ARB Research Seminar
This page updated June 19, 2013
Characterization and Improvement of the Versatile Aerosol Concentration Enrichment System (VACES)
Anthony Wexler, Ph.D., Director, Air Quality Research Center, Crocker Nuclear Center, and San Joaquin Valley Aerosol Health Effects Research Center, University of California, Davis
June 02, 2010
Cal EPA Headquarters, 1001 "I" Street, Sacramento, CA
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).