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Characterization of bioaerosols using chemical tracers, single-particle fluorescence spectroscopy, and passive sampling
Dissertation   Open access

Characterization of bioaerosols using chemical tracers, single-particle fluorescence spectroscopy, and passive sampling

Chamari Buddhika Ariyawansa Mampage
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2023
DOI: 10.25820/etd.006857
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Abstract

Bioaerosols like pollen grains (10–100 µm), pollen fragments (< 2.5 µm), fungal spores (1–30 µm), and bacteria (0.25–8 µm) can affect human health as allergens, pathogens, and inflammatory agents. Intact pollen grains can rupture to produce smaller pollen fragments. Because of their smaller size, pollen fragments can penetrate deeper into the respiratory tract compared to intact pollen grains and may trigger severe asthma symptoms. Pollen fragments and fungal spores are hypothesized to be a cause of thunderstorm asthma outbreaks that occur when many people experience severe asthma symptoms with the passage of a thunderstorm. To mitigate the health impacts of these bioaerosols, a robust understanding of their size distributions, the meteorological conditions causing their release, and the spatial and temporal variability in their concentrations is required. Herein, this thesis is focused on advancing our understanding of the effect of meteorology on the release and size distribution of bioaerosols, including pollen fragments released by ragweed pollen grains, and the spatial variability of pollen concentrations. To assess the temporal variability, size distributions, and effect of meteorological conditions on atmospheric bioaerosol concentrations, measurements of chemical and biological tracers were conducted during the spring tree pollen season in the Midwestern United States. The concentrations of fructose, a pollen tracer, in particles smaller than 2.5 µm (PM2.5) were higher in the late spring, with the warmer temperatures favorable for the release of pollen. The first co located measurements of fluorescent particles, fructose, and Bet v 1 (birch pollen allergen) indicated that pollen fragment diameters span <0.25 to 2.5 µm, which is roughly one to two orders of magnitude smaller than the intact pollen grains. Peak pollen fragments were observed on rainy days during spring, with peak concentrations coinciding with extreme weather conditions like severe thunderstorms and a tornado. Endotoxin (from gram-negative bacteria) was enhanced on rainy days, primarily in particles 1.0–10 µm. The concentration of mannitol (a fungal spore tracer) in PM2.5 was higher in the late spring, with warmer temperatures favoring the release of fungal spores. Mannitol was primarily elevated in particles 1.0–10 µm, in agreement with intact spore diameters. Overall, our findings showed that, depending on the timing of rain events, high concentrations of pollen fragments and bacteria can be expected during rain and fungal spores overnight. Field measurements of bioaerosol concentration and size distributions were likewise conducted in the summertime, corresponding to the weed pollen season in the Midwestern United States. Similar to springtime, the concentration of mannitol increased following rain, primarily in particles 1.0–10 µm, agreeing with the spore diameters. The concurrent increase of the fluorescent particle concentrations in the size range 1.0–10 µm showed that fungal spores were primarily released overnight, due to the favorable conditions for spore release. Endotoxin from gram-negative bacteria was also enhanced during rain, primarily in particles 1.0–10 µm. Fluorescent particles relevant to bacteria contributed to 0.5–1.0 µm diameter particles, while a combination of bacteria and fungal spores contributed to fluorescent particles with diameters 1.0–10 µm released during rain. Chemical tracers for pollen indicated a lack of pollen rupturing during the weed pollen season, indicating that this phenomenon is more common during thunderstorms in the spring tree pollen season. The simultaneous release of fungal spores and bacteria during summer rain events suggests the potential for co-exposure to these bioaerosol types. Due to the recurrence of the thunderstorm asthma outbreaks, field measurements were conducted in Melbourne, Australia, to use chemical tracers to track pollen rupturing during grass pollen season. Elevated concentrations of chemical tracers of pollen in PM2.5, which is indicative of pollen fragments, primarily occurred on dry and warm days, suggesting the release of pollen fragments. However, thunderstorm asthma was not recorded during the study period, suggesting the released pollen fragment concentrations were below the threshold levels to cause severe asthma symptoms. Mannitol concentrations were primarily observed in 1.0–10 µm sized particles, consistent with the fungal spore diameters. This study showed the possibility of using chemical tracers to track the pollen rupturing events in Melbourne, Australia, where thunderstorm asthma is a persistent health concern. Ragweed pollen, a prevalent allergen in the summer, can trigger allergic rhinitis and asthma symptoms. In a laboratory study, the pollen fragments released from rupturing giant ragweed pollen were characterized for the first time using scanning electron microscopy. The observed pollen fragments ranged in diameter from 20 nm to 6.5 µm, with 82% of the observed pollen fragments being smaller than 1.0 µm in diameter. This study showed the ability of giant ragweed pollen grains to release pollen fragments that are up to three orders of magnitude smaller than the intact pollen. Traditional pollen counting methods are expensive and typically use one location to reflect the pollen concentration in larger surrounding areas. Herein, a low-cost and scalable passive sampling technique was used to evaluate the distribution of pollen concentrations across various landscapes during weed pollen season in Johnson County, Iowa, US. Microscopic slides mounted on custom-made slide holders placed inside a shelter were deployed at 34 sites in August September 2021. The pollen concentrations were heterogeneous in concentrations across the sites. Our findings indicated that higher pollen concentrations were observed at sites with abundant pollen sources nearby. Sites located in developed areas with well-maintained lawns and paved surfaces showed the lowest pollen concentrations. Collectively, this thesis has provided novel insights into the dynamics of atmospheric bioaerosols. Through extensive real-time and offline field measurements conducted in the Midwestern United States and Melbourne, Australia, a deeper understanding of how meteorological conditions influence bioaerosol concentrations and size distributions was gained. The ambient observation of pollen fragments demonstrated the ability to use chemical tracers and real-time fluorescent particle measurements to track the timing of their release and their size distribution in the atmosphere. Additionally, the development of a cost-effective and scalable passive sampling technique to assess pollen concentrations across different landscapes highlights how the surrounding environment affects pollen distribution. Ultimately, this research contributes to our understanding of the atmospheric distribution of bioaerosols and will aid in mitigating the adverse health effects of aeroallergens.  
 Bacteria Bioaerosols Chemical tracers Fungal spores Particulate matter Pollen fragments

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