Particulate Amine Salts as Dynamic Hubs for Atmospheric Nitrogen Exchange

Abstract

Amine salts in atmospheric particulate matter have long been regarded as chemically inert and nonvolatile sinks for organic nitrogen. Here, we overturn this paradigm by demonstrating that these salts instead serve as dynamic hubs for nitrogen exchange. By using integrated ab initio calculations and high-resolution mass spectrometry, we show that dimethylamine (DMA) and trimethylamine (TMA) salts are efficiently initiated by •OH, generating reactive radical cation intermediates, which is not considered in previous organic nitrate chemistry. Meanwhile, sulfates dramatically accelerate the subsequent oxidation process by lowering free energy barriers by orders of magnitude compared to water-mediated pathways. Quantitative branching ratios reveal a bifurcated fate for oxidized amine salts: over 47.5% yields low-volatility amides (CH2NCHO and (CH3)2NCHO) that preferentially remain in the particle phase, and the remaining 52.5% produces volatile species (CH3NH2 and CH3NC) that partition to the gas phase. The latter branch is a previously unrecognized nitrogen re-release source, which explains the missing sources of HCN in remote background and amide in aged particles. These findings fundamentally revise the role of particulate amine salts in atmospheric organic nitrogen budgets, long-range transport, and nitrogen deposition in remote ecosystems.