From quantum chemical formation free energies to evaporation rates

I K Ortega,V Loukonen,H Vehkamäki,O Kupiainen,M J Mcgrath,T Kurtén,O Wilkman,T Olenius

doi:10.5194/acp-12-225-2012

Abstract

Abstract. Atmospheric new particle formation is an important source of atmospheric aerosols. Large efforts have been made during the past few years to identify which molecules are behind this phenomenon, but the actual birth mechanism of the particles is not yet well known. Quantum chemical calculations have proven to be a powerful tool to gain new insights into the very first steps of particle formation. In the present study we use formation free energies calculated by quantum chemical methods to estimate the evaporation rates of species from sulfuric acid clusters containing ammonia or dimethylamine. We have found that dimethylamine forms much more stable clusters with sulphuric acid than ammonia does. On the other hand, the existence of a very deep local minimum for clusters with two sulfuric acid molecules and two dimethylamine molecules hinders their growth to larger clusters. These results indicate that other compounds may be needed to make clusters grow to larger sizes (containing more than three sulfuric acid molecules).

Highlights

Climate change is one of the central scientific issues in the modern world
We have found that dimethylamine forms much more stable clusters with sulphuric acid than ammonia does
We have used the formation free energies obtained with this approach to calculate the evaporation rates of sulfuric acid clusters containing either ammonia or DMA molecules, with up to four acids and bases

Summary

Introduction

While the effects of long-lived greenhouse gases are fairly well known, the radiative forcing associated with atmospheric aerosols is still very uncertain (Baker and Peter, 2008). Atmospheric aerosols have adverse effects on human health and deteriorate visibility (Nel, 2005; Pope and Dockery, 2006). While new-particle formation is observed everywhere in the Earth’s atmosphere (Kulmala et al, 2004), the actual birth mechanism of particles is still uncertain. In terms of the molecular species participating in nucleation, the only thing known for certain is that sulfuric acid is somehow involved (Kuang et al, 2008; Sihto et al, 2006). Sulfuric acid alone cannot explain the observed particle formation rate, and several other candidates, both organic and inorganic, have been proposed. Base molecules like ammonia and especially amines are one of the strongest candidates to be relevant in atmospheric nucleation; they have been the focus of numerous studies in recent years (Kurten et al, 2008; Loukonen et al, 2010; Nadykto et al, 2011; Ge et al, 2011, 2011a)

Methods

Results

Conclusion