Gas-Particle Partitioning and Photochemical Processing of Amines in a Megacity of China

Abstract:

Gas-/aerosol-phase (PM_2.5) amines/aminium salts and their photochemical oxidation products were simultaneously measured with a FIGAERO-HRToF-CIMS using water clusters of hydronium ions (H₃O⁺(H₂O)_n, n = 1, 2,3) as reagent ions. Measurements were performed from 26 August to 17 September 2014 in Nanjing, a typical megacity in China. The major detected oxidation products of amines included amides, imines, and nitrosamines. All species except imines were predominantly present in the aerosol phase. The relative abundances (in molarity) of aerosol-phase ammonium, aminium, amidium, iminium, and nitrosaminium ions respectively ranged from 98.84%, 0.50%, 0.26%, 0.32%, and 0.07% in a clean day case to 84.32%, 4.23%, 2.84%, 2.78%, and 5.82% in a polluted day case. Diurnal variations of particulate aminium ions and amidium ions were found anti-correlated. Aminium ions were lower in the daytime and started building up during nighttime. Amidium daytime profiles, on the contrary, followed closely with the solar radiation intensity, indicating that amides were most likely produced by photochemistry instead of multiphase chemistry. The thermograms of aminium ions and their oxidation products in ambient samples showed broadened peaks spanning from ~60 °C to ~120 °C. However, laboratory tests using pure methylaminium sulfate salt produced sharp peaks centered around 158 °C. These findings indicated that aminium ions might not exist as thermostable sulfate salts in the aerosol phase as suggested by their strong basicity.