Grantee Research Project Results
2000 Progress 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 Period Covered by this Report: December 1, 1999 through November 30, 2000
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 objectives of the project are to: (1) develop a novel particle monitoring technique that measures particle mass, density, mean diameter, and bound water; a technique based on a Continuous Ambient Mass Monitor (CAMM) that we have developed; and (2) conduct laboratory experiments to explore the possible use of a modified CAMM as a Continuous Fine Particle Surface Monitor (CFPSM). Existing particle size measuring instruments are to be used to measure particle surface area, and to validate and calibrate the CFPSM.Progress Summary:
Based on the results of the tests of hygroscopic properties of artificial sulfate and bisulfate aerosols completed in the third year of this project, we concluded that our original goals of continuous measurements of bound water, density, and mean diameter were no longer reasonable. Thus, in the fourth (and last) year of this project (using the no-cost extension of the grant), we pursued the objective of semi-continuous measurement of particle surface area, using an altered configuration of the CAMM that had been tested and validated during the first 3 years of the project. The output pressure drop measurements of the altered CAMM were compared to surface measurements made using commercially available instruments for measurement of particle size distributions. These instruments include Aerodynamic Particle Sizer (APS, Model 3310, TSI, Inc., St. Paul, MN) for particles between 0.5 and 10 m, and the Scanning Mobility Particle Sizer (SMPS, Model 3934, TSI, Inc., St. Paul, MN) for particle sizes below 0.5 m. 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 altered CAMM, and the sum of the surface area measured by the two particle sizing instruments.
Laboratory Test Methods
The CFPSM is a modified CAMM. For the laboratory tests, the CAMM was modified by removing both the PM2.5 size-selective inlet (designed for a flow of 16.6 LPM), and the series of two virtual impactors (designed to increase the concentration of larger particles proportionately to their aerodynamic diameter). For the CAMM to have a response that is proportional to mass concentration, it has to use the virtual impactors because the method of measuring the increase in pressure drop across a membrane filter is proportional to the surface area of the particles. This can be derived from equation (1) (Babich, et al., 2000) for the pressure drop (P) of the instrument without virtual impactors:
where Cm is the mass concentration of the aerosol, t is time, dp is the particle diameter, and Cc is the Cunningham slip correction factor. Assuming that the particles are spherical, and given that the volume is proportional to the mass, we can use the relation between the diameter, total surface area, St, and volume of a sphere to derive equation (2), which indicates that the pressure drop (PD) across the filter per unit time is directly proportional to the particle surface area:
All three instruments-the modified CAMM, the APS, and the SMPS-were set to download to a computer for each 5-minute interval. Monodisperse polystyrene particles with nominal diameters from 0.050 to 2.1 µm (density = 1.05 g/cm3) were aerosolized using a nebulizer (Ding and Koutrakis, 2000). The generated aerosol was introduced into a mixing chamber and diluted with dry filtered room air. Each experiment consisted of 10 consecutive 5-minute tests for each test with monodisperse aerosol.
The two particle measuring instruments were used to simultaneously determine the size and count distribution over the range of 0.05 to 2.1 µm. Because the PSL particles are spherical, the size and count distributions allow accurate determination of surface area. The total surface was calculated from the counts of APS and SMPS according to equation (3):
where St is total surface, di is size diameter in channel i, Ni is the particle count in channel i, and n is the total number of channels.
For each interval, the response factor (RF) of the modified CAMM, was calculated by dividing the net pressure drop, P (the difference between the final pressure drop and the initial pressure drop) by the total surface area, St, measured by the APS plus SMPS, using equation (4):
For each test, the following parameters also were calculated: surface geometric mean diameter (SGMD); the surface geometric standard deviation (SGSD); the 16 percent surface lower diameter limit (16 percent SLD), the 84 percent surface upper diameter limit (84 percent SUD); and the mass concentration (MC). The surfactant surface area, Ss, was estimated using the total value of the surface for sizes smaller than the peak of the monodisperse aerosol. It was not possible to determine the surfactant surface area for the 0.05 µm particles because the surfactant particles had a size distribution that extensively overlapped that of the monodisperse particles.
For the smaller sizes aerosol (0.05, 0.1, and 0.2 µm) the RF was initially higher, but it stabilized after about 30 minutes. Figure (1) shows a plot of RF (only values for stabilized time periods) versus the calculated SGMD. It shows that for larger particles, (above 0.7 µm), the RF is very stable, with an overall average of 0.00198. For the smaller particles, (below 0.5 µm) the RF tends to increase with decreasing size, and also has more variability, with an overall average of 0.00291. According to equation (2), the RF should decrease as diameter decreases because of the slip correction term in the equation. These results contradict this prediction, and would suggest that there is no slip correction effect for the pressure drop resulting from particles collected on this type of membrane filter. These results also could be explained by a relatively lower response by the SMPS in the lower size range (below 0.2 µm).
