This paper presents results from a field evaluation of a mobile high-capacity particle size classifier (HCPSC) which samples at 8501 min(-1) and classifies ambient particles in three size groups: coarse, fine (or accumulation) and ultrafine modes. Particles in coarse and fine modes are collected by impaction on a small, uncoated surface area (14 cm(2)) and can be extracted with a small amount of solvent for toxicological tests and chemical characterization. Particle bounce is eliminated by using cavity-type impaction substrate configurations. Polyurethane foam is used as the impaction substrate material. Ultrafine particles are collected on 20 x 25 cm quartz or Teflon filters(TM) (PTFE). The performance of the HCPSC was evaluated in a field study conducted in the Netherlands from May to August 1999. Field tests showed that mass, chloride, nitrate and sulfate concentrations in the coarse mode (2.5-10 mum) measured by means of the HCPSC and a collocated Micro-Orifice Uniform Deposited Impactor (MOUDI) were in very good agreement (within 10% or less). Comparison between fine mode PM concentrations obtained with the HCPSC and MOUDI indicated that approximately 10-15% of fine mode particles are carried over to the ultrafine mode due to imperfect collection efficiency characteristics of rectangular geometry impactors (HCPSC) compared to round nozzle impactors (MOUDI), Comparisons between the PM-2.5 nitrate concentrations obtained with the HCPSC and MOUDI and those with a collocated Harvard/EPA Annular Denuder Sampler (HEADS) yielded average nitrate sampling efficiencies of about 80 and 70% for HCPSC and MOUDI, respectively. Nitrate losses from Teflon filters during this study were on the average higher than 50%. (C) 2000 Elsevier Science Ltd. All rights reserved.