Photooxidative aging of water-soluble pollen extracts and their primary constituents in an oxidation flow reactor

Environmental context Atmospheric transformations of biological aerosols were investigated in this study using an oxidation flow reactor. Pollen extracts, as well as common biological molecules (including free amino acids and saccharides) were tested. These experiments indicate that free amino acids may be reactive at atmospherically relevant conditions, emphasizing the need for further research to determine the extent of bioaerosol transformations in the atmosphere. Rationale Chemical aging of biological aerosols (bioaerosols) and their constituents in the atmosphere have not been well characterized so far. Thus, the goal of this research is to test the hypothesis that bioaerosol constituents undergo chemical transformations by simulated atmospheric oxidation by the hydroxyl radical (OH˙). Methodology In this work, an oxidation flow reactor (OFR) was used to expose pollen constituents and extracts of two pollen species (lodgepole pine and aspen) to OH˙ to simulate atmospheric pollen that is ~4.5 days old. Thermo-optical analysis and liquid and gas chromatography coupled to mass spectrometry were used to quantify changes in chemical composition upon oxidation. Results The organic mass to organic carbon ratio (OM/5OC) of pollen extracts increased. Saccharides did not decrease appreciably upon oxidation in the OFR. By contrast, oxidation of the free amino acid proline resulted in hydroxyproline formation. Changes in the abundance of individual amino acids upon oxidation of the aerosolized pollen extracts were observed, notably an increase in hydroxyproline and otherwise general decreases in abundance of other free amino acids. Discussion The results indicate that bioaerosol chemical composition can change significantly due to atmospheric photochemical transformations. Such transformations may lead to OM/OC ratios higher than those of other aerosol types and those generally assumed for bioaerosols. These findings provide new estimates for elemental signatures of aged bioaerosols for future atmospheric budgeting studies. The observed formation of photochemical oxidation products (e.g. hydroxyproline) suggests that such products could serve as chemical tracers for aged bioaerosols. The observed chemical changes imply that health effects of bioaerosols could also be modified in the atmosphere, necessitating further investigation.