the hygroscopic properties (bound water) of ambient particles. To accomplish this, the behavior of these particles needed to be understood as a function of relative humidity. Consequently, during the second year of the study the second objective was added (i.e., to conduct laboratory and field studies to validate the performance of the new technique and to develop a better understanding of the hygroscopic properties of ambient particles). Several existing techniques for measurement of particle mass properties were to be used in these laboratory and field studies. The results of these tests indicated that the contribution to the total mass of particle-bound water is quite significant, even after standard filter weighing equilibration protocols are followed, and that the water mass is likely to be directly dependent on the relative humidity of the sample aerosol.
During the third year of the study, the commercial development of the CAMM was underway, requiring extensive involvement and interaction with the production process. Significant improvements for key features of the laboratory prototype were achieved, especially to obtain adequate sensitivity for the method. During this same period, extensive followup laboratory work using artificially generated aerosols of sulfate and bisulfate salts was conducted to further investigate and develop a better understanding of hygroscopic particles that are similar to those in ambient air.
The results that were found for the effects of relative humidity on the artificial sulfate and bisulfate aerosols were very consequential for interpretation of measurements of ambient particle mass, which are performed following protocols of the Federal Reference Method, which is the standard method used for both routine and research methods for ambient particles. At the same time, these findings also had strong implications for the ability of the CAMM technology to be used for semicontinuous measurements of particle-bound water. Even at very low relative humidity, a substantial fraction of the water remains bound to these aerosol particles, and the concept of measuring the bound water by using the difference in response of the CAMM no longer seemed feasible. The original proposal included testing particle mass for samples with and without passing through a diffusion dryer (the particles remaining suspended in the sample air in both cases), with the difference in mass to be attributed to bound water. Because the particles are expected to retain substantial amounts of bound water after passing through a diffusion dryer, it was concluded that this objective could not be achieved. Moreover, because determination of density and mean diameter also are dependent on the ability to measure the bound water, it was concluded that it was not feasible to use the CAMM to determine these particle properties.
As a consequence of the limitations associated with measurements of bound water, during the last year of the study (with a no-cost extension), the focus of the project turned to a different feature of the ambient particles, namely the total particle surface area. Thus, the following objective was added: to conduct laboratory experiments to explore the possible use of a modified CAMM as a Continuous Fine Particle Surface Monitor (CFPSM). Two existing particle size measuring instruments were to be used to measure particle surface area as well as to validate and calibrate the CFPSM. Tests were conducted using aerosolized spherical monodisperse polystyrene latex particles (PSLs). The results showed a reasonably consistent relation between the pressure drop change per unit time for the CFPSM, and the sum of the surface area measured by the two particle sizing instruments.
Summary/Accomplishments (Outputs/Outcomes):
Based on the initial experimental results, it is likely that the CAMM system, modified as a CFPSM, will be a good method to measure total surface of particles. Even though there appears to be a higher response for particles below 0.3 m compared to larger particles, further research is needed to determine if this difference is real or if it is due to inaccurate measurements by either the APS or the SMPS. Previous research suggests that there is a different response for the two reference instruments. Estimation of the performance of these instruments should be done in parallel with future surface experiments.With respect to the original objectives of the project, it has been concluded that the basic method of measuring particle mass concentration using the pressure drop of the collected particles across a membrane filter is unsuitable for determination of bound water, density, and particle mean diameter. The reason for this is that it was assumed that passing the sample air with hygroscopic ambient air fine particles through a diffusion dryer would allow quantitative removal of the bound water. Experiments with artificially created sulfate and bisulfate aerosol particles proved that even under more vigorous incubation for long periods of time at virtually zero relative humidity, a substantial amount of water remains bound to these particles.
