Mechanisms of Atmospherically Relevant Cluster Growth

Area of Science:

• Atmospheric Chemistry
• Climate Science
• Aerosol Science

Background:

• Atmospheric aerosols significantly influence global climate by scattering solar radiation and affecting cloud properties.
• New particle formation (NPF) is a major source of atmospheric particles, yet the underlying chemical processes and growth mechanisms remain poorly understood.
• Predicting particle number concentrations is challenging, hindering accurate climate modeling.

Purpose of the Study:

• To elucidate the molecular mechanisms governing the growth of atmospheric clusters involved in new particle formation.
• To establish a framework for cluster growth incorporating key species like sulfuric acid, ammonia, amines, and water.
• To understand the role of different bases (ammonia vs. amines) in cluster growth kinetics and energetics.

Main Methods:

• Experimental investigation of atmospherically relevant molecular clusters using Fourier transform mass spectrometry.
• Computational modeling using density functional theory to confirm experimental findings and study uncharged/hydrated clusters.
• Analysis of cluster composition, growth pathways, and base incorporation kinetics.

Main Results:

• Cluster growth primarily occurs via an ammonium/aminium bisulfate coordinate, maximizing proton transfer.
• Growth of positively charged clusters involves sequential acidification and neutralization steps, with significant activation barriers for ammonia neutralization.
• Amines facilitate initial cluster growth but become less important as clusters enlarge, especially with hydration.

Conclusions:

• A unified framework for cluster growth involving sulfuric acid, ammonia, amines, and water has been established.
• The interplay between gas-phase basicity and binding energetics governs the preference for amines over ammonia in charged clusters.
• Combined experimental and computational approaches are vital for advancing our understanding of atmospheric NPF and its climate implications.