Environmentally Persistent Free Radicals in Outdoor Surface Grime during Pollen Season

On outdoor surfaces, grime is an important substrate for the deposition and reactive loss of semivolatile and particulate bound pollutants. A previously unexplored source of grime reactivity is environmentally persistent free radicals (EPFRs), which are precursors to reactive oxygen species (ROS). To measure EPFRs in grime, grime was collected in Spring 2023 on glass beads deployed in suburban North Carolina, where possible primary EPFR sources include pollen and nearby prescribed burning. EPFRs are measured in all collected grime samples via electron paramagnetic resonance spectroscopy after extraction of the grime onto filters. Pollen contains the majority of the collected grime mass, and when isolated from the smaller component of the grime constituents via filtration, it comprises most of the measured EPFRs. Endogenous EPFRs were measured in commercial pollen samples (Quercus rubra, Quercus velutina, and Betula pendula), which supports the conclusion that pollen contributed to the overall EPFRs measured in the grime. Grime samples were collected under shaded conditions, which limits photochemistry as a secondary source of EPFRs in the field samples; to further investigate light mediated EPFR formation mechanisms, we also measured EPFRs in grime following laboratory photoaging. EPFRs are formed from irradiation in all grime samples; however, extended irradiation eventually reduces the number of EPFRs to their starting numbers, a trend which was also observed with the irradiation of the commercial pollen. The increase of EPFRs over grime age and with irradiation implies that ROS production in grime from EPFRs may also be sensitive to aging over time.