Arvydas Juozaitis

Method and test system for evaluation of bioaerosol samplers

Abstract A method and test system have been developed for the laboratory evaluation of the performance of bioaerosol samplers. The method differentiates between the overall physical sampling efficiency (which reflects the inlet and collection efficiencies) and the biological sampling efficiency (which reflects the survival of the test microorganisms during the sampling process). The number concentrations of laboratory-generated bioaerosol particles are measured with an aerosol size spectrometer up- and downstream of the bioaerosol sampler being tested. In a bioaerosol impactor, which was specially designed for testing microbiological aspects of bioaerosol sampling, the inlet and collection efficiencies are differentiated by measuring downstream of the collection surface location with and without the collection surface in place. The number of recovered particles is counted as microcolonies with a microscope after sampling the bioaerosol particles into agar and culturing them. The total recovery of these bioaerosol particles is determined as a ratio of the number of viable microorganisms recovered to the number of bioaerosol particles present in the air sampling volume upstream from the sampler. This total recovery is a measure of the ratio of culturable to non-culturable bacteria present in the air. By measuring physical and microbiological aspects simultaneously, information is gained on aspects of bioaerosol sampling that cannot be determined by either of these branches of science alone. This is exemplified by tests on the influence of relative humidity and desiccation time on colony count. The newly-developed system can be used to test any bioaerosol sampler. A special single-stage impactor was designed, built and used to study how different sampling and analysis variables affect the total recovery of bioaerosol particles. The designed impactor was calibrated using PSL particles. Its inlet sampling efficiency was found to be within the range of 96–99.5%, depending on the sampling conditions and particle size, if the latter is less than 8 μm (this range represents single bacteria, bacterial agglomerates, and fungi). The collection efficiency was found to be about 100% when collecting PSL particles larger than 0.7 μm in diameter at 201 min −1 or higher air flows. The total recovery of microorganisms measured under these conditions is characterized only by the “survivability” of microorganisms during their sampling. It was found that relative humidity had a pronounced effect on total Pseudomonas fluorescens recovery. Experimental data also showed that the sampling time may be limited due to bacterial desiccation and subsequent loss in viability of collected microorganisms.

Factors affecting microbiological colony count accuracy for bioaerosol sampling and analysis.

The effects of the following variables on the occurrence of colony masking (the indistinguishable merging or overlap of sufficiently close colonies) were evaluated experimentally using the bacterium Bacillus subtilis: spore density on a collection surface, concentration of nutrients in the culture medium, sample incubation time, and ability of an observation system to distinguish overlapped colonies. Increasing spore surface density and incubation time increased colony masking, whereas lowering nutrient concentration decreased colony diameter and, therefore, masking but also limited spore germination and growth. Overall, full-strength medium was best for accurate counting of early microcolonies examined with the aid of a microscope, whereas half- or quarter-strength medium was better for counting older readily observable macrocolonies. Masking bias was determined for varying spore surface densities and colony diameters and was applied to two widely used slit-to-agar bioaerosol impactors. Appropriate collection times have been determined for these samplers to minimize colony masking for expected bioaerosol concentrations. It was found, for example, that 6-min samples collected from an environment with an air concentration of 10(3) CFU m-3 would result in colony surface densities, for 3-mm colonies, of 1.5 and 3.9 microorganisms cm-2 for the two samplers with respective masking biases of < 10% and < 20%.

Investigations of gas-to-particle conversion in the atmosphere

Continental aerosol particle size distributions were measured as a function of time at the background station Aisetas and in the suburb of Vilnius. Marine aerosol particle formation was studied in outdoor photochemical chamber experiments performed on the coast of the Baltic sea. The functional dependence of the particle diameter growth rate on the particle size was calculated from the aerosol particle size distribution data and was compared with the theoretical predictions for the particle growth due to vapor condensation or due to the adsorption of aerosol precursor gases on the particle surface or within an airbome droplet. It was shown that during summer months condensation of low pressure vapors formed by gas phase homogeneous chemical reactions was predominant mechanism of the continental aerosol particle growth. While in winter growth of a submicrometer continental aerosol particle was frequently governed by two competing processes: condensation of low pressure vapors and heterogeneous oxidation of aerosol precursor gases inside the liquid droplet. Ozone was found to be an active oxidant in the heterogeneous aqueous phase chemical reactions. The outdoor photochemical chamber experiments showed that aqueous phase chemical reactions were responsible for the formation of marine sulfuric acid aerosol particles from the dimethylsulfide. After these particles were partially neutralized by ammonia to the ammonium sulfate or bisulfate, their further growth was observed to be surface limited.

Inlet sampling efficiency of bioaerosol samplers

Abstract The inlet sampling efficiencies of several commercial bioaerosol samplers have been calculated by use of available and newly-developed equations. Under certain sampling conditions, the bioaerosol concentration was found to be significantly over- or underestimated.

Filtration, loading and faceseal leakage characteristics of filtering facepieces

Abstract The filtration and loading characteristics of filtering facepieces have been measured and analyzed, and a new fit testing technique has been developed that measures the aerosol penetration at high and low flowrates and derives from that the size of the leak.