Decomposition and Growth Pathways for Ammonium Nitrate Clusters and Nanoparticles.

Area of Science:

• Atmospheric Chemistry
• Physical Chemistry
• Materials Science

Background:

• Aerosolized ammonium nitrate contributes to haze formation, impacting air quality and climate.
• Accurate modeling of aerosol thermodynamics and kinetics requires understanding ammonium nitrate formation and decomposition.
• Investigating cation and anion clusters provides insights into early-stage nucleation and growth pathways.

Purpose of the Study:

• To identify structures of ammonium nitrate clusters observed in mass spectra.
• To understand the growth and decomposition mechanisms of aerosolized ammonium nitrate.
• To develop accurate models for intermolecular forces in ammonium nitrate systems.

Main Methods:

• Simulated annealing Monte Carlo optimization to predict minimum-energy structures.
• Density functional theory (DFT) optimizations, specifically ωB97X-D3, to calculate interaction energies.
• Validation of DFT calculations using CCSD(T) and MP2 methods for small clusters.

Main Results:

• Successfully identified the structures for all detectable species in positive and negative ion mass spectra of ammonium nitrate.
• Demonstrated thermodynamic control over particle growth and decomposition via loss of ammonia or nitric acid.
• Presented structures and interaction energies for larger ammonium nitrate nanoparticles, including novel trigonal pyramidal morphologies.

Conclusions:

• The study provides the first comprehensive identification of ammonium nitrate cluster structures in mass spectra.
• Findings advance the understanding of aerosol haze formation and guide the development of more accurate atmospheric models.
• The research offers insights into nanoparticle formation and morphology, relevant for atmospheric science and materials research.