A Two-Stage Virtual Impactor for Continuous Concentration of Particles in an Aerosol

Abstract

A two-stage virtual impactor has been designed and fabricated to concentrate aerosol particles. In this impactor, the minor flow from the first stage of the impactor is introduced into the second stage as the primary flow. The cut-point of the impactor is designed to be ~0.3 μm. The sample aerosol is introduced into the impactor at a flow rate of 8.0 atm- ℓ min.⁻¹ and after the two stages of separation, particles (> 0.5 μm diameter) are collected into the minor flow of 0.06 atm- ℓ min⁻¹ with 84% efficiency. The tandem arrangement of the two stages increases the concentration of larger particles by a factor of > 100. The small size and the low flow rates make the impactor compatible with the sampling requirements of instruments used for the chemical and physical characterization of particles from low-particle number density aerosols. In particular, the high particle concentration factor of the impactor will make it possible to analyze individual particles in real time from aerosols containing < 10 particles cft⁻¹ by the mass spectrometric methods developed in our laboratory.