Evaluation of Epifluorescence Methods for Quantifying Bioaerosol in Air Quality Samples

摘要

Currently U.S. EPA are enforcing the National Ambient Air Quality Standards (NAAQS) to regulate the annual and 24-hour average concentrations of PM2.5 and PM10 in the air. Both PM2.5 and PM10 contain multiple components from multiple sources. Primary biological aerosol particles (PBAP) is an important component of PM, but there is limited knowledge about how PBAP contributes to PM2.5 and PM10 concentrations. There is also a lack in research about the incidence and prevalence of diseases related to PBAP exposure. The main barrier to assess PBAP concentrations and health-related effects is the absence of efficient methodology for quantifying PBAP or its surrogate. Fluorescence methods have been used to quantify bioaerosol containing fluorophores in PM samples, but the long turnaround time and uncharacterized performance limited its applications. This study evaluates different protocols for quantifying PBAP in PM2.5 and PM10 collected on polycarbonate filters using an epifluorescence microscope. PBAP was stained with DNA markers directly on filter (direct-stain) or after they had been washed off from the fitter (extract-stain), followed by a series of fixation, microscopic imaging, and automated particle counting. The methods were first validated using reference samples prepared by depositing known concentrations of E. coli or Aspergillus fumigatus spore on blank polycarbonate fitters. It was found that the direct-stain method required the least sample preparation while achieving a linear response over two orders of magnitude (r2 = 0.9) for PBAP > 0.3-μm diameter and a counting accuracy within ±25% when particle concentrations are >10 times the detection limit. The linear relationship held for PM-preloaded samples, showing the feasibility of measuring PBAP associated with ambient PM. During a two-month monitoring campaign at Las Vegas, Nevada in spring 2017, PBAP concentrations in PM10 ranged from 0.1 to 1.5 #cm-3 while 38% of PBAP were attributed to coarse particles between 2.5 and 10 urn. The extract-stain yielded only about half of PBAP counting, likely due to particle loss during extraction. The direct-stain method was also applied to MOUDI (multi-stage impact sampling) samplers for size-segregated measurement, showing that PBAP level peaked at 2.5 - 5.6 μm diameter. The controlling factors for PBAP episodes at Las Vegas will be discussed.