This study presents results from a field evaluation of a mobile versatile aerosol concentration enrichment system (VACES), designed to enhance the ambient concentrations of ultrafine (less than 0.18 mum), fine (0-2.5 mum), and coarse particles (2.5-10 mum) for in vivo and in vitro toxicity studies. The VACES may be coupled to an exposure chamber system to assess exposure-dose effects of any one, or all, of ambient aerosol on either human subjects and/or animals. Alternatively, concentrated ultrafine, fine and coarse particles can be directly collected by impaction onto a medium suitable for application to cell cultures for in vitro evaluation of their toxic effects. The enrichment and preservation of ambient ultrafine, fine and coarse particles by size and chemical composition was determined by comparisons made between the VACES and a co-located multistage MOUDI impactor, used as a reference sampler. Furthermore, preservation of the ultrafine fraction is measured by the enrichment based on ultrafine particle numbers, morphological characteristics as well as their elemental carbon (EC) content. The results suggest that the concentration enrichment process of the VACES does not differentially affect the particle size or chemical composition of ambient PM. The following fractions: (1) mass (coarse and fine PM); (2) number (ultrafine PM); (3) sulfate (fine PM); (4) nitrate (fine PM, after correcting for nitrate losses within the MOUDI); (5) EC (ultrafine PM); and (6) selected trace elements and metals (coarse and fine PM), are concentrated very close to the "ideal" enrichment value of 22-thereby indicating a near 100% concentration efficiency for the VACES. Furthermore, ultrafine particles are concentrated without substantial changes in their compactness or denseness, as measured by the fractal dimension analysis.