Estimation of the nucleation barrier in a multicomponent system with intermolecular potential

Abstract

The formation of a “critical nucleus” prior to phase change is a crucial step for new particle formation (NPF) in the atmosphere. However, the nucleation occurring below ∼1 nm is hard to observe directly. As an effective alternative, theoretical nucleation models have been widely studied. An energy barrier is involved in the nucleation and is the fundamental factor for the nucleation model. Typical atmospheric nucleation agents such as H₂SO₄, H₂O and NH₃ are dipole molecules, whose intermolecular interactions are non-ignorable. Herein, a dipole–dipole potential model is adopted to determine the interaction between molecules instead of the traditional hard sphere model, and graph theory is used to describe the structure of the cluster and the cluster–molecule interaction. The nucleation barriers (ΔE_b) of H₂SO₄–H₂SO₄, H₂SO₄–H₂O, H₂SO₄–NH₃ and H₂SO₄–H₂O–NH₃ are derived and compared to each other. In the presence of H₂O and NH₃, the ΔE_b value is decreased by 17–28% compared to that in the pure H₂SO₄ nucleation system. NH₃ is identified to be a key factor for ternary nucleation based on an orthogonal test. Atmospheric concentrations of H₂SO₄, H₂O and NH₃ are considered to investigate the influence of [H₂O + NH₃]/[H₂SO₄] on ΔE_b and the related effective collision coefficient (α). The α value in the ternary nucleation system reaches the range of (2.5–25) × 10⁻⁵, which is 3–4 orders of magnitude higher than that in the pure H₂SO₄ system. Due to a significant enhancement of α, NH₃ and H₂O should be focused on in future aerosol particle estimation and control.