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Reactive oxygen species (ROS) play a central role in chemistry in cloud water, as well as in other aqueous phases such as lung fluid and in wastewater treatment. Recently, work simulating nascent cloud droplets showed that aerosol particles produce a large burst of OH radicals when they first take up water. This activity stops abruptly, within two minutes. The source of the OH radicals is not well understood, but it likely includes the aqueous phase chemistry of ROS and/or organic hydroperoxides and redox active metals such as iron and copper. ROS and their precursors are in general highly reactive and labile, and thus may not survive during traditional sampling methods, which typically involve multi-hour collection on a filter or direct sampling into water or another collection liquid. Further, these species may further decay during storage. Here, we develop a technique to grow aerosol particles into small droplets and capture the droplets directly into a vial containing the terephthalate probe in water, which immediately scavenges OH radicals produced by aerosol particles. The method uses a Liquid Spot Sampler. Extensive characterization of the approach reveals that the collection liquid picks up substantial OH/OH precursors from the gas phase. This issue is effectively addressed by adding an activated carbon denuder. We then compared OH formation measured with the direct-to-reagent approach vs. filter collection. We find that after a modest correction for OH formed in the collection liquid, the samples collected into the reagent produce about six times those collected on filters, for both PM2.5 and total suspended particulate. This highlights the need for direct-to-reagent measurement approaches to accurately quantify OH production from ambient aerosol particles. 

Ascorbic acid is among the most abundant antioxidants in the lung, where it likely plays a key role in the mechanism by which particulate air pollution initiates a biological response. Because ascorbic acid is a highly redox active species, it engages in a far more complex web of reactions than a typical organic molecule, reacting with oxidants such as the hydroxyl radical as well as redox-active transition metals such as iron and copper. The literature provides a solid outline for this chemistry, but there are large disagreements about mechanisms, stoichiometries and reaction rates, particularly for the transition metal reactions. Here we synthesize the literature, develop a chemical kinetics model, and use seven sets of laboratory measurements to constrain mechanisms for the iron and copper reactions and derive key rate constants. We find that micromolar concentrations of iron(III) and copper(II) are more important sinks for ascorbic acid (both AH₂and AH⁻) than reactive oxygen species. The iron and copper reactions are catalytic rather than redox reactions, and have unit stoichiometries: Fe(III)/Cu(II) + AH₂/AH⁻ + O₂→ Fe(III)/Cu(II) + H₂O₂ + products. Rate constants are 5.7 × 10⁴and 4.7 × 10⁴ M⁻² s⁻¹for Fe(III) + AH₂/AH⁻and 7.7 × 10⁴and 2.8 × 10⁶ M⁻² s⁻¹for Cu(II) + AH₂/AH⁻, respectively.