1Atmospheric Composition Research, Finnish Meteorological Institute, 00101, Helsinki, Finland
2Cooperative Institute for Research in the Environmental Sciences, 80309, Boulder, USA
3Department of Chemistry and Biochemistry, University of Colorado at Boulder, 80309, Boulder, USA
4Department of Physics, University of Helsinki, Post Office Box 64, 00014, Helsinki, Finland
5Department of Civil and Environmental Engineering, University of Southern California, Los Angeles, CA, 90089, USA
6Aerodyne Research Inc., Billerica, MA 01821, USA
*now at: Tofwerk AG, 3600 Thun, Switzerland
Received: 08 Feb 2014 – Published in Atmos. Meas. Tech. Discuss.: 21 Mar 2014
Abstract. The performance of the miniature Versatile Aerosol Concentration Enrichment System (m-VACES; Geller et al., 2005) was investigated in laboratory and field studies using online instruments. Laboratory tests focused on the behavior of monodisperse ammonium sulfate (AS) or dioctyl sebacate (DOS) particles in the m-VACES measured with the aerodynamic particle sizer (APS) and scanning mobility particle sizer (SMPS). The ambient measurements were conducted at an urban site in Helsinki, Finland, where the operation of the m-VACES was explored in conjunction with a Soot Particle Aerosol Mass Spectrometer (SP-AMS) in addition to the SMPS. In laboratory tests, the growth of particles in water vapor produced a stable droplet size distribution independent of the original particle size. However, when the droplets were dried with the goal of measuring the original size distribution, a shift to larger particles was observed for small particle sizes (up to ~ 200 nm in mobility diameter). That growth was probably caused by water-soluble organic compounds absorbed on the water droplets from the gas phase, but not evaporated in the drying phase. In ambient measurements, a similar enrichment was observed for nitrate and sulfate in the m-VACES whereas the presence of acidic ambient particles affected the enrichment of ammonium. Gaseous ammonia was likely to be absorbed on acidic particles in the m-VACES, neutralizing the aerosol. For organics, the enrichment efficiency was comparable with sulfate and nitrate but a small positive artifact for hydrocarbons and nitrogen-containing organic compounds was noticed. Ambient and concentrated organic aerosol (OA) was analyzed further with positive matrix factorization (PMF). A three-factor solution was chosen for both of the data sets but the factors were slightly different for the ambient and concentrated OA, however, the data set used for the PMF analysis was limited in size (3 days) and therefore had substantial uncertainty. Overall, the operation of the m-VACES was not found to lead to any severe sampling artifacts. The effect of acidity could be an issue in locations where the aerosol is acidic, however, in those cases the use of a denuder (which was not used in this study) is recommended. Further ambient tests are needed for the characterization of the m-VACES as the time period for the ambient measurements was only 5 days in this study. Especially for OA additional tests are important as the chemical properties of organics can differ widely depending on time and location.
Revised: 27 May 2014 – Accepted: 28 May 2014 – Published: 15 Jul 2014
Saarikoski, S., Carbone, S., Cubison, M. J., Hillamo, R., Keronen, P., Sioutas, C., Worsnop, D. R., and Jimenez, J. L.: Evaluation of the performance of a particle concentrator for online instrumentation, Atmos. Meas. Tech., 7, 2121-2135, doi:10.5194/amt-7-2121-2014, 2014.