Title:
Understanding bioaerosols atmospheric lifecycle, abundance variability and impacts
Understanding bioaerosols atmospheric lifecycle, abundance variability and impacts
Author(s)
Negron, Arnaldo Andres
Advisor(s)
Nenes, Athanasios
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Abstract
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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Date Issued
2019-12-18
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Text
Resource Subtype
Dissertation