Determination of the ratio of diffusion charging-based surface area to geometric surface area for spherical particles in the size range of 100–900 nm

Diffusion charging-based surface area for spherical particles was measured and compared with geometric surface area in the submicrometer size ranging from 100 to 900 nm. Spherical aerosol particles (polystyrene latex particles (PSL) and droplets of diethylhexyl sebacate (DEHS)) were generated by electrosprays for 100–600 nm particles and by a condensation generator for 700–900 nm particles. Two commercially available diffusion chargers (DCs) (DC2000CE, Ecochem, USA; LQ1-DC, Matter Engineering, Switzerland) were challenged with monodisperse uncharged spherical aerosols. Results showed that the surface areas measured by the two DCs were proportional to mobility diameter to power 1.22 and 1.38, respectively, in the size range from 100 to 900 nm. Comparison of the DC-based surface area with theoretical active surface area resulted in reasonable agreement within ±30%, indicating that the DCs underestimate geometric surface area of particles. The deviation of the DC-based surface area from the geometric surface area was quantitatively measured and was found to be up to 94% in the size range studied. Three types of aerosol particles were used to validate the correction of the DC deviation from the geometric surface area for particles larger than 100 nm based on the fit obtained for spherical particles in this study: spherical silver particles, carbon nanofibers, and titanium dioxide agglomerates. Comparison of the corrected DC-based surface area to Brunauer–Emmett–Teller (BET)-measured surface area indicated that the DC surface area reasonably agrees with the BET value for the particles tested except carbon nanofibers with 300 nm modal diameter.