Comparing simulated and experimental molecular cluster distributions.

Area of Science:

• Atmospheric Chemistry
• Aerosol Science
• Computational Chemistry

Background:

• Secondary atmospheric aerosol particle formation begins with gas-phase molecules forming small molecular clusters.
• Experimental detection of electrically neutral clusters, especially for chemical composition, remains challenging below 1 nm.
• High-resolution mass spectrometry excels at characterizing charged clusters but struggles with neutral ones.

Purpose of the Study:

• To simulate electrically neutral and charged molecular clusters of sulfuric acid and ammonia.
• To develop a dynamic collision and evaporation model for these clusters.
• To compare model predictions with experimental data and provide insights into neutral clusters.

Main Methods:

• Simulated neutral and charged clusters using sulfuric acid and ammonia.
• Employed a dynamic collision and evaporation model with classical kinetics for collision frequencies.
• Derived evaporation rates from first-principles quantum chemical calculations without fitting parameters.

Main Results:

• Achieved good agreement between modeled steady-state concentrations of negative cluster ions and experimental APi-TOF data.
• Validated the model against state-of-the-art Atmospheric Pressure interface Time-Of-Flight mass spectrometer (APi-TOF) measurements.
• Demonstrated the model's capability to interpret experimental results for charged clusters.

Conclusions:

• The developed model accurately represents the behavior of charged molecular clusters.
• The model provides valuable information on electrically neutral clusters, which are difficult to measure directly.
• This work advances the understanding of initial steps in atmospheric aerosol formation.