Mechanism for Rapid Conversion of Amines to Ammonium Salts at the Air–Particle Interface

The effect of sulfuric acid (SA) concentrations on heterogeneous reactions of amines such as methylamine (MA), dimethylamine (DMA), and trimethylamine (TMA) at the air-particle interface is investigated using combined classical molecular dynamics, Born-Oppenheimer molecular dynamics, and quantum chemical calculations. The results show that the mixtures of these amine vapors can accumulate at the air-particle interface and then participate in two types of heterogeneous reactions depending on the SA concentrations in the aqueous particles. At high SA concentrations, amines are neutralized by H₃O⁺ and form ammonium salts within only a few picoseconds. At low SA concentrations, amines mainly proceed by hydrolysis reactions and produce ionic pairs of ammonium and OH^-. However, the formed ionic pair is extremely unstable, and the reverse reaction takes place. Considering that the salt conversion time scales of amines at high SA concentrations are 2.5-15 times faster than those at low SA concentration, amine accumulation at high acidity particles is more favored.