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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 PM 2.5 at the air-water interface under ultraviolet A irradiation, with the OH generation rate estimated to be 1.5 × 10 16 molecule·s -1 ·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.

The multiphase oxidation of SO 2 to sulfate in aerosol particles is a key process in atmospheric chemistry. However, there is a large gap between the observed and simulated sulfate concentrations during severe haze events. To fill in the gaps in understanding SO 2 oxidation chemistry, a combination of experiments and theoretical calculations provided evidence for the direct, spin-forbidden excitation of SO 2 to its triplet states using UVA photons at an air-water interface, followed by reactions with water and O 2 that facilitate the rapid formation of sulfate. The estimated reaction energy for the whole process, 3 SO 2 + H 2 O + 1/2O 2 → HSO 4 - + H + (298 K, 1 M), was Δ G r = -107.8 kcal·mol -1 . Moreover, calculations revealed that this was a multistep reaction involving submerged, small energy barriers (∼10 kcal·mol -1 ). These results indicate that photochemical oxidation of SO 2 at the air-water interface with solar actinic light may be an important unaccounted source of sulfate aerosols under polluted haze conditions.