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

Particulate nitrate photolysis regenerates reactive oxidized nitrogen species (HONO and NO_<i>x</i>) 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 (<i>j</i>) 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). <i>j</i> 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 (<i>r</i>* = 706 μm), below which <i>j</i> scales inversely with the droplet radius and above which <i>j</i> 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, <i>r</i>* = 706 μm), the surface-driven size-dependent photolysis likely explains the variations in atmospheric particulate nitrate photolysis rates.