Understanding bioaerosols atmospheric lifecycle, abundance variability and impacts

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Negron, Arnaldo Andres
Nenes, Athanasios
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Bioaerosols are ubiquitous in the atmosphere and may have important impacts on human health, cloud formation, the hydrological cycles and biogeochemical cycles. Measuring and characterizing bioaerosols remains a challenge owing to their low atmospheric concentration. During this thesis, we have developed an approach to collect large amounts of bioaerosols (e.g., on ground-based or airborne platforms) in a liquid suspension over the sub-hour to multiple hour timescale using a modified high-volume wet cyclone. The bioaerosols are then subsequently characterized using flow cytometry and other biology tools, results leading to robust quantifications of bioaerosol populations. Together with the observations from rapid autofluorescence detection techniques, they can provide powerful insights on the concentration, composition, and activity of bioaerosol with rapid time resolution. The new characterization approach was applied to study bioaerosol populations in multiple, distinct environments: i) an urban environment in the Southeast United States surrounded by heavy forestation (Atlanta, GA), ii) the marine boundary layer, free troposphere, terrestrial environments near California during the BOAS 2015 aircraft campaign, and, iii) the remote Eastern Mediterranean sea influenced by the European continental outflow and Saharan dust events. In the Southeast United States, we observed that the bioaerosol population is highly dynamic and driven by the prevailing meteorology. We detect high concentrations of large bioaerosol population rich in nucleic acid (consistent with wet-ejected fungal spores) during humid and warm days after rain events, while other days are characterized by smaller bioaerosol (consistent with bacteria) that are low in nucleic acid content. During the airborne deployment at the California Coast, small bacterialike particles that are low in nucleic acid content are ubiquitous and tend to be enhanced in the marine free troposphere compared to the boundary layer thought to be the source. Concentrations of microbes in the marine boundary layer are about 10 times less than those found in the airmasses characterized by terrestrial emissions, while the cell types from flow cytometry and light induced fluorescence indicate very different populations. In the Eastern Mediterranean, bioaerosol is dominated by small bioaerosol with low nucleic acid content (consistent with bacterial cells). Interestingly, the highest concentration is not observed during periods where continental outflow airmasses are sampled, but during dust events. The observations carried out during this thesis show that bioaerosol associated with air masses influenced by terrestrial (and especially dust) emissions carry the largest bioaerosol concentrations. We also see that smaller bioaerosol consistent with microbes (with a diameter ~ 1 μm and low nucleic content) are ubiquitous at concentrations ranging between 104 m-3 and 105 m-3. Microbes in the marine boundary layer off the coast of California are about 10 times lower than that observed in terrestrial environments (103 m-3 to 104 m-3), although in the Eastern Mediterranean, bioaerosol concentrations can be as high as in terrestrial environments. Occasionally, we observe concentrations of larger nucleic acid-rich particles (consistent with fungal spores), especially after rain events. The extent to which the fungal spores travel is surprisingly large – given that they are observed at the remote Eastern Mediterranean, hundreds (and maybe thousands) of kilometers away from their terrestrial origin. The impacts of these concentrations and types of bioaerosol in all the environments sampled can be significant. We estimate for example that the phosphorous delivery from bioaerosol to the Eastern Mediterranean Sea, although much lower than recent model estimates, can still explain the concentrations that are associated with background levels of atmospheric phosphorus. In terms of their impacts on clouds, the concentration of marine bioaerosol is high enough to potentially influence ice nucleation in warm mixed-phase cloud, especially given that secondary ice processes are favored and can promote any initial low levels of primary ice. The above mentioned potential impacts of bioaerosol, however, may be modulated by atmospheric processing – very few studies of which exist. Towards this, we studied the response of microbes to simulated atmospheric acidification (a process that occurs everywhere in the atmosphere) by quantifying their cultivability and ability to express ice nucleation capacity as a function of pH levels observed for micron-sized particles in the atmosphere. For this, a droplet freezing assay was developed and used to study the effect of aerosol pH on an ice active P. syringae strain. Surprisingly, the microbes could resist considerable levels of acidification, as they retain their cultivability and ice nucleation capability to pH levels as low as 4. Upon increased acidification, however, (e.g., pH=2.5 or less), the ice active P. syringae lost cultivability and reduced their ice nucleation temperature close to -15oC, approaching the properties of Arizona test dust. Repeated freezing-thawing cycles over the same strains exhibit repeatable ice nucleation results. These results show that models of ice nucleation that consider the effects of bioaerosol need to consider the impacts of atmospheric acidification; the smooth dependence of ice nucleating characteristics (freezing temperature vs. pH) suggests that such effects can be parameterized using the approach developed during this thesis. The methods and scientific results produced during this thesis show that the simple yet powerful methods developed here can be readily used to sample bioaerosol, characterize their population characteristics, metabolic state, ice nucleation activity, and response to a variety of atmospheric stressors.
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