Rayleigh light scattering properties of atmospheric molecular clusters consisting of sulfuric acid and bases

Abstract

The Rayleigh light scattering properties of (H₂SO₄)_a(NH₃)_b and (H₂SO₄)_a((CH₃)₂NH)_b atmospheric molecular clusters have been investigated using a response theory approach. Using density functional theory the molecular structures and stepwise formation free energies of clusters with a and b up to 4 have been re-investigated. The Rayleigh scattering intensities are calculated from the dipole polarizability tensor α using the CAM-B3LYP functional by applying linear response methods. The intrinsic scattering properties of (H₂SO₄)_a(NH₃)_b and (H₂SO₄)_a((CH₃)₂NH)_b indicate that amine containing clusters scatter light significantly more efficiently then their ammonia containing counterparts. Using the Atmospheric Cluster Dynamics Code (ACDC) the steady state cluster concentrations are estimated and the effective scattering is calculated. The effective scattering is shown to be highly dependent on the estimated concentrations and indicates that there exist competitive pathways, such as nucleation and coagulation, which influence the cluster distributions. The frequency dependence of the scattering is found to depend on the cluster composition and show increased responses when clusters contain more bases than acid molecules. Based on structures obtained using semi-empirical molecular dynamics simulations the Rayleigh scattering properties of clusters with up to 20 acid–base pairs are evaluated. This study represents the first step towards gaining a fundamental understanding of the scattering properties of small atmospheric clusters in the ambient atmosphere.