Uptake of ammonia by mixed sulfate-bisulfate cluster cations under multicollisional conditions: approaching equilibrium particle formation in ion traps.

Ammonia is a key base driving atmospheric new particle formation by stabilizing sulfur-containing clusters through acid-base interactions. Here we investigate ammonia uptake by mass-selected sodium sulfate-bisulfate cluster cations using an ion trap operated under multicollisional conditions at room temperature, complemented by quantum chemical calculations. Mixed sulfate-bisulfate clusters serve as a model system that allows systematic control of the number of acidic protons, providing insight into the mechanistic aspects of ammonia uptake. The results show that the maximum ammonia uptake strongly correlates with the number of bisulfate units in the cluster, demonstrating that acidic protons govern ammonia stabilization through ammonium-(bi)sulfate ion pair formation. Thermochemical calculations confirm that the ammonium formation is energetically highly favorable. Kinetic measurements demonstrate sequential ammonia uptake, in which the first adsorption step is rate-limiting and concurrently stabilizes the cluster, leading often to lower evaporation rates in subsequent steps. Increasing the bisulfate content enables additional uptake steps but reduces the overall reaction rate. The Gibbs free energies of ammonia uptake derived from experiment are in very good agreement with the calculated values, suggesting that the system is approaching equilibrium under the experimental conditions.