Matrix-Dependent Photolysis of Neonicotinoid Insecticides: An Overlooked Source of Reactive Volatile Organic Compounds and Brown Carbon.

Neonicotinoid insecticides (NNs) generate matrix-dependent photoproducts with significant atmospheric implications. Through high-resolution mass spectrometry analysis of five NNs (nitenpyram, thiamethoxam, imidacloprid, clothianidin, and dinotefuran), we demonstrate that solid-phase photolysis produces 2.5 times more gaseous products (430 vs 170 compounds) than liquid-phase reactions. Solid-state transformations yield concerning chlorinated VOCs (13% of products) and 59 light-absorbing brown carbon (BrC) species, while aqueous-phase reactions favor oxygenated CHO compounds (72-100% abundance). Mechanistic studies reveal that confined radicals in solids promote hazardous byproducts, including high-DBE (≥4) Cl-VOCs and BrC chromophores (DBE/C > 0.5). Volatility analysis shows that 85% of products are VOCs/IVOCs, but low-volatility BrC from solids may nucleate secondary aerosols. These findings identify NNs residues on crops and soils as previously overlooked emission sources for reactive compounds affecting air quality and the climate. Regulatory assessments must account for matrix-dependent photochemical pathways to fully evaluate NNs' environmental impacts.