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Ambient Aerosols: Collection of Source-Oriented Samples for Toxicity Testing (Part I)
Characterization and Improvement of the Versatile Aerosol Concentration Enrichment System (VACES) (Part II)
This page updated April 21, 2010
Chair’s Air Pollution Seminar |
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Wednesday, June 2,
2010
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Part I
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Anthony Wexler, Ph.D., Director
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PART I
Current
particulate matter regulations control the mass concentration of
particles in the atmosphere regardless of composition, but some primary
and/or secondary particulate matter components are no doubt more or
less toxic than others. Testing direct emissions of pollutants from
different sources neglects atmospheric transformations that may
increase or decrease their toxicity. This work describes a system that
conditionally samples particles from the atmosphere depending on the
sources or source combinations that predominate at the sampling site at
a given time. A single particle mass spectrometer (RSMS-II), operating
in the 70-150 nm particle diameter range, continuously provides the
chemical composition of individual particles. The mass spectra indicate
which sources are currently affecting the site. Ten ChemVol® samplers
are each assigned one source or source combination, and the RSMS-II
controls which one operates depending on the sources or source
combinations observed. By running this system for weeks at a time,
sufficient sample is collected by the ChemVols for comparative
toxicological studies. This presentation describes the instrument and
algorithmic design, implementation, and first results from operating
this system in Fresno, California, during summer 2008 and winter 2009.PART II
The
performance of the Versatile Aerosol Concentration Enrichment System
(VACES) was assessed in terms of the enrichment factor for particles
and high solubility vapors. For ambient particles and those composed of
oleic acid, polystyrene latex (PSL), and ammonium sulfate, the VACES
enrichment factor (EF(PM)) ranged between 5 and 25 in typical northern
California climate conditions during winter-spring. The EF(PM) values
depended on the combined effects of ambient conditions (temperature and
relative humidity) and VACES operational parameters. Gases ranged in
their behavior from a slight enrichment for ammonia (EF(NH3) = 1.9 ±
0.8) to strong depletion of nitric acid (EF(HNO3) = 0.12 ± 0.06). H2O2
fell in between, with EF(H2O2) averaging 0.37 (± 0.25) and ranging from
0.07 and 0.91 depending on the conditions. Detailed results for H2O2
indicate that there are two competing processes at play: soluble gases
are lost to condensed water in the VACES, particularly in the saturator
water bath but also other locations, depleting outlet gas-phase
concentrations. Working in the opposite direction, H2O2 and other
soluble gases are also concentrated together with particles. Depending
on conditions, depletion and concentration play larger or smaller
roles. Presumably, the gases are absorbed into the particles as they
take up water, pass through the concentration step, and are released
once particles are re-dried. The relative importance of these competing
processes appear to follow in order of Henry’s law solubilities, with
losses more important for the most soluble gases and particle-mediated
concentration dominating for less soluble ammonia. This
presentation will outline the experimental design and results, and
discuss some changes made to the VACES that enhance its performance.
Anthony Wexler, Ph.D., is Professor of Mechanical and Aerospace Engineering, Civil and
Environmental Engineering, and Land, Air and Water Resources.
Dr. Wexler is also Director of the Air Quality
Research Center (AQRC), Crocker Nuclear Laboratory (CNL), and USEPA’s
San Joaquin Valley Aerosol Health Effects Research Center (SAHERC).
For information
on this Series please contact: For
a complete listing of the ARB Chairman's Series and the related
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