Formation of Reactive Oxygen Species by Ambient Particulate Matter: Probing causative agents and the underlying mechanism

Aerosol aging plays an important role in modifying aerosol chemical composition, hygroscopicity, cloud condensation nuclei activity and optical properties. Aqueous phase aerosol aging often involves reactive oxygen species (ROS), which include hydroxyl radicals (OH), hydrogen peroxide (H2O2), superoxide anion (O2.-) and organic peroxides. Similar oxidation processes in cloud water may provide a pathway for secondary organic aerosol (SOA) formation through cloud processing during nighttime. Very limited studies have reported the direct quantification of OH and H2O2 production by ambient particles; and the contribution from different redox-active species to formation of ROS is not well understood. To address these questions, we combine field studies with laboratory experiments. Two field campaigns were carried out at Claremont, CA in summer and Fresno, CA in the winter. Samples collected on Teflon filters were extracted in an atmospherically relevant solution (water at pH3.5) and physiologically relevant solutions (surrogate lung fluid, pH 7.4). Marine samples of engine emissions from a research vessel when operating on Ultra Low Sulfur Diesel (ULSD) and Hydrogenation-Derived Renewable Diesel (HDRD) were collected during dedicated cruises in 2014 and 2015, including aged samples collected by re-intercepting the ship plume. These samples were analyzed for particle mass, H2O2 and OH generation, soluble transition metals and speciated soluble iron (iron speciation and H2O2 was not quantified in the marine samples). Our results show that soluble iron was about 50% Fe(II) and 50% Fe(III) in the Claremont samples but that the balance was slightly in favor of Fe(II) in the Fresno samples (65%) in pH3.5. The mass concentration was somewhat higher in Fresno, and on a per-mass basis, the Fresno PM generated a similar amount of H2O2 but far more OH than the Claremont PM. This high OH production was driven by the nighttime and morning samples, which were about 2.5 times more active than the Claremont samples; afternoon samples in Fresno were less active than Claremont. Nighttime and morning samples in Fresno had a strong signature from biomass burning HUmic LIke Substances (BBHULIS), and this material was very strongly correlated with OH formation, r2=0.89. Because of fluorescence interference in the OH assay from BBHULIS, a method was developed to separate the BBHULIS from the OH probe (4-hydroxyterephthalic acid) using Hydrophilic Lipophilic Balance (HLB) cartridges. Particles generated from HDRD combustion had slightly to significantly (5-50%) higher OH generation activity when extracted under surrogate lung fluid (SLF) than those from ULSD and freshly emitted particles exhibited lower activity than aged plumes. To better understand the mechanism of ROS formation by ambient samples, the ability of synthetic mixtures containing quinones, metals and Humic/Fulvic acids (surrogates for BBHULIS) to produce ROS was investigated. When added alone, most of the redox active species tested did not produce ROS at atmospherically relevant concentrations. Addition of quinones did not show or only showed slight enhancement of ROS formation from Fe. In the contrast, the synergism of Cu and 1,4-Naphthoquinone produced more ROS than Cu alone