Evaluation of the performance of a particle concentrator for on-line instrumentation
Authors: Saarikoski S. , Carbone S. , Cubison M. J. , Hillamo R. , Keronen P. , Sioutas C. , Worsnop D. R. , Jimenez J. L. 
Published: March 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 on-line 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, similar enrichment factors (EFs) were observed for nitrate, sulfate, organics and refractory black carbon. Size-dependent EFs showed a small shift in the accumulation mode peak size after the m-VACES. The presence of acidic ambient particles affected the enrichment of ammonium and chloride. Gaseous ammonia was observed to be absorbed on acidic particles in the m-VACES, neutralizing the aerosol. As a result the contribution of ammonium to particle mass increased from 6% for ambient to 9% for concentrated aerosol. The opposite trend was observed for chloride, since a fraction of chloride evaporated from acidic particles upon neutralization. Organic artifacts were quite small but a small positive artifact for hydrocarbons and nitrogen-containing organic compounds was observed. However, the oxidation state of organics remained nearly the same before and after the m-VACES. Ambient and concentrated 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. That could reflect the sensitivity of PMF to minor changes in OA composition; however, the data set used for the PMF analysis was limited in size and therefore had substantial uncertainty. Eight trace elements (Al, V, Fe, Zn, Rb, Sr, Zr and Cd) were detected with the SP-AMS of which three (Sr, Zr and Cd) were observed only with the m-VACES. Trace elements seemed to be enriched in the m-VACES similar to major inorganic and organic species. 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.