Abstract
Abstract. In the present-day atmosphere, sulfuric acid is the most important vapour for aerosol particle formation and initial growth. However, the growth rates of nanoparticles (<10 nm) from sulfuric acid remain poorly measured. Therefore, the effect of stabilizing bases, the contribution of ions and the impact of attractive forces on molecular collisions are under debate. Here, we present precise growth rate measurements of uncharged sulfuric acid particles from 1.8 to 10 nm, performed under atmospheric conditions in the CERN (European Organization for Nuclear Research) CLOUD chamber. Our results show that the evaporation of sulfuric acid particles above 2 nm is negligible, and growth proceeds kinetically even at low ammonia concentrations. The experimental growth rates exceed the hard-sphere kinetic limit for the condensation of sulfuric acid. We demonstrate that this results from van der Waals forces between the vapour molecules and particles and disentangle it from charge–dipole interactions. The magnitude of the enhancement depends on the assumed particle hydration and collision kinetics but is increasingly important at smaller sizes, resulting in a steep rise in the observed growth rates with decreasing size. Including the experimental results in a global model, we find that the enhanced growth rate of sulfuric acid particles increases the predicted particle number concentrations in the upper free troposphere by more than 50 %.
Highlights
Sulfuric acid (H2SO4) is the major atmospheric trace compound responsible for the nucleation of aerosol particles in the present-day atmosphere (Dunne et al, 2016)
The newly formed particles need to grow rapidly in order to avoid scavenging by larger, pre-existing aerosols and, thereby, contribute to the global cloud condensation nuclei (CCN) budget (Pierce and Adams, 2007). The dynamics in this cluster size range of a few nanometres determines the climatic significance of atmospheric new particle formation (NPF), which is the major source of CCN (Gordon et al, 2017) and can affect urban air quality (Guo et al, 2014)
While the effect of temperature expected from theory is small and cannot be discerned within the statistical uncertainties of our measurements (Nieminen et al, 2010), the insignificant influence of ammonia and the ionization level on the growth rate differs from previous findings (Lehtipalo et al, 2016)
Summary
Sulfuric acid (H2SO4) is the major atmospheric trace compound responsible for the nucleation of aerosol particles in the present-day atmosphere (Dunne et al, 2016). Sulfuric acid participates in new particle formation (NPF) in the upper troposphere (Brock et al, 1995; Weber et al, 1999; Weigel et al, 2011), stratosphere (Deshler, 2008), polar regions (Jokinen et al, 2018), urban or anthropogenically influenced environments (Yao et al, 2018), and when a complex mixture of different condensable vapours is present (Lehtipalo et al, 2018). In the initial growth of small atmospheric molecular clusters, sulfuric acid is likely of crucial importance (Kulmala et al, 2013). The main pathway of cluster and particle growth is condensation of low volatility vapours, like sulfuric acid or oxidized organics (Stolzenburg et al, 2018). Nanoparticle growth rates depend on both the evaporation rates of the condensing vapours and the molecular collision frequencies
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