Dichotomous Virtual Impactors for Large Scale Monitoring of Airborne Particulate Matter.

摘要

The bimodal particle size distribution in the urban aerosol suggests the use of a dichotomous sampling method with a cut point near two microns. The collection of the two size fractions separately should aid the identification of pollutant sources and the evaluation of fine particle-related health hazards. A sampler in which size segregation is effectd by inertial impaction across a virtual surface into a volume of relatively stagnant air was constructed. Following a two stage separation,the particle size fractions are deposited on membrane filters. One filter collects 95% of uncontaminated fine particles while the other collects all of the coarse particles along with 5% of the fine fraction. The sampling scheme is tailored to match the requirements of elemental analysis by X-ray fluorescence and total mass measurements by beta gauging. The design parameters of the virtual impactor have been optimized for minimum loss and sharp cut characteristics. A servo flow controller is utilized to maintain a constant sampling rate of 50 l/min with a paricle size cut at 2.4mu m Stokes'diameter. Fully automated units have been developed for a computer controlled monitoring network. The compatibility of the virtual impactor with other analytical instrument and its adaptability to automation suggest that it may be suited for wider use.