Robust quantification of the burst of OH radicals generated by ambient particles in nascent cloud droplets using a direct-to-reagent approach

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.

Automated summary

Reactive oxygen species (ROS) play a crucial role in various aqueous phases, including cloud water, lung fluid, and wastewater treatment. Recent studies on simulating nascent cloud droplets have shown that aerosol particles produce a significant amount of OH radicals upon taking up water, but the source of these radicals is not well understood. Traditional sampling methods may not effectively capture ROS due to their reactivity and lability. In this study, a technique using a Liquid Spot Sampler was developed to directly capture aerosol particles into a vial containing a terephthalate probe in water, allowing immediate scavenging of OH radicals. Comparisons between this direct-to-reagent approach and filter collection showed that the reagent collection method produced about six times more OH radicals, highlighting the importance of accurately quantifying OH production from ambient aerosol particles using direct-to-reagent measurement approaches.