Collection of Bioaerosol Particles by Impaction: Effect of Fungal Spore Agglomeration and Bounce

Abstract

Calibration and performance evaluations of bioaerosol impactors are usually conducted with non-biological test aerosols, such as polydisperse liquid oleic acid particles or monodisperse solid polystyrene latex (PSL) particles. This study was undertaken to investigate to what degree surface properties and agglomeration of bioaerosol particles may result in different performance characteristics of impactors. The single-stage impaction of biological and non-biological particles on a sticky surface was studied utilizing Air-O-Cell sampling cassettes that are widely used to collect airborne fungal spores. The aerosol concentrations upstream and downstream of the sampler were measured with an aerodynamic particle size spectrometer. The collection efficiency was determined for the sampler operating at different flow rates ranging from 10 to 30 L/min. The tests were performed with aerosol particles of about 1 to 4 μm in diameter, including two fungal species of different surface properties (Penicillium brevicompactum and Penicillium melinii), and two types of non-biological aerosols (oleic acid and PSL). The 50% cut-off sizes determined experimentally with non-biological particles differed from the theoretical predictions by 11% or less. The data obtained with biological test particles, however, were found to show higher (at low sampling flow rates) or lower (at high flow rates) collection efficiencies than determined through the use of conventional non-biological test particles. E.g., at 30 L/min, the difference is about 50%. The differences were attributed to the presence of spore aggregates and their possible deaggregation during impaction. Inertial impaction, deaggregation, and bounce of fungal spores from the collection surface were studied experimentally and estimated theoretically utilizing experimental data on the percentages of singlets, doublets and triplets in specific bioaerosols. It is concluded that the calibration and performance of bioaerosol impactors may strongly depend on the surface characteristics, initial percentage of aggregates, and deaggregation rate of the specific bioaerosol particles being sampled.