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
Wednesday, June 2,
Anthony Wexler, Ph.D., Director
PART ICurrent 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 IIThe 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).
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