Determining the effective density of airborne nanoparticles using multiple charging correction in a tandem DMA/ELPI setup

Increasing numbers of workers are exposed to airborne nanoparticles, the health effects of which remain difficult to evaluate. Effective density is considered to be a key characteristic of airborne nanoparticles due to its role in particle deposition in the human respiratory tract and in the conversion of number distributions to mass distributions. Because effective density cannot be measured directly, in this study the electrical mobility and aerodynamic equivalent diameters of airborne nanoparticles were measured simultaneously (tandem DMA/ELPI). Test aerosols consisted of spherical Di-Ethyl-Hexyl-Sebacate nanoparticles produced by nebulization (PALAS AGK 2000). To take into account the presence of multiply-charged particles at the DMA outlet, a theoretical model was developed in which the successive mechanisms undergone by particles are accounted for. Using this model, it is possible to determine the proportion of each population exiting the DMA (p = 1, 2, …5 elementary charges) in each channel of the overall ELPI signal. Thus, particle effective density can be estimated for each population. The results indicate that using the ELPI signal alone could lead to significant misevaluation of particle effective density, with biases up to 150%. However, when the proportion of each population is taken into account, particle effective density is determined within ± 15% of the theoretical value.