Heterogeneous uptake of amines onto kaolinite in the temperature range of 232–300 K

Abstract. Amines contribute to atmospheric reactive nitrogen (N_r) deposition, new particle formation and the growth of nano- and sub-micron particles. Heterogeneous uptake of amines by ammonium compounds and organic aerosols has been recognized as an important source of particulate organic nitrogen. However, the role of mineral dust in the chemical cycle of amines is unknown because the corresponding reaction kinetics are unavailable. In this study, the heterogeneous uptake of methylamine (MA), dimethylamine (DMA) and trimethylamine (TMA) by kaolinite was investigated in the temperature range of 232–300 K using a Knudsen cell reactor. Lewis acid sites on kaolinite were identified as dominant contributors to the uptake of amines, utilizing Fourier transform infrared spectroscopy. The uptake coefficients (γ) were derived from the mass accommodation coefficients based on the temperature dependence of the γ. The initial effective uptake coefficients (γ_eff) were (2.27 ± 0.26) × 10⁻³, (1.71 ± 0.26) × 10⁻³ and (2.95 ± 0.63) × 10⁻³, respectively, for MA, DMA and TMA on kaolinite at 300 K, while they increased ~3-fold with decreasing temperature from 300 K to 232 K. The adsorption enthalpies (△H_obs) of MA, DMA and TMA on kaolinite were −7.8 ± 0.8, −9.9 ± 2.9 and −9.4 ± 1.0 kJ mol⁻¹, respectively, and the corresponding entropy values (△S_obs) were −77.1 ± 3.2, −84.1 ± 11.8 and −80.6 ± 3.7 J·K⁻¹·mol⁻¹. The lifetimes of MA, DMA and TMA attributable to heterogeneous uptake by mineral dust were estimated to be 7.2, 11.5 and 7.7 h, respectively. These values were comparable to the lifetimes of amines consumed by OH oxidation. Our results reveal that uptake by mineral dust should be considered in models simulating the chemical cycle of amines in the atmosphere. The results will also aid in understanding the possible impacts of amines on human health, air quality, and climate effects.