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2001 Progress Report: A Portable Device for Real-Time Measurement of the Size and Composition of Atmospheric Aerosols
EPA Grant Number: R826769Title: A Portable Device for Real-Time Measurement of the Size and Composition of Atmospheric Aerosols
Investigators: Johnston, Murray V. , Eiceman, Gary A.
Institution: University of Delaware , New Mexico State University
Current Institution: University of Delaware , New Mexico State University - Main Campus
EPA Project Officer: Shapiro, Paul
Project Period: October 1, 1998 through September 30, 2001
Project Period Covered by this Report: October 1, 2000 through September 30, 2001
Project Amount: $580,963
RFA: Air Pollution Chemistry and Physics (1998)
Research Category: Engineering and Environmental Chemistry
Description:
Objective:The goal of this research project is to develop and field test a portable device for real-time size and composition measurements of atmospheric aerosols. Individual particles are sized with a commercial aerodynamic sizer and then ablated with a pulsed laser. Ions produced by the ablation process are analyzed with an ion mobility spectrometer. Alternatively, photons produced by the ablation process are analyzed with an optical spectrometer. In either case, each particle gives a spectrum that can be related to its chemical composition. Instrument development work emphasizes the adaptation of proven, field-worthy technologies toward the goal of correlated size and composition measurements. Fundamental work establishes and validates the link between spectral features and chemical composition.
Progress Summary:During previous report periods, two devices were built and tested: one to perform laser ablation ion mobility spectrometry of bulk solids and one to perform laser ablation of individual aerosol particles in real time. In addition, the mobility spectrometer for bulk solids was coupled with a mass spectrometer to further characterize the ions. During the current report period, these devices were used to study the link between spectral features and chemical composition.
Laser ablation ion mobility spectra have been obtained for a variety of organic and inorganic species. In most cases, the spectra of bulk solids and individual particles are similar if the chemical compositions are similar. While the signal intensity is too low to perform mass analysis of ions produced from single particles, mass analysis of the ions from bulk solids can give significant insight into the ion formation process. Polycyclic aromatic hydrocarbons (PAHs) give molecular ions (molecule minus an electron) corresponding to the compounds present in the sample. Unlike laser ablation mass spectrometry of particles in a vacuum, little if any fragmentation is observed with ion mobility spectrometry. Inorganic solids, however, give ions derived from trace impurities in the surrounding gas (e.g., water cluster ions). Evidently, the ions produced by laser ablation of the solid quickly charge exchange with the surrounding gas, leaving no memory of the original composition. These results highlight both the capabilities and limitations of laser ablation ion mobility spectrometry to determine the chemical composition of condensed phase materials.
An alternative means of determining chemical composition also was studied during the report period. The single particle laser ablation device was re-configured for detection of photons emitted from the laser ablation process. If the laser-induced plasma is sufficiently robust, then atomic species are produced that emit radiation at specific, characteristic wavelengths. This approach was tested with approximately 750 nm diameter particles containing salts of sodium, potassium, magnesium, calcium, copper, and iron. In each case, the metal ions present in the particle could be identified based upon the emission wavelengths detected.
Future Activities:A field instrument has been built and will be tested for ambient particle analysis. The instrument is designed around an aerodynamic particle sizer. The aerosol is sampled through a critical orifice where individual particles achieve a size-dependent velocity. The velocity is measured by time-of-flight between two continuous laser beams oriented perpendicular to the aerosol flow. The velocity measurement also allows the ablation laser pulse to be synchronized with the arrival of a particle. The ablation laser and aerosol counter-propagate along one axis of the device. The continuous laser beams and scatter optics are placed perpendicular to the aerosol axis. In the ion mobility mode, dual drift tubes for simultaneous positive and negative ion detection are placed along the third orthogonal axis. In the plasma emission mode, a fiber optic collector and optical spectrometer are positioned along the third orthogonal axis. In either mode, the chemical composition of an individual particle can be correlated with its size as determined by laser velocimetry.
Journal Articles:No journal articles submitted with this report: View all 6 publications for this project
Supplemental Keywords:ambient air, particulates, metals, polycyclic aromatic hydrocarbons, PAHs, organics, measurement methods , Air, Scientific Discipline, RFA, Engineering, Chemistry, & Physics, indoor air, air toxics, particulate matter, Environmental Chemistry, Environmental Monitoring, aerosols, real time monitoring, indoor air quality, pulsed laser, chemical composition, air sampling, monitoring, aerodynamic sizer, particulates, atmospheric particles, spectroscopy, field monitoring, spectroscopic studies, ion exchange, atmospheric aerosol particles, ambient aerosol, particle size
Progress and Final Reports:
1999 Progress Report
2000 Progress Report
Original Abstract
Final Report