Generation of Nitrite from Residual Urea on Leafy Vegetables Caused by Water Vapor Condensation

Leaves are ubiquitous substrates for multiphase reactions, with growing evidence indicating that various multiphase reactions occurring on leaf surfaces influence the transformation of chemicals adsorbed by leaves. However, the critical role of ambient water in mediating such multiphase reactions on leaf surfaces has often been overlooked. Here, using isotopic tracing techniques, we demonstrate that reactive oxygen species generated in the leaf surface condensate can mediate the oxidation of urea adsorbed on leaves, leading to the formation of nitrite (NO₂^-). This phenomenon is widespread across various leafy vegetables. At 5 °C and an 80% relative humidity (RH), condensate-mediated oxidation of residual urea on leaf surfaces resulted in NO₂^- accumulation exceeding the food limit standard. Hydroxyl radicals (^•OH), amino radicals (^•NH₂), and nitric oxide radicals (^•NO) were produced during water vapor condensation, collectively driving the conversion from urea to NO₂^-. Density functional theory calculations verified that radical-mediated transformation occurs spontaneously. Further studies indicate that controlling urea residues on vegetable leaves below 60 ppm and maintaining RH in cold storage below 60% can effectively suppress NO₂^- formation. Our results demonstrate that water vapor condensation on leaf surfaces can influence the oxidation of organic compounds adsorbed by leaves, providing new insights into the environmental transformation of organic pollutants on plant surfaces.