Accelerated Photolysis Mechanism of Neonicotinoids at the Air–Water Interface of Microdroplets

Neonicotinoid insecticides are extensively applied worldwide as broad-spectrum pesticides, yet approximately 30–50% of them may be lost to the air during spray application. While photolysis dominates the degradation of neonicotinoids in atmospheric liquid water, existing studies focus primarily on bulk-solution reactions, overlooking the ubiquitous atmospheric microdroplets. Here, we employed in situ single-droplet ultraviolet absorption spectroscopy, revealing that the photolysis rate of neonicotinoids in microdroplets was 25–76 times faster than in bulk solution. A comprehensive characterization of photoproduced reactive intermediates demonstrated that direct photolysis proceeded through denitrification pathways. Mechanistic investigation showed that interfacial enrichment combined with an ultrahigh electric field promotes photolysis, as evidenced by in situ FTIR, surface-enhanced Raman spectroscopy, and DFT calculations. Free energy barriers for excited-state generation decreased linearly with increasing electric field strength at a rate of 104–105 kJ mol–1 au–1. Moreover, we established quantitative structure–activity relationship models to predict photolysis rate constants of neonicotinoids. These findings bridge the critical knowledge gap between traditional bulk solution and microdroplet-mediated atmospheric reactions, providing a scientific foundation for accurately evaluating the environmental fate and transboundary ecological threats of neonicotinoids.