Nature and Use of a Cascade Impactor

Nature and use of a Cascade impactor

So much has been written about what a cascade impactor is very few have ever commented as to what it is used for.

A Cascade Impactor is an aerodynamic sampling device which is intended to utilize, selectively, the forces of inertia to permit the determination of the size distribution parameters of an aerosol. In modern usage, the size distribution of an aerosol is assumed to be best approximated by the lognormal distribution. Therefore, the two parameters used to define most distributions are the Mass Median Aerodynamic Diameter (MMAD) and Geometric Standard Deviation (GSD). These parameters are determined from impactor stage mass loadings by a standardized form of data analysis. From these parameters the portion of the distribution that corresponds to all the “key diameters” of interest can be obtained. At the present time these are:

1 All of the diameters are applied to the unofficial area of Indoor Air Quality.
2 This diameter has no official status but is considered important with regard to hexavelent Chromium.
3 This diameter has gained widespread acceptance as a ‘critical value’ in several fields.
4 This diameter has no official status but is considered important with regard to aerosols of Diesel origin.

Aerodynamic Equivalent Diameter Aerodynamic Equivalent Diameter normalizes particles with different densities by adjusting the actual diameter of a particle to the equivalent diameter of a particle that has unit density. All impactors utilize the concept of an aerodynamic diameter when determining the cut-points for each stage. This obviously negates the need to determine stage cut-points for each particle type (density).

Mass Median Diameter is the median of the distribution curve of the aerosol based upon particle mass (as opposed to number, surface or volume values, among many). It is now considered to be the most significant measurement.

Geometric Standard Deviation is the size distribution parameter that indicates the width of an aerosol distribution. A value of 1 indicates no spread – all the particles are the same diameter. Almost all discovered distributions lie between 1.5 and 3

Given an impactor with stage cut-diameters close to, or coincidental with, the “key diameters” noted above, there is a desire to determine the concentration of particles associated with that diameter by simply calculating the concentration from the mass collected on the stage. However, this method will not yield correct results.

An example of this error may be viewed at: Data Analysis. For this reason we have endeavored to provide spreadsheets for all Cascade Impactors which we sell. They may be found on the web page corresponding to each model.