Formation of Reactive Nitrogen Species Promoted by Iron Ions Through the Photochemistry of Neonicotinoid Insecticide

Abstract. Nitrous acid (HONO) and nitrogen oxides (NOx = NO + NO₂) are important atmospheric pollutants and key intermediates in the global nitrogen cycle, but their sources and formation mechanisms are still poorly understood. Here, we investigated the effect of soluble iron (Fe³⁺) on the photochemical behavior of a widely used neonicotinoid (NN) insecticide, nitenpyram (NPM), in the aqueous phase. The yields of HONO and NOx increased significantly when NPM solution was irradiated in the presence of iron ions (Fe³⁺). We propose that the enhanced HONO and NO₂ emissions from the photodegradation of NPM in the presence of iron ions result from the redox cycle between Fe³⁺ and Fe²⁺ and the generated reactive oxygen species (ROS) by the electron transfer between the excited triplet state of NPM and the molecular oxygen (O₂). Using the laboratory-derived parametrization based on kinetic data and gridded downward solar radiation, we estimate that the photochemistry of NPM induced by Fe³⁺ releases 0.50 and 0.77 Tg N year^-1 of NOx and HONO to the atmosphere, respectively.

This study suggests a novel source of HONO and NOx during daytime and potentially helps to narrow the gap between the field observations and model outcomes of HONO in the atmosphere. The suggested photochemistry of NPM can be an important contribution to the global nitrogen cycle affecting the atmospheric oxidizing capacity as well as the climate change.