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## Q. 8.1

Clouds of Water Droplets
Consider four different monodisperse aerosols consisting of water droplets (ρ = $10^9$ mg/m³) in air, as in Table E8.1. As can be seen in the table, the number concentration gets higher as the aerosol particles get smaller, such that the mass concentration of each aerosol is the same, c = 0.5236 mg/m³.

 Table E8.1 Four sample monodisperse aerosols of water droplets, each with a different number concentration, but with the same mass concentration. aerosol $D_P$(μm) $C_{number}$ (particleS/m³) c (mg/m³) A 1,000 1 0.5236 B 100 $10^3$ 0.5236 C 10 $10^6$ 0.5236 D 1 $10^9$ 0.5236

To do: Verify that mass concentration is insufficient to distinguish one aerosol from another.

## Verified Solution

Aerosol A consists of one large water drop ($D_p$ = 1,000 Jim, about the size of a rain drop) suspended in a cubic meter of air, yet produces the same mass concentration as aerosol D which consists of $10^9$ water droplets ($D_p$ = 1 μm) in a cubic meter of air! Obviously mass concentration alone does not satisfactorily differentiate one aerosol from another.
Discussion: Other factors such as average mass, average diameter, or median diameter, along with other statistical properties are required in order to discriminate one polydisperse aerosol from another.