Collection of Airborne Spores by Circular Single-stage Impactors With Small Jet-to-plate Distance

<div class="line" id="line-5"> 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, <span style="font-size: 117%;"> S/W </span> , 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 <span style="font-size: 117%;"> 2.5&mu; </span> m (Air-O-Cell and Burkard samplers) or greater while some fungal species produce spores of ca. 1.8&ndash; <span style="font-size: 117%;"> 2.5&mu; </span> m in aerodynamic diameter. In this study, we evaluated the single-stage circular-jet impactors with very small jet-to-plate distances ( <span style="font-size: 117%;"> S/W&LessLess;1 </span> ). The laboratory and field data obtained with test particles of different sizes and different origin (biological and non-biological) demonstrated the feasibility of these &ldquo;incorrectly designed&rdquo; 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 ( <span style="font-size: 117%;"> d </span> <span style="font-size: 70.7%;"> 50 </span> ): from <span style="font-size: 117%;"> 2.5&mu; </span> m to about <span style="font-size: 117%;"> 1&mu; </span> m. This reduction of cut-off size makes such an impactor efficient for collecting spores of all fungal species ( <span style="font-size: 117%;"> &ges;1.8&mu; </span> m) and even some bacterial species ( <span style="font-size: 117%;"> &ges;1&mu; </span> m). 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 &ldquo;diametric slice&rdquo;. 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 <span style="font-size: 117%;"> S/W&asymp;0.099 </span> , 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.</div>