Rapid Check of Cascade Impactor Cut-Sizes using a Polydisperse Challenge Aerosol Calibration Method

Abstract

A method has been developed to check the 50% cut-size values of cascade impactor stages. The method involves generating a broadsize, log-normally distributed aerosol that covers the range of 50% cut-sizes for the impactor being tested. The amount of deposit on each impaction plate is analyzed and a histogram of the resulting aerosol size distribution plotted, using the amount of aerosol collected on each impaction plate and the published values for the cut-sizes of the impactor in question. If the particle size distribution indicated by the histogram does not result in a log-normal distribution, one or more of the assumed cut-sizes of the impactor are in error. The incorrect cut-sizes of the impactor can then be adjusted until the curve is log-normal, and these adjusted cut-sizes are the correct values for the impactor stages. INTRODUCTIONThe traditional method of calibrating an impactor stage is to generate a monodisperse aerosol and pass the aerosol through the impactor. The amount of particles collected on the impactor stage and the amount of particles penetrating the stage is determined by a variety of methods depending upon the nature of the challenge aerosol. The fraction of the sum of these two quantities of particles that are collected on the impaction plate is the particle collection efficiency for that particle size and that stage (Marple et al. 2003b). A number of these tests are run with a variety of monodisperse sizes to determine the particle collection efficiency curve of the stage from near 0% efficiency to near 100% efficiency. The value of the collection efficiency corresponding to 50% efficiency is defined as the cut-size of the stage.The above method is labor intensive and rather time consuming since the quantity of particles collected for each test must be determined. Thus, other methods have been developed that are not so time consuming.One of these methods is to generate monodisperse aerosols and measure the concentration of particles upstream and down-

Second Generation Micro-Orifice Uniform Deposit Impactor, 120 MOUDI-II: Design, Evaluation, and Application to Long-Term Ambient Sampling

“…The maximum spatial aerosol concentration variation was found to be less than the measurement uncertainly of ±4% and the temporal uniformity was found to be constant as a function of time. A high-output polydisperse aerosol generator (Marple et al 2006) was used to generate a polydisperse aerosol from a solution of 5% oleic acid tagged with an uranine dye tracer in methanol. After the solution was aerosolized, it was dried using dilution air and brought to electrostatic charge equilibrium using a Po 210 radioactive neutralizer.…”