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The performance of the 13-stage MOUDI-II cascade impactor (MSP Model 122,30 L/min) was studied for the collection efficiency and particle loss.Monodisperse test particles in the aerodynamic size range between 17 to 450 nm were used for the tests,which were generated by a constant output atomizer (TSI Model 3076) using solutions of Oleic Acid (OA) or Dioctyl Sebacate (DOS) of different concentrations.From the experimental data,it was observed that the cutoff aerodynamic diameter (dpa50) of 7,8 and 9th stage of the MOUDI-II were 10,8 and 3% lower than the nominal values given by Marple et al.,(1991) and particle loss increased from 13.85 to 31.96 % as the particle diameter was decreased from 83.5 to 17 nm for the inlet to the 8-th stage (cutoff diameter is 180 nm).The loss is may be due to the high velocity jet from the nozzles,which creates a flow field recirculation region near the rear end of the nozzles leading to diffusional deposition of nanoparticles.To increase the accuracy of the cutoff diameters,new nozzle plates with smooth micro-orifice nozzles were fabricated for the 7th to 10th stages using a semiconductor technique in hoping to reduce possible clogging inside the nozzles.The values of S/W (S: jet to plate distance,W: nozzle diameter) of these lower stages were adjusted so that dpa50 values were close to the nominal values of the MOUDI-II.Results showed that S/W,3.5 (7th stage),3.6 (8th stage) and 13.8 (9th stage) were different from those specified by Marple et al.,(1991),which were 4.1 (7th stage),6.4 (8th stage) and 10.6 (9th stage).It is expected that the cascade impactor with new micro-orifice plates are able to obtain more accurate mass distributions of ambient aerosols.