However, in the process of all of the experimental research, both in modification and commercialization of the CAMM, with the result of a very sensitive instrument for measuring ambient fine particle mass concentrations, it was realized that there was an equally important application of this method for the characterization of ambient fine particles. Specifically, it was suggested that a modification of the CAMM would allow the continuous measurement of ambient fine particle total surface area. Even in the early development of the CAMM, experiments had been conducted that would have yielded this same suggestion, but in the intense effort to produce an instrument capable of continuously measuring mass concentration, the potential for measuring surface area had been essentially neglected. Consequently, during the last year of the project, preliminary tests were performed that gave a strong indication that it will be possible to use the modified CAMM to make meaningful continuous measurements of the total surface area of ambient fine particles. This technology has a big advantage over existing instruments, in particular, the same instruments that were used to characterize its performance. First of all, the combined cost of the Scanning Mobility Particle Sampler (SMPS) and the Aerosol Particle Sampler (APS) is close to $100,000, but that of the CAMM is about $25,000. In addition, the SMPS requires use of a noxious solvent, butanol, but the CAMM uses no chemicals at all. Also, the CAMM is designed for continuous use outdoors, but the other instruments can only be used in a location with typical indoor temperature ranges, and may require more frequent service to be able to operate on a full-time basis. More importantly, the size distribution of ambient fine particles is such that there is frequently a maximum concentration in the size range that is just in between the maximum size measurable by the SMPS and the minimum size measurable by the APS. Consequently, the potential for errors in measurement are much greater for the combination of these two instruments compared to the single CAMM. Moreover, the measurement principle of both the SMPS and the APS is to determine the number (count) concentrations of particles, and then the surface area is calculated assuming that the particles are spherically shaped. This assumption is quite valid for the spherical artificial polystyrene latex particles used for the evaluation of the CAMM using the two instruments. However, because a large fraction of ambient particles are expected to be nonspherical, using the combined SMPS and APS to measure surface area of the ambient particles might be less accurate than measurements made with the CAMM. The net decrease in cross-sectional area by any given particle depends more on its actual area than on the area determined based on the aerodynamic diameter of the particle.
The final important issue is the question of how this research adds to the understanding of or solutions for environmental problems, or is otherwise of benefit to the environment and human health. In the last 4 years or so, there have been intensive efforts to determine which of the physicochemical properties of ambient particles are responsible for the epidemiologically determined associations between adverse human health outcomes and ambient particles. Results from experiments conducted at the Harvard School of Public Health using our Ambient Particle Concentrator (APC) have shown that the mass concentration of particles is insufficient to explain the mechanism(s) that may bring about the health effects from inhalation of the ambient particles. Although existing technology has allowed for the measurement of many of the physical and chemical characteristics of ambient air particles, and to discriminate between the composition for some features such as mass and elemental carbon concentrations, as well as total particle count (which is a surrogate for number concentration of ultrafine particle?those less than about 0.1 m) on an hourly or less basis, until now measurement of the particle surface area was not possible. Researchers have hypothesized that the health impact of ambient particles may be more dependent on species present on the surface of these particles than on their bulk composition. This is because the surface composition of a large fraction of these particles is different than the composition of their interiors. This phenomenon is related to the mechanisms of formation of the fine particles in the atmosphere in particular. Most of the material in fine particles are the secondary products of combustion and similar processes. The initially formed particles frequently subsequently acquire either smaller particles on their surfaces, or else reactions of pollutant gases add to the materials on the surfaces. The net result is that some of the more reactive substances may be more concentrated on the particle surfaces. Thus, bulk measurements of particles first collected on filters, then later extracted and dissolved, could fail to identify differences in the particle surface versus interior compositions. With the technology identified and tested in this project, namely, the modified CAMM, researchers will have a powerful tool to explore the potential importance of particle surface area in determining the mechanism(s) of the harmful human health outcomes associated with ambient particle exposures.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
Other project views: | All 6 publications | 3 publications in selected types | All 3 journal articles |
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Babich P, Wang PY, Allen G, Sioutas C, Koutrakis P. Development and evaluation of a continuous ambient PM2.5 mass monitor. Aerosol Science and Technology 2000;32(4):309-324. |
R825336 (Final) |
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Sioutas C, Koutrakis P, Wang P-Y, Babich P, Wolfson JM. Experimental investigation of pressure drop with particle loading in nuclepore filters. Aerosol Science and Technology 1999;30(1):71-83. |
R825336 (1997) R825336 (1998) R825336 (2000) R825336 (Final) |
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Sioutas C, Abt E, Wolfson JM, Koutrakis P. Evaluation of the measurement performance of the scanning mobility particle sizer and aerodynamic particle sizer. Aerosol Science and Technology 1999;30(1):84-92. |
R825336 (2000) R825336 (Final) |
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Supplemental Keywords:
monitoring, continuous, aerosols, water, measurement, atmospheric chemistry,, RFA, Scientific Discipline, Air, Ecosystem Protection/Environmental Exposure & Risk, Ecology, particulate matter, Environmental Chemistry, Chemistry, Monitoring/Modeling, Engineering, particle size, bound water, ambient particle properties, chemical characteristics, particulate, particles, urban environment, hydroscopic aerosols, thermodynamics, air quality, atmospheric chemistry, validationProgress 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.