Grantee Research Project Results
2000 Progress Report: Real-Time Analysis of PAH Bound to Size-Resolved Atmospheric Particles by Tandem Time of Flight Mass Spectrometers
EPA Grant Number: R825391Title: Real-Time Analysis of PAH Bound to Size-Resolved Atmospheric Particles by Tandem Time of Flight Mass Spectrometers
Investigators: Smith, Kenneth A. , Worsnop, Douglas R.
Institution: Massachusetts Institute of Technology , Aerodyne Research Inc.
Current Institution: Massachusetts Institute of Technology
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1996 through September 30, 1999 (Extended to November 30, 2000)
Project Period Covered by this Report: October 1, 1999 through September 30, 2000
Project Amount: $375,000
RFA: Exploratory Research - Air Engineering (1996) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Land and Waste Management , Air , Safer Chemicals
Objective:
Polycyclic aromatic hydrocarbons (PAHs) adsorbed on atmospheric particles are of concern because PAHs are known to be mutagenic. Measurements of the amount of PAH associated with different aerosol size fractions are critical for a complete understanding of the environmental fate of and human exposure to fine particles containing PAH. The goal of this research is to develop and demonstrate an aerosol mass spectrometer capable of quantifying PAH associated with individual size-segregated atmospheric particles in real time.Progress Summary:
The instrument operates by drawing ambient aerosol into a vacuum system through an aerodynamic lens which focuses particles into a narrow beam and efficiently transmits the particles into a vacuum system where they impact on a resistively heated surface. The PAH components in/on the particle flash vaporize upon contact with the heater and the gas phase molecular constituents are then photo-ionized by UV laser resonance-enhanced multi-photon ionization (REMPI). The ionized molecular PAH species are classified using time-of-flight molecular mass spectrometric analysis. The technique provides complete mass spectrometric information for individual particles. Particle aerodynamic size is determined by measuring particle time-of-flight using a mechanical particle beam chopper. This is performed by firing the laser at specific time intervals synchronized to the phase of the particle beam chopper with subsequent detection of ion signals in the TOF spectrometer. This particle sizing scheme takes advantage of the size-dependent distribution of particle velocities generated by the expansion of the aerosol into the vacuum.
During the reporting period, we have further developed and quantified the aerodynamic particle sizing technique. Size-dependent particle velocity, particle beam width and particle transmission and collection efficiency have been measured as a function of particle size. Particles in the size range of 60-1000 nm travel at velocities ranging from 260 to 90 m s-1, respectively. The experimental measurements are supported by fluid dynamics modeling results. Our modeling results also indicate that the collection efficiency can be essentially unity over the entire particle size range of ~50 to 5000 nm if the inlet geometry of the aerodynamic lens system is optimized. This will significantly enhance the applicability of the instrument in analyzing ambient particles.
Also during this reporting period, we have improved single
particle detection sensitivity,
by moving the vaporization heater closer to
the axis of the molecular TOF tube. This requires that the heater (which is a
metal surface) be floated to the potential of the TOF ion optics which is
approximately 4000 V. In addition, the performance of the AMS also has been
tested using atomized pure pyrene particles, freshly generated soot particles
from a propane flame and soot particles exhausted from a Honda EG1400 gasoline
engine. A typical spectrum for soot particles from a propane diffusion flame is
shown in Figure 1. Figure 1A is a plot of an average ion current signal over 512
laser firing events versus time-of-flight, where the solid line is for soot
particles and the dashed line is for atomized pure pyrene particles for
molecular weight calibration. In the plot, the peak at t=0 is electronic
"pickup" indicating the time of the laser firing. One important feature in the
pyrene spectrum is that it shows only one ion peak at 17.5 µs time of flight,
which corresponds to the pyrene parent mass peak (202 AMU). The pyrene spectrum
indicates that with the current laser beam intensity (107 W/cm2), the REMPI
process is "soft" (i.e., ion fragments are minimal). Another feature in the
upper plot is that there is a dominant ion peak in the soot spectrum
corresponding to the parent ion peak of pyrene. This indicates that pyrene also
is a major semi-volatile component from propane flames.
The spectrum in Figure 1A is plotted in Figure 1B versus molecular weight. The plot shows that the lowest PAH peak is at 140 AMU corresponding to an 11 carbon molecule (C11H8). The plot also shows all PAH peaks from 11 carbon molecules (C11H8) to 42 carbon molecules (C42H16). It also is observed that PAHs containing odd numbers of carbon atoms produce smaller peaks. This is consistent with the observation of gas phase PAHs from a methane diffusion flame by Siegmann and Sattler (Journal of Chemical Physics 2000;112:698-709), in which it was explained that the PAHs containing an odd number of C atoms cannot be completely conjugated or benzenoided and, therefore, they are less stable than PAHs of comparable structure containing an even number of C atoms.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
| Other project views: | All 10 publications | 3 publications in selected types | All 3 journal articles |
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Jayne JT, Leard DC, Zhang XF, Davidovits P, Smith KA, Kolb CE, Worsnop DR. Development of an aerosol mass spectrometer for size and composition analysis of submicron particles. Aerosol Science and Technology 2000;33(1-2):49-70. |
R825391 (1999) R825391 (2000) R825391 (Final) R825253 (Final) R828172 (Final) |
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Zhang X, Smith KA, Worsnop DR, Jimenez J, Jayne JT, Kolb CE. A numerical characterization of particle beam collimation by an aerodynamic lens-nozzle system: Part I. An individual lens or nozzle. Aerosol Science and Technology 2002;36(5):617-631. |
R825391 (2000) R825391 (Final) |
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Supplemental Keywords:
exposure, risk, risk assessment, health effects, ecological effects, carcinogen, environmental chemistry, monitoring, transportation., RFA, Scientific Discipline, Air, particulate matter, air toxics, Environmental Chemistry, mobile sources, Engineering, Chemistry, & Physics, monitoring, fate, particulates, aerosol particles, flight mass spectrometer, fine particles, atmospheric particles, air quality models, emissions measurement, fine particulates, ambient emissions, PAH, human exposure, combustion, ultraviolet excimer laser, vapor plumeProgress 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.