Absorption and Scattering Properties of Atmospheric Molecular Clusters

Information about the optical properties of atmospheric molecular clusters is scarce as they are challenging to measure using current experimental techniques. Here we explore the absorption and Rayleigh scattering properties of acid-base molecular clusters using quantum chemical methods. We studied 127 small (acid)_1-2(base)_1-2 cluster systems, with the acids sulfuric acid (SA), methanesulfonic acid (MSA), nitric acid (NA), and formic acid (FA) in all combinations of the bases ammonia (AM), methylamine (MA), dimethylamine (DMA), trimethylamine (TMA), and ethylenediamine (EDA). To further explore the effect of cluster size on the optical properties, we studied the large (SA)_<i>n</i>(AM)_<i>n</i> cluster systems, with <i>n</i> up to 15 acid-base pairs. We calculated the polarizability tensors and the 10 lowest excitation energies at the CAM-B3LYP/aug-cc-pVTZ level of theory. We find that the isotropic polarizability is almost linearly dependent on the cluster size, with small variations depending on the cluster composition. The anisotropic polarizability is plateauing as a function of cluster size. The larger the cluster, the more dominant the isotropic contribution becomes in the calculation of the Rayleigh light scattering activity. As a consequence, the Rayleigh scattering activity will increase quadratically as a function of cluster size. We stress that future studies on the scattering properties should be evaluated as effective scattering, taking the concentrations of the clusters into account. We find that the clusters absorb infrared (IR) radiation in the atmospheric spectral window region but speculate that their lifetime is too short to be competitive with common greenhouse gases. Due to the lack of strong chromophores in the studied acid-base clusters, the ultraviolet-visual (UV-vis) absorption is found to occur in the deep UV. Hence, clusters with more organic content should be studied in the future. Finally, we outline several directions in which the field of studying the optical properties of clusters and aerosols using response theory methods could evolve.