Collection of airborne spores by circular single-stage impactors with small jet-to-plate distance

Most of the commonly used bioaerosol samplers are single-stage impactors that meet the conventional Marple's design criteria: their non-dimensional jet-to-plate distance, S/W, is greater than the established threshold (1.5 for rectangular nozzles and 1 for the circular ones). Recent studies have shown that these samplers underestimate the concentration of airborne fungal spores because their cut-off size is about 2.5μm (Air-O-Cell and Burkard samplers) or greater while some fungal species produce spores of ca. 1.8-2.5μm in aerodynamic diameter. In this study, we evaluated the single-stage circular-jet impactors with very small jet-to-plate distances (S/W≪1). The laboratory and field data obtained with test particles of different sizes and different origin (biological and non-biological) demonstrated the feasibility of these "incorrectly designed" impactors for the spore collection and total enumeration (viable + non-viable spores). A decrease in the jet-to-plate distance resulted in a critical decrease of the impactor's cut-off size (d50): from 2.5μm to about 1μm. This reduction of cut-off size makes such an impactor efficient for collecting spores of all fungal species (≥1.8μm) and even some bacterial species (≥1μm). Since the spore surface density across the circular deposit area was non-uniform, three sample reading procedures were evaluated: the entire area count, random partial count, and a partial count on a rectangular "diametric slice". The collection efficiency data suggested that a relatively small jet-to-plate distance is likely to result in excessive shear forces in the impaction zone, thus enhancing the spore deaggregation and bounce. The coefficient of inter-sample variation of the field samples, collected by commercially available impactors with S/W≈0.099, did not exceed 50% for the total spore count. The highest variability was observed for Arthrospores, which were more aggregated than other types of fungi.