Structural Characteristics of Medium‐Sized Sulfuric Acid–Ammonia Cluster Cations and Anions and their Atmospheric Implications

Sulfuric acid-ammonia clusters play important roles in atmospheric nucleation processes. However, experimental structural analysis of relative medium-sized or large-sized clusters at the molecular level has always been lacking. Herein, tandem mass spectrometry and IR photondissociation (IRPD) spectroscopy are applied to mass-selected medium-sized cluster ions that have diameters of 1.0-1.2 nm. The differences in their dissociation behaviors and IRPD spectra of the cluster cations and anions are attributed to their structural characteristics, which are further revealed by the combination of theoretical calculations. The protonated clusters can be treated as structures composed of ammonium cations and hydrogen sulfate anions with an alternate acid-base arrangement, which makes their dissociation characterized by the loss of the acid-base pair. For deprotonated clusters, the mobility of intra-cluster protons lowers energy barriers of isomerization, making their dissociation pathways more complicated. The differences and commonalities in the formation of organic acid- and inorganic acid-ammonia clusters are also discussed, as well as their impacts on nucleated orders and sizes. The method can be further extended to other clusters for obtaining the lacking information on relative key species in the formation of new particles in the atmosphere.