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

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 via simulated atmospheric oxidation by the hydroxyl radical (OH٠). Methodology: An oxidation flow reactor (OFR) was used in this work to expose pollen constituents and extracts of two pollen species (lodgepole pine and aspen) to OH٠ approximating 4.5 days of equivalent atmospheric age. 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/OC) of pollen extracts increased. Saccharides did not decrease appreciably upon oxidation in the OFR. In 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, notably an increase in hydroxyproline and otherwise general decreases in abundance of other free amino acids, were observed. 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 previously assumed for bioaerosols, potentially informing new estimates for elemental signatures of aged bioaerosols in future atmospheric budgeting studies. The observed formation of photochemical oxidation products (e.g. hydroxyproline) suggests potential chemical tracers for aged bioaerosols, and implies that health effects of bioaerosols could be modified while in the atmosphere, necessitating further investigation.