The aerodynamic particle sizer (APS) model 3321 (TSI, Inc., St. Paul, MN) aims to resolve issues identified with an earlier APS, model 3320. These issues include discrepancies between concentrations measured in summing and in correlated modes, and the creation of "anomalous", large particles caused by recirculation within the detection region. In the present work, the number concentration of a laboratory aerosol was measured with the APS 3321 to be statistically the same in summing mode and in correlated mode for all particles except those in bin 1, < 0.523 mum. Further, anomalous large particles were not measured with the APS 3321. The counting efficiency of the APS 3321 was lower than that for the APS 3320 and ranged from 40% to 60% for particles from 0.8 to 4 mum, respectively. Thus, concentrations reported by the APS 3321 were lower than those measured by the impactor. However, because counting efficiencies were roughly constant with particle size and anomalous particles were absent, the shape of the size distribution was similar to that obtained using the impactor. (C) 2003 Elsevier Science Ltd. All rights reserved.