Fast Hydroxyl Radical Generation at the Air-Water Interface of Aerosols Mediated by Water-Soluble PM2.5 under Ultraviolet A Radiation

Due to the adverse health effects and the role in the formation of secondary organic aerosols, hydroxyl radical (OH) generation by atmospheric fine particulate matter (PM) has been of particular research interest in both bulk solutions and the gas phase. However, OH generation by PM at the air-water interface of atmospheric water droplets, a unique environment where reactions can be accelerated by orders of magnitude, has long been overlooked. Using the field-induced droplet ionization mass spectrometry methodology that selectively samples molecules at the air-water interface, here, we show significant oxidation of amphiphilic lipids and isoprene mediated by water-soluble PM2.5 at the air-water interface under ultraviolet A irradiation, with the OH generation rate estimated to be 1.5 x 1016 molecule center dot s-1 center dot m-2. Atomistic molecular dynamics simulations support the counter-intuitive affinity for the air-water interface of isoprene. We opine that it is the carboxylic chelators of the surface-active molecules in PM that enrich photocatalytic metals such as iron at the air-water interface and greatly enhance the OH generation therein. This work provides a potential new heterogeneous OH generation channel in the atmosphere.

Automated summary

This study reveals that atmospheric fine particulate matter (PM) can generate a significant amount of hydroxyl radicals (OH) at the air-water interface. The researchers observed the oxidation of amphiphilic lipids and isoprene by water-soluble PM2.5 under ultraviolet A irradiation, with an estimated OH generation rate of 1.5 x 1016 molecule center dot s-1 center dot m-2. Atomistic molecular dynamics simulations support the unexpected affinity of isoprene for the air-water interface. This work suggests a potential new heterogeneous OH generation channel in the atmosphere.