Surface-sensitive spectrometry: new insights into radical reactions at interfaces

Oxidized organics on atmospheric particle surfaces strongly influence water uptake, toxicity, and heterogeneous reaction kinetics. However, the nature of surface species and their interaction with gas-phase radicals are not well-understood. Here, experiments probed the impact of gas-phase HO₂ and RO₂ radicals on the surface products formed in the reaction of gas-phase OH with solid glutaric acid (GA) particles at 298 K and 1 atm in air. Hydroxyl radicals were formed from the reaction of tetramethylethylene (TME) with ozone in a flow system and reacted with atomized, dried GA particles. Surface products including alcohols, carbonyls, hydroperoxides, and organic peroxides were detected using Matrix Assisted Ionization in Vacuum - Mass Spectrometry (MAIV-MS), an emerging surface-sensitive technique. A product tentatively identified as an ester from reactions between surface-bound GA and gas-phase radicals was also observed. Concentrations of gas-phase radicals (RO₂ and HO₂) were varied by altering TME concentrations or by adding methanol or acetone, significantly impacting the observed product distribution. Particle size was also varied to alter the surface density of RO_2(surf) and explore the role of surface availability. The results show that the fates of surface-bound radicals are largely determined by reactions with gas-phase HO₂ or RO₂. This complex competition is central in determining the surface composition of organic particles, and therefore the chemistry and environmental impacts of oxidized airborne organic particles.