Grantee Research Project Results
1997 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, 1996 through November 30, 1997
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:
(1) to develop a novel particle monitoring technique that measures particle mass, density, mean diameter and bound water. This technique (New Continuous Particle Mass Monitor, or NCPMM) is based on a Continuous Ambient Mass Monitor (CAMM) that we have recently developed.(2) to 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 will be used in these laboratory and field studies; this will allow a high level of confidence in the expected results in data from the system validation studies.
Progress Summary:
In the second year of this project, we have completed the evaluation of the original continuous mass pressure drop method referred to as the CAMM and performed initial tests on the amount of water associated with laboratory generated sulfate aerosols. As described in the Year 1 Progress Report, the CAMM had been modified to use a Fluoropore membrane filter for particle collection instead of the original design which utilized Nuclepore filter material. This method is the basis of the proposed work for the NCPMM. A description of the present instrument configuration and laboratory evaluations of the method for PM2.5 is included in Babich et al. (1998). This paper also includes the results of an extensive field evaluation of the CAMM method for PM2.5 that was conducted in seven U.S. cities with very different aerosol composition during 1997 and 1998: Bakersfield, CA, Boston, MA, Chicago, IL, Dallas, TX, Philadelphia, PA, Phoenix, AZ and Riverside, CA. These tests were necessary to demonstrate that the performance of the CAMM is not overly sensitive to different mixes of urban ambient aerosols. Having a fully validated CAMM method was imperative in order to proceed with the development and characterization of the proposed NCPMM system.Laboratory Test Methods
During the fall of 1998, laboratory tests using CAMMS and two other independent methods were conducted in order to determine the contributions of particle-bound water to mass measurements. Hygroscopic particles of ammonium sulfate [(NH4)2SO4)] or ammonium bisulfate [NH4HSO4] were generated using a Retec X-70/N pocket nebulizer. The generated aerosol was sampled with the NCPMM (for these tests, CAMM without the sample aerosol drier) and also with a 47mm Teflon filter used for both gravimetric and sulfate analysis. Tests were performed under a broad range of relative humidities (28 to 93%) by controlling the humidity of the dilution air of the generated aerosol. The sampled mass concentrations were in the range of 20 to 260 ?g/m3, with sampling durations from 60 to 300 minutes. The mass collected by the Teflon filter was determined gravimetrically as well as by measuring the sulfate ion (SO4-2) using ion chromatography (IC). Ammonium ion was not measured, but the mass was taken into account, resulting in an IC determined "mass", a measurement of the test aerosol mass concentration without any water. Gravimetric mass was measured after filter equilibration for at least 48 hours at 40% RH in compliance with US EPA Federal Reference Method requirements for PM2.5 samples. The results of these tests are shown in figures 1 and 2 below for ammonium bi-sulfate, and figures 3 and 4 for ammonium sulfate. The experimental data are shown in tables 1 and 2.
