Nitrite to Nitrate Spontaneous Oxidation in Suspended Aqueous Droplets

Abstract We report experimental evidence of the spontaneous formation of NO 3 − in suspended aqueous droplets containing NO 2 − , driven by hydroxyl radicals and NO 2 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 υ 1 (N–O) Raman mode of NO 3 − detected around 1,050 cm −1 as a function of time. The influence of pH, particle size, initial NO 2 − 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 NO 3 − through the interaction of NO 2 and OH radicals.