Microdroplet Surface Drives and Accelerates Proton-Controlled, Size-Dependent Nitrate Photolysis

Particulate nitrate photolysis regenerates reactive oxidized nitrogen species (HONO and NO_x) in the atmosphere, influencing tropospheric ozone, atmospheric oxidation capacity, and particulate matter formation. However, reported nitrate photolysis rates vary by orders of magnitude, and the roles of protons and the air-water interface in accelerating nitrate photolysis in deliquesced aerosols (i.e., microdroplets) remain unclear. Using confocal Raman spectroscopy and aerosol optical tweezer, we measured nitrate photolysis rate coefficients (j) in microdroplets, ranging from 5.28 × 10^-8 to 9.07 × 10^-6 s^-1, depending on pH (3.2-7.4) and radius (3.7 μm to 6.0 mm). j remained pH-independent under proton-sufficient conditions (pH < 6.5) but decreased with increasing pH under proton-deficient conditions (pH > 6.5), resolving discrepancies in previous studies. Moreover, we identified a tipping-point radius (r* = 706 μm), below which j scales inversely with the droplet radius and above which j plateaus at the aqueous bulk value (6.73 ± 2.12 × 10^-8 s^-1). These findings demonstrate that nitrate photolysis occurs predominantly at the air-water interface in microdroplets. With the atmospheric aerosol pH and radii typically below these tipping points (pH* = 6.5, r* = 706 μm), the surface-driven size-dependent photolysis likely explains the variations in atmospheric particulate nitrate photolysis rates.