Results and Discussion
The mass on the Teflon filter as measured by the IC (correcting the measured sulfate ion mass for the additional mass of associated ammonium ions) was less than the mass measured both gravimetrically and with the CAMM for all tests (figures 1 and 3). This is expected to some degree, since some water is always associated with sulfate aerosols even at relative humidities below deliquescent points. However, both "wet" CAMMS mass and the equilibrated gravimetric mass were very similar. This result was not expected, since it has long been assumed that most of the water is removed from the Teflon filter during the equilibration process. Furthermore, the ratio of the gravimetric mass (or CAMMs mass) to the IC "dry" mass increased linearly as the relative humidity of the sampled aerosol increased (figures 2 and 4). These results indicate that at least for these homogeneous aerosol samples, the higher the relative humidity of the sample aerosol, the higher the percentage of the particle-bound water associated with the sulfate, even though filters were equilibrated at 40% RH for at least 48 hours prior to being weighed. v From the results of the laboratory tests using ammonium sulfate, it was observed that for the lower range of sample relative humidities (29 - 33%, below the deliquescent point of ammonium sulfate), approximately 10% of the gravimetrically measured mass can be attributed to the particle-bound water. For the highest range of relative humidities used in these tests (88 to 93%), the contribution of particle-bound water to the mass on an equilibrated filter was approximately 30%. A regression of the sample relative humidities and the gravimetric mass to IC mass ratio yields a Pearson correlation coefficient of 0.84, demonstrating that this response is relatively linear throughout the relative humidity range that was sampled. Similar results were observed for ammonium bisulfate, with the particle-bound water contributing approximately 15% of the gravimetric mass at the low RH range (28 - 39%), and approximately 23% of the gravimetric mass at the higher RH range (55-63%). Again, this relationship was reasonably well correlated (r = 0.79). These linear relationships lend confidence to the results and our interpretation of them, since if we were looking at large but random bound-water values vs sample RH variability, we would have to conclude that our methods were not working properly. Furthermore, the filters were also weighed immediately after sample collection, at the start of the 2-day 40% RH post-exposure equilibration period; the masses measured at that time were not significantly higher than those reported here. Taken together, these data suggest that sulfate aerosols retain substantially more water after standard equilibration conditions than the literature suggests. This has potentially significant implications with respect to several aspects of PM monitoring and health effect assessment, as well as mass closure techniques ("reconstructed mass") that estimate the PM from selected measured components and compare the results to integrated gravimetric PM measurements.
Accomplishments and Research Results: We have completed a thorough laboratory and field evaluation of the modified CAMM method using the Fluoropore filter material. Field tests show a uniform relationship with integrated PM2.5 samples across several urban areas at different times of the year. These results are reported in Babich et al. (1999). Initial laboratory tests using hygroscopic particles indicate that the contribution to the total mass of particle-bound water may be significant even after standard filter weighing equilibration protocols are followed, and that the water mass is directly dependent on the relative humidity of the sample aerosol. Results from laboratory tests are presented in the tables and figures shown below.
Future Activities:
Based on the preliminary laboratory test results reported here, we need to further characterize our initial gravimetric mass measurement bound-water results before proceeding further with development of the NCPMM system. Areas to be investigated for the gravimetric method include different filter media (such as Nuclepore), aerosol composition effects (similar tests, but with other hygroscopic aerosols), and different filter equilibration times at varying RH equilibration conditions. All of these factors could have effects on the water measured on gravimetric mass samples. We anticipate using much drier RH levels than what the US EPA presently allows for ambient measurements. Gravimetric analysis at low RH would normally be very difficult with filter media like Nuclepore due to severe static problems, but we have recently implemented improved static controls as part of our weighing method. A coronal discharge bi-polar ion generator is used to control static charge on filters during gravimetric analysis; this technique is much more effective than the traditional use of 210Po alpha particle sources for this purpose.Once the issues related to integrated gravimetric PM sample bound water are sufficiently characterized, we expect to be able to quantify the particle-bound water in real-time by implementing two channels in our NCPMM system; one sampling at ambient relative humidity conditions and the other with very dry sample air. With the performance of the modified CAMMS method fully validated and the issues with gravimetric filter equilibration characterized, we will continue with the development and laboratory characterization of the NCPMM. In place of the Nuclepore filters, we plan to use a combination of multiple virtual impactors and Fluoropore filters to accomplish the performance goals described in the original proposal.
As noted in the first progress report for this project, we experienced significant delays in year one of this project related to the modifications to the core CAMMS mass measurement method that required a change in the filter material used in the sampler from Nuclepore to Fluoropore, as well as other significant design changes. Initial tests with the Nuclepore filter material indicated measurement errors with certain types of aerosols; these tests and additional detail on the filtration properties of Nuclepore filter media are described in Sioutas, et al. (1999). As a result of these delays in year 1, the time schedule of project tasks has pushed back at least one year. We continue to anticipate requesting a no-cost extension at the end of the funding period to complete the work as originally proposed.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 6 publications | 3 publications in selected types | All 3 journal articles |
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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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Supplemental Keywords:
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.