Table 1 summarizes some physical properties of the generated monodisperse aerosol (including the mean values for the tests with stable RF values). The relatively monodisperse nature of the generated aerosols is demonstrated by the values of the SGSD, which range from 1.49 to 1.70 for all of the aerosols except where there was a relatively large amount of surfactant aerosol. For the cases where the SGSD is low, the SGMD is close to the nominal size. In the cases where SGSD was higher than 2, the amount of surfactant aerosol is considerably higher (about 20 percent of the total surface). The presence of surfactant results in a bimodal distribution, with the surfactant peak having a lower size diameter (ranging from 30 to 100 nm) than the peak from the PSL particles. This increases the value of the SGSD and also shifts the SGSD to lower values compared to the nominal size (in the case of the aerosol of nominal size 0.451 µm, the SGSD was only 0.279 µm). To get a measurable amount of surface (from 1000 to 3000 µm2/cm3), the amount of aerosol mass concentration is relatively high (compared to typical ambient air concentrations) for smaller particles, and is even higher for the larger particles. This is understandable, because larger particles provide relatively smaller amounts of surface for a given mass concentration compared with smaller particles. To achieve similar signal to noise ratios for ambient samples, the noise level of the CAMM will have to be reduced (unpublished results suggest that this is feasible). The manufacturer of the PSL particles has indicated that the type and amount of surfactant is not controlled, which can explain the variable amounts that were found.
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, about 0.3 µm, compared to higher sizes, further research is needed to determine if this difference is real or 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 (Sioutas, et al., 1999). Estimation of the performance of these instruments should be done in parallel with future surface experiments.
Figure 1. Response factor of the CFPSM for monodisperse aerosol of different sizes
Table 1. Physical properties of generated monodisperse serosols
Nominal Size µm | SGMS µm | SGSD | 16% SLD µm | 84% SUD µm | Total Surface Concentration µm2cm3 | Total Mass Concentration µg/m3 | % Ss | RF |
0.048 | 0.071 | 1.580 | 0.045 | 0.113 | 3515 | 41.8 | 0.0028 | |
0.048 | 0.053 | 1.624 | 0.033 | 0.087 | 1005 | 11.1 | 0.0030 | |
0.098 | 0.088 | 1.566 | 0.056 | 0.138 | 2099 | 41.5 | 4.7 | 0.0026 |
0.202 | 0.180 | 2.113 | 0.085 | 0.381 | 813 | 32.0 | 19.8 | 0.0028 |
0.451 | 0.279 | 2.882 | 0.097 | 0.805 | 2265 | 159.5 | 22.2 | 0.0034 |
0.792 | 0.562 | 2.644 | 0.212 | 1.485 | 3807 | 675.6 | 21.1 | 0.0020 |
1.053 | 1.002 | 1.503 | 0.666 | 1.506 | 2375 | 460.2 | 3.1 | 0.0020 |
1.444 | 1.103 | 1.702 | 0.648 | 1.878 | 5637 | 1239.1 | 10.9 | 0.0019 |
2.134 | 1.814 | 1.490 | 1.217 | 2.703 | 2582 | 1045.9 | 0.9 | 0.0021 |
Future Activities:
Although this is the last annual report for this project, it is worthwhile to discuss the future direction of this area of research. Response factors were estimated for aerosols with a concentration high enough to generate a good response of the modified CAMM. To use the monitor in ambient conditions, the sensitivity needs to be increased. Possible solutions are to increase sampling time, increase the sample flow, and eliminate leaks in the sample/reference channels. In addition, it also may be possible to reduce the amount of surfactant enough to have a negligible contribution to the total surface of the aerosol. Alternatively, the PSL particles could be separated by centrifugation from the aqueous phase, and a blank aerosol could be generated with the surfactant solution alone, allowing subtraction of the blank from the measurements with PSL particles.
Once the performance of the reference instruments has been characterized more accurately, and after the sensitivity of the CAMM has been adequately improved, it will be feasible to perform comparison tests for ambient air particles. There are additional variables with ambient air compared to the monodisperse PSL particles, including non-homogeneous composition, typically non-spherical shapes, and polydisperse size distributions.
The surface monitor can be used in further research to characterize total-surface variation of ambient urban fine particles, including temporal, seasonal, geographic, and diurnal variations. This information will be very useful for exploring potential associations between particle surface area and human health outcomes.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 6 publications | 3 publications in selected types | All 3 journal articles |
---|
Type | Citation | ||
---|---|---|---|
|
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) |
Exit Exit |
|
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) |
not available |
Supplemental Keywords:
particle-bound water, particle surface area, continuous measurements, partculate matter, PM., 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.