In the atmosphere, a well-known and conventional pathway toward the formation of ammonium sulfate is through the neutralization of sulfuric acid with ammonia (NH 3 ) in water droplets. Here, we present direct ab initio molecular dynamics simulation evidence of the formation of ammonium bisulfate (NH 4 HSO 4 ) from the hydrated NH 3 and SO 3 molecules in a water trimer as well as on the surface of a water droplet. This reaction suggests a new mechanism for the formation of ammonium sulfate in the atmosphere, especially when the concentration of NH 3 is high (e.g., ~10 μg m −3 ) in the air. Contrary to the water monomer and dimer, the water trimer enables near-barrierless proton transfer via the formation of a unique loop structure around the reaction center. The formation of the loop structure promotes the splitting of a water molecule in the proton-transfer center, resulting in the generation a NH 4 + /HSO 4 − ion pair. The loop-structure promoted proton-transfer mechanism is expected to be ubiquitous on the surface of cloud droplets with adsorbed NH 3 and SO 3 molecules and, thus, may play an important role in the nucleation of aerosol particles (e.g., fine particles PM 2.5 ) in water droplets.
Reference REF presented that NH3 accelerates the formation of ammonium bisulfate and ammonium sulfate in the atmosphere, playing a key role in the formation of PM2.5.