A method for measuring the density of irregularly shaped biological aerosols such as pollen

This paper introduces a method to measure the density of irregularly shaped particles. The technique, demonstrated on corn pollen, is based on measuring the settling velocity of the particles at low Reynolds numbers (Stokes flow) in two fluids with different density and viscosity. If different samples are used, they should have statistically similar shapes and size distributions, and should not absorb the fluids. The settling velocities are measured using digital holography. Based on the mean velocities, the density of corn pollen is 1.14×103±0.05×103 kg/m3. The uncertainty is mostly caused by natural size variations, measured using optical and scanning electron microscopy. The optically measured diameter is 78.1 μm. Pre-treatment for SEM in a vacuum chamber shrinks the grains to 71.2 μm, whereas in water they swell to 89.2 μm and become spherical. The paper contains a comparison to previous measurements, and explanations on causes for discrepancies.