Abstract We report experimental evidence of the spontaneous formation of NO3− in suspended aqueous droplets containing NO2−, driven by hydroxyl radicals and NO2 produced by the intrinsic properties of the air‐water interface. This oxidation of nitrite anions is observed in single droplets of micrometer size using optical tweezers, with the υ1(N–O) Raman mode of NO3− detected around 1,050 cm−1 as a function of time. The influence of pH, particle size, initial NO2− concentration, and different gaseous atmospheres (air and nitrogen) has been systematically studied. In sharp contrast, no bulk oxidation was observed under similar conditions. By using KSCN as an OH radical scavenger, we demonstrate the critical role of hydroxyl radicals generated at the air‐water interface in nitrate formation. These experiments suggest that a particle effect is indeed at play, leading to the spontaneous formation of NO3− through the interaction of NO2 and OH radicals.