Image processing method to adjust the ELPI distribution : application to the measurement of aerosol density emitted from combustion sources
Résumé
Fly ashes from combustion sources are great of interest since they can be a risk for health and environment. Number size distribution of aerosols can be obtained using the Electrical Low Pressure Impactor (ELPI), which is dependent on the density ρ. However, the density ρ of the particles is specified by the user and assumed to be constant for the twelve stages. Besides, ρ cannot be measured with usual methods since there is not enough matter collected on each stage of the ELPI. ρ is proportional to the square of the aerodynamic diameter dae over the equivalent (Stokes) diameter deq. Our approach uses electronic microscopy to evaluate ρ at each stage to increase the accuracy of the number size distributions given by the ELPI. Firstly, particles were collected on glass substrates stuck by capillarity on the impaction stages. Then, images were acquired with an electron microscope, and finally, processing tools were applied : pretreatment (noise and contrast), segmentation (discrimination of the particles) and measurement (form factor to check the circularity and area to deduce deq). This method was applied to Silica particles which are defined to have a constant density (ρ = 2.5 g.cm-3 with the pycnometer) : density was found to be 2.2 g.cm-3 and 2.4 g.cm-3 for stages 2 (dae 57nm to 95nm) and 3 (dae 95nm to 158nm), respectively. The results match reality for fly ashes from wood combustion with ρ varying from 1.1 g. cm-3 to 2.4 g. cm-3, respectively for impaction stages 2 to 8.
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