Final Report: Real-time Measurement of the Size and Composition of Atmospheric Particulate Matter

EPA Grant Number: R826234
Title: Real-time Measurement of the Size and Composition of Atmospheric Particulate Matter
Investigators: Wexler, Anthony S. , Johnston, Murray V.
Institution: University of Delaware
EPA Project Officer: Hahn, Intaek
Project Period: December 15, 1997 through December 14, 2000
Project Amount: $374,833
RFA: Ambient Air Quality (1997) RFA Text |  Recipients Lists
Research Category: Air Quality and Air Toxics , Air

Objective:

The objective of this research project was to develop and test an instrument that can size and analyze individual aerosol particles online and in real time in the respirable size range from 10 nm to 1 micron. Although numerous studies show a correlation between PM2.5 and health effects, the mechanism has not yet been elucidated. The atmosphere contains particles ranging in size and composition. Establishing if the health effects are due to size, composition or a combination of both depends on developing an ability to measure these quantities for individual particles.

Summary/Accomplishments (Outputs/Outcomes):

We had already built an instrument capable of analyzing individual particles. Particles are drawn through a set of orifices that reduce the pressure of the gas and efficiently transmit the particles. The particles then pass through an orifice that focuses a selected size to the source region of a mass spectrometer. The pressure upstream of this second orifice governs the particle size that is focussed. An excimer laser is free fired at about 80 Hz and directed colinear and opposite in direction to the particle beam. If a particle is in the source region of the time-of-flight (TOF) mass spectrometer when the laser fires, the particle is ablated, ionized, and the ions are analyzed.

Prototype Light Scattering Add-On Completed and Tested. There are more small particles in the atmosphere than larger ones. To increase the efficiency for sampling the larger particles, a light scattering apparatus was built and field-tested. With the light scattering apparatus, the excimer laser pulse can be synchronized with the particle arrival; however, this only works with particles large enough to be detected by light scattering.

Improved Nozzle Completed. An improved inlet section was built and tested. The new inlet has a higher transmission efficiency and transmits particles in a well-collimated beam. We tested the focusing characteristics of the inlet and found that they are consistent with the theory. The new inlet design can selectively transmit particles over sizes ranging from about 10 nm to 2 microns, which correspond to the size range most relevant to human health studies.

First Field Trial Completed. The instrument was transported to the Calspan smog chamber test facility in Ashland, NY. The Naval Research Laboratory was running aerosol dynamics tests there and we used this opportunity to test the single particle instrument on both aerosols in the smog chamber and rural ambient aerosols in the vicinity of the facility. Previous work shows that the aerosols must be dried to improve their analysis and the diffusion dryers were not able to dry in high humidity situations.

Drier Completed. A Nafion drier was built and added to the system to: (1) remove the need to recharge our previous silica-based dryer; and (2) reduce the relative humidity to reasonable levels under fog and cloud conditions.

Preliminary Tests on Soot Completed. Soot is a potential culprit in the PM-health link. In collaboration with a combustion group in the Mechanical Engineering department, we formed soot particles in the 5 nm to 50 nm size range and sampled them into the instrument. This allowed us to test the capabilities of the instrument for very small particles and to understand more about the spectra arising from analysis of soot particles.

Second Field Trial Completed. A portable laboratory was purchased and used to transport the instrument to Atlanta for the Atlanta Supersite measurements during August 1999. About 17,000 spectra were obtained, ranging from 17 to more than 500 nm. The results will be published in a special issue of the Journal of Geophysical Research that is devoted to the Atlanta supersite measurements.

Flow/Pressure Controller. The flow and pressure were controlled with a bank of 10 critical orifices and a computer-controlled rotary valve to select the pressure/flow. The orifices have very small particle losses because they are sharp orifices opening into a relatively large chamber so that there are very few surfaces for the particle to impinge on.

Instrument Control Software. Software has been developed to operate the instrument for extended periods of time so that data can be acquired without continuous operator intervention. This software cycles the instrument through each of the sampling particles sizes, using the flow/pressure controller. The goal of the sampling is to obtain enough spectra in each composition class that a statistically significant average can be obtained on the major composition classes. To do this, the software uses the ART-2a algorithm to group the spectra into similar classes. The instrument samples at a given size, grouping the spectra into composition classes in real time. When the major classes have a sufficient number of spectra, the software writes the data files and starts sampling at the next particle size.

Laboratory Test of ART-2a Algorithm. Particles of known size and composition were generated in the laboratory and analyzed with the instrument. ART-2a was used to classify the particles. We found that: (1) generating particles of known composition is very difficult and small impurities may seriously influence the classification; (2) sulfate particles are extremely difficult to classify well because the sulfate matrix does not analyze well using our current techniques; and (3) most of the particle compositions that we tested were accurately classified by ART-2a.

New Excimer Laser. The MPB PSX-100 excimer laser that we originally used had a number of shortcomings, including short gas lifetime and poor beam quality. The new laser, a GAM EX10, has a much improved gas lifetime and beam quality, and much greater pulse power. Also, the EX10 can be completely software controlled so that the instrument control software can also control and monitor laser performance.

Third Field Experiment Completed. The instrument and portable laboratory were used at the Houston supersite experiment. We collected more than 20,000 spectra during the experiment, classifying the spectra on site. The most dominant particle class contained primarily silicon dioxide and ranged in size from 30 to 100 nm. We posit that these particles arise from atmospheric oxidation of silane, because other processes could not produce SiO2 particles that are so small.

Improved A/D Converter. During the Houston supersite experiment, the A/D converter failed. A new A/D converter was purchased from GAGE Applied and the software was altered to collect data using this new hardware.

Laser Control Software. A first draft of the laser control software has been completed.

Conclusions:

The primary goal of this grant was to take a laboratory-based instrument that sizes and performs single particle analysis on ultrafine particles, convert it into a field-portable configuration, and deploy it to validate its performance. The objectives have been met and the results either have been published or will be published. A similar instrument design also will be deployed at the Baltimore and Pittsburgh supersites.


Journal Articles on this Report : 5 Displayed | Download in RIS Format

Other project views: All 10 publications 5 publications in selected types All 5 journal articles
Type Citation Project Document Sources
Journal Article Mallina RV, Wexler AS, Johnston MV. High speed particle beam generation: experimental evaluation of dynamic mechanisms. Journal of Aerosol Science 1998;29(Suppl 1):S431-S432. R826234 (Final)
not available
Journal Article Mallina RV, Wexler AS, Johnston MV. High-speed particle beam generation: simple focusing mechanisms. Journal of Aerosol Science 1999;30(6):719-738. R826234 (1999)
R826234 (Final)
R823980 (Final)
  • Abstract: Science Direct Abstract
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  • Journal Article Mallina RV, Wexler AS, Rhoads KP, Johnston MV. High speed particle beam generation: a dynamic focusing mechanism for selecting ultrafine particles. Aerosol Science and Technology 2000;33(1-2):87-104. R826234 (1999)
    R826234 (Final)
  • Abstract: InformaWorld Abstract
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  • Journal Article Phares DJ, Rhoads KP, Wexler AS, Kane DB, Johnston MV. Application of the ART-2a algorithm to laser ablation aerosol mass spectrometry of particle standards. Analytical Chemistry 2001;73(10):2338-2344. R826234 (Final)
    R827352 (Final)
  • Abstract from PubMed
  • Full-text: ACS-Full Text HTML
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  • Abstract: ACS-Abstract
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  • Other: ACS-Full Text PDF
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  • Journal Article Phares DJ, Rhoads KP, Wexler AS. Performance of a single ultrafine particle mass spectrometer. Aerosol Science and Technology 2002;36(5):583-592. R826234 (Final)
  • Abstract: InformaWorld Abstract
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  • Supplemental Keywords:

    ambient air, atmosphere, tropospheric, health effects, particulates, PAHs, PNAs, metals, heavy metals, sulfates, organics, monitoring, analytical, measurement methods., RFA, Scientific Discipline, Air, particulate matter, Environmental Chemistry, Environmental Monitoring, Atmospheric Sciences, particle size, particulates, PM10, mass spectrometer, fine particles, PM 2.5, field-portable instruments, Volatile Organic Compounds (VOCs)

    Relevant Websites:

    http://www.me.udel.edu/wexler/spa_refs.html Exit

    Progress and Final Reports:

    Original Abstract
  • 1998
  • 1999 Progress Report