Mechanistic understanding of rapid H₂SO₄-HNO₃-NH₃ nucleation in the upper troposphere

The upper troposphere (UT) nucleation is thought to be responsible for at least one-third of the global cloud condensation nuclei. Although NH₃ was considered to be extremely rare in the UT, recent studies show that NH₃ is convected aloft, promoting H₂SO₄-HNO₃-NH₃ rapid nucleation in the UT during the Asian monsoon. In this study, the roles of HNO₃, H₂SO₄ (SA), and NH₃ in the nucleation of SA-HNO₃-NH₃ were investigated by quantum chemical calculation and molecular dynamic (MD) simulations at the level of M06-2×/6-31 + G (d, p). The nucleation ability of SA-HNO₃-NH₃ is suppressed as the temperature increases in the UT. The results indicated that bisulfate (HSO₄^-), nitrate (NO₃^-), and ammonium (NH₄⁺) ionized from SA, HNO₃, