Charging equivalent diameter relation for sodium chloride and soot aggregates using electrical low-pressure impactor

The average number of charges on non-spherical particles after charging is much higher than for spherical particles of the same mobility equivalent diameter. This makes it difficult to interpret measurements from instruments employing chargers like the electrical low-pressure impactor (ELPI+), which is often used to measure aerosols of varying shapes. Previous efforts to determine this difference have used non-spherical particles classified by a differential mobility analyzer (DMA). However, the presence of multiple charged particles introduces uncertainty, in part given that the multiple charged fractions from the classifier itself are impacted by particle morphology. To largely eliminate this issue, we classify the particles first with an Aerodynamic Aerosol Classifier (AAC) prior to classification with a DMA. Three different aerosols - polyethylene glycol (PEG - spherical), sodium chloride (NaCl - cube-like), and soot aggregates were tested. PEG and NaCl were generated using an atomizer while soot was produced from a laminar inverted flame with either ethylene or propane as the fuel. Both NaCl and soot particles showed a higher average charge than PEG. The charge carried by these particles is a function of the details of the charger, including penetration and ion concentration time, which is minimized by using charging equivalent diameter (d_qe). We found this relation to be d_qe(NaCl) = 1.106 d_m^1.014 and d_qe(Soot) = 1.489 d_m^1.088 (where d_m is mobility diameter in μm). Copyright © 2025 American Association for Aerosol Research