2000 Progress Report: A Portable Device for Real-Time Measurement of the Size and Composition of Atmospheric AerosolsEPA Grant Number: R826769
Title: 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 - Main Campus
EPA Project Officer: Shapiro, Paul
Project Period: October 1, 1998 through September 30, 2001
Project Period Covered by this Report: October 1, 1999 through September 30, 2000
Project Amount: $580,963
RFA: Air Pollution Chemistry and Physics (1998) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air , Engineering and Environmental Chemistry
Objective:The goal of this research 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 by ion mobility. Each particle gives a unique mobility spectrum that can be related to chemical composition. Instrument development work will emphasize the adaptation of proven, field-worthy technologies toward the goal of correlated size and composition measurements. Fundamental work will establish and validate the link between mobility spectra and chemical composition.
During the report period, two devices were used in this work?one to perform laser ablation of bulk solids and one to perform laser ablation of individual aerosol particles.
The bulk solid work centered around two instrumental improvements: incorporation of an ion shutter to improve resolution, and the addition of a mass spectrometer to identify mobility peaks. The ion shutter was found to increase resolution with an acceptable drop in signal intensity. With no shutter, a resolution of 4 was typically achieved. With the shutter, a resolution of 20 could be achieved with approximately a 30 percent drop in signal and a resolution of 60 could be achieved with a 60 percent drop in signal relative to the spectrum without a shutter. Polycyclic aromatic hydrocarbons (PAHs) from anthracene to coronene were studied by drying a small volume of solution containing these materials on the sample probe. Each compound gave a single peak whose reduced mobility was consistent with the molecular ion. With the ion shutter operating under moderate resolution conditions, these peaks could be fully resolved in the mobility spectra. The location of the ion shutter relative to the laser ablation source was found to be an important design criterion. If the shutter was placed too far away, then some mobility separation of the ions occurred before the shutter and it was difficult or impossible to operate the shutter under conditions that would transmit a wide range of ion mobilities.
A mass spectrometer was attached to the mobility analyzer to identify ions produced by laser ablation. For example, laser ablation of borosilicate glass gives a broad peak in the mobility spectrum. Mass analysis of this peak indicates the presence of several cluster ions in the 100-250 m/z range. These ions correspond to various combinations of boron, silicon, and oxygen. Modifications to the mass spectrometer interface should improve the ion transmission efficiency so that a wider range of materials can be studied.
Single particle mobility spectra were obtained for a variety of inorganic salts (ammonium, alkali and transition metals, sulfate, nitrate). Some spectra exhibited a single peak whose reduced mobility was similar to that obtained from the bulk solid. Other spectra exhibited a single, very sharp peak having a width that was limited by the rise time of the electronics and a reduced mobility that varied from particle to particle. These sharp peaks are thought to arise from highly photocharged particles. The peak area and reduced mobility allow the charge and size of each particle to be determined.
A second-generation single particle mobility spectrometer has been designed. The aerosol expands through a critical orifice and achieves 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. Dual drift tubes for simultaneous positive and negative ion detection are placed along the third orthogonal axis. This device currently is under construction and will be tested shortly.