Chemical composition, volatility, and oxidative aging of pollen in the atmosphere
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Authors
Axelrod, Kevin Mark
Issue Date
2023
Type
Dissertation
Language
Keywords
amino acid , atmospheric aging , bioaerosol , pollen , saccharide , volatility
Alternative Title
Abstract
Biological aerosols, also known as bioaerosols, are atmospheric aerosols that are biological in origin. Examples include pollen, fungi, bacteria, archaea, algae, viruses, plant or animal debris, and biological molecules (e.g. lipids, proteins, saccharides), and can be emitted from both biogenic (natural) and anthropogenic (human-influenced) sources. While bioaerosols are of very high interest due to their connection with the spread of infectious diseases, most recently the COVID-19 pandemic, the overall impact of bioaerosols in the Earth system is much broader. Bioaerosols are emitted into the atmosphere in highly varying quantities and massively influence the spread and reproduction of organisms as well as serve as cloud and ice condensation nuclei in the atmosphere with implications on meteorology and climate. Bioaerosols may also participate in atmospheric chemistry processes, many of which remain uncharacterized and can significantly influence how bioaerosols behave, physically, chemically, and biologically in the environment.This dissertation research focused on the bioaerosol pollen due to its large but highly variable emissions in the atmosphere, its role in plant reproduction, and its human allergen component. Pollen was studied for its chemical composition, volatility, and chemical transformations via oxidative aging with a wide variety of analytical chemistry and aerosol measurement and sampling techniques in a laboratory setting. Pollens of several individual tree, grass, and shrub species were characterized for their chemical composition due to the current lack of knowledge regarding the differing chemical contributions that different species may make in the atmosphere’s total organic compound budget. Analytical chemistry techniques, which included gas chromatography mass spectrometry and liquid chromatography mass spectrometry, were used to assess the profile of saccharides, amino acids, anhydrosugars, and various other acids in the individual pollen species. Saccharides were found to be the most abundant chemical species in most pollens, while amino acid profile varied widely among different pollens. Water soluble extracts of the pollens of two tree species (aspen and lodgepole pine), as well as main pollen chemical constituents, were analyzed for their volatility (tendency to reside in the gas phase at atmospherically relevant conditions), in order to gauge their potential to participate in gas phase chemistry in the atmosphere. Different aerosol volatility measurement techniques, which implemented various laboratory setups involving scanning mobility particle sizers and a thermodenuder, were used to model important volatility parameters of compounds such as saturation vapor concentrations. In this research, we found that several chemicals found in pollen displayed a wide range of saturation vapor concentrations, some of which were in the semivolatile range, indicating potential for gas-phase chemical reactions in the atmosphere. We also found that two different species of pollen have different volatility profiles, indicating that different pollen species can contribute to gas-phase organic matter in the atmosphere differently. Depending on a variety of environmental factors, they may contribute appreciably to semivolatile concentrations in ambient air. We also simulated atmospheric oxidation of these pollens and their constituents via a laboratory experiment utilizing an oxidation flow reactor which. Via the collection of various filter and sorbent media in these experiments, we compared the difference in chemical composition between fresh and aged (oxidized) aerosol. We found that two different species of pollen extracts both increase their organic mass to organic carbon ratio due to oxidation, and many contribute to high ratios of organic mass to organic carbon in ambient air, both freshly-emitted and atmospherically aged. In summary, this work sheds light on the differences in chemical composition between different pollen species, establishes certain pollen extracts and constituents as semivolatile, and indicates that pollen organic compounds undergo oxidative aging in laboratory experiments. However, future research is necessary in order to better identify and quantify the specific chemical reactions and products of the aging of these bioaerosols in the atmosphere.