Thermodynamical and Optical Properties of Atmospheric pre-nucleation Cluster Containing Sea Salt, Water and Sulfuric Acid: A Density Functional Theory Study

Sea salts are one of the natural aerosols that are abundantly found in the atmosphere. Their interactions with other atmospheric molecules, particularly H₂O and H₂SO₄, are important, as they can alter the chemical compositions of the atmosphere, impacting nucleation processes and aerosol growth. A comprehensive quantum-chemical study has been performed to analyse the structural stability and thermodynamics using M06-2X/6-311++G(3df,3pd) and optical characteristics with M06-2X and ωB97XD. NaCl and MgCl₂ are treated with basis set 6-311++G(3df,3pd) and aug-cc-pVTZ, while KCl is treated with def2-TZVPP. Among these, ωB97X-D/6-311++G(3df,3pd) demonstrate the best performance. A thermodynamic study reveals that these clusters have the potential to form stable clusters in the atmosphere. Analysis of cluster growth with H₂O and H₂SO₄ molecules suggests that the successive cluster formation is thermodynamically favourable at standard conditions. It is inferred from optical properties that a H₂SO₄ molecule increases the Rayleigh scattering intensities more (∼200%) compared to the H₂O molecule (∼48%). This indicates that the more H₂SO₄ molecules in clusters, the higher contributions to the extinction properties and an enhanced cooling effect of aerosols by reflecting more sunlight to space. The anisotropy and depolarisation calculation suggests H₂O-rich clusters have a similar trend between these parameters, and H₂SO₄-rich clusters do not show such a trend. By linking the molecular-scale properties of scattering to large-scale radiative effects, it can improve the representation of aerosols in climate models, ultimately reducing uncertainties in climate predictions.