CONCENTRATION UNITS FOR AEROSOLS

1. CONCENTRATION UNITS FOR AEROSOLS YvesAlarie, Ph.D Professor Emeritus University of Pittsburgh,USA

2. A. MASS CONCENTRATION • mg/m3 for an individual substance or a mixture of substances, i.e., sulfuric acid, coal dust. This refers to the amount (mass) of the substance (mg) dispersed in one cubic meter of air (m3).

3. B. SIZE CONCENTRATIONSDistribution, physical size, vs. mass sizeIf a sample of an aerosol is taken it is very unlikely that the size will be the same for all particles. In practice it has been found that the size distribution is best represented by a log-normal distribution and from this, the median size is obtained and the standard deviation is also obtained. Since this is a log-normal distribution the standard deviation is the geometric standard deviation, abbreviated σg. The distribution can be for the physical size (i.e., count median diameter, CMD) or for the mass size (i.e., mass median diameter, MMD).

4. a) Physical Size (Count Mean Diameter) This is obtained by sampling the aerosol on a filter or appropriate media and “sizing” the collected particles under a light microscope or electron microscope. Also a variety of other methods are available using laser, etc. b) Mass Size (Mass Median Diameter) Most commonly, it is obtained experimentally by using an impactor which has been previously calibrated using spherical particles of a chemical of unit density. When this is done the shape as well as the density of the particle being measured are taken into account.

5. Since both shape and density are taken into account this can be called “mass median aerodynamic diameter” (MMAD), or “mass median aerodynamic equivalent diameter” (MMAED). The concept here is that if a particle of a given shape and given density impacts where a 1 μm spherical particle of unit density impacts, it is said to “behave aerodynamically”, as a 1 μm spherical of unit density particle behaves.

6. Since particle penetration and deposition in the respiratory tract is due to two factors, impaction and sedimentation, and for both mechanisms of deposition the size, shape and density of the particle are primary factors it follows that MMAD must be obtained. This is true for particles > 0.5 μm. For particles < 0.5 μm a third factor is important: diffusion or brownian movement. For these particles their diffusion coefficient is important and this varies with their size, but is independent of their shape or density (i.e., they behave like gas molecules). We will discuss this again below.

7. C. AERODYNAMIC SIZE: DIRECT MEASUREMENT We want to know how the particles behave aerodynamically since this will influence deposition in the respiratory tract. Thus two identical spherical particles (> 0.5 μm) will behave differently if their density is different. Also we seldom have spherical particles. Thus different shapes as well as densities must be taken into consideration. This can be measured experimentally by using a calibrated impactor.

8. The mass of particles deposited on each stage of the impactor is obtained and distribution by mass is then derived. A variety of impactors are available. One is described on the next page and results are given on the following page.

9. Impactors such as the one described on the next page have become very useful for exposure assessment since you can do two things with them:a) you get the particle size distribution and from this, you can determine where the particles are likely to be deposited and retained in the respiratory tract.

10. b) you get the exposure concentration (mg/m3) by adding the mass deposited on each stage of the impactor and on the last stage which is a filter collecting the particles too small to be captured by any stage of the impactor.

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