Atmospheric Sulfuric Acid-Water-Ammonia Particle Formation Using Quantum Chemistry

3 Computing formation free energies for nucleating clusters 422 

The scope of this paper is to provide an overview of methods used to study properties of electrically neutral molecular clusters initiating particle formation in the troposphere, with focus on quantum chemistry. The review of results is intended to be complete with regard to water-sulfuric acid-ammonia clusters. Concerning studies on clusters including other molecular species, we review representative examples and newest publications. Ionic clusters and clusters involving iodine, related to coastal nucleation, are mentioned in passing. 

Atmospheric aerosols are liquid or solid particles floating in the air. Their size ranges from nanometer-scale molecular clusters to, for example, road dust with a 

Particle formation events from gaseous precursors are observed frequently almost everywhere in the troposphere, both in polluted cities and remote clean areas [4]. It is likely that different nucleation mechanisms are at work in different conditions, but no formation mechanism has been identified so far. It is, however, clear that particles are formed by nucleation of a multicomponent vapor mixture. Water vapor is the most abundant condensable gas in the atmosphere, but it can not form particles on its own homogeneous nucleation requires such a high supersaturation, that heterogeneous nucleation on omnipresent pre-existing particles always starts first and consumes the vapor. However, vapor that is un- 

When the formation free energy is plotted against the number of molecules, in a one-component vapor the critical cluster size is the location of the maximum of 

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Investigating Atmospheric Sulfuric Acid-Water-Ammonia Particle Formation Using Quantum Chemistry... 

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