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.