Solid–Water–Air Interface Accelerated Photochemical Reactions in Mineral Dust Microdroplets: An Overlooked Source of Atmospheric •OH

The hydroxyl radical (•OH) plays a pivotal role in atmospheric chemistry, yet the generation mechanisms of •OH at solid–water–air interfaces (SWAIs) under real atmospheric conditions remain incompletely understood and require further in-depth investigation. This study identifies mineral dust (e.g., hematite) microdroplets as an important •OH source via SWAIs photochemical reactions. Remarkably, under simulated sunlight and at pH 3, hematite microdroplets enhance •OH production by 2 orders of magnitude compared to the bulk solution. This enhanced activity is governed by their exposed facets, which regulate the iron redox cycle. SWAIs improve O2 exchange efficiency, and smaller microdroplets accelerate •OH generation owing to improved O2 accessibility. DFT results further suggest that interfacial electric fields can reduce the band gap, enhance photoinduced electron excitation, and thereby may facilitate •OH generation. Our results further demonstrate that the enhanced transformation of phenol and SO2 underscores the atmospheric significance of SWAIs. Our findings demonstrate that mineral-dust-bearing microdroplets constitute a previously overlooked yet critical source of •OH. This elucidates the crucial mediating role of SWAIs in climate-chemistry feedback loops, influencing the formation and transformation processes of sulfate aerosols, secondary organic aerosols, and greenhouse gas emissions.