The effect of an orifice on the collection efficiency and wall loss of a slit virtual impactor was investigated both numerically and experimentally. The ratios of the collection nozzle width (W c ), distance between acceleration nozzle and collection nozzle (S), length of acceleration nozzle (T), inlet width (D), and nozzle span (l) to the acceleration nozzle width (W a ) were fixed at W c /W a = 1.4, S/W a = 1.5, T/W a = 1.1, D/W a = 6, and l/W a = 10, respectively. The minor-to-total flow ratio was set to 0.1 in laminar flow regime. The collection efficiency and wall loss of the slit virtual impactor were found to be characterized by the square root of the Stokes number. For the slit virtual impactor without an orifice, the square root of the Stokes number corresponding to the cut-off diameter was determined to be (Stk 50 ) 1/2 = 0.77 and the maximum wall loss at the collection nozzle reached 30% or 40%. When an orifice having the same width as the acceleration nozzle was placed upstream of the acceleration nozzle at a distance of 20W a , the value of (Stk 50 ) 1/2 decreased to 0.68 and the wall loss at the collection nozzle decreased below 5%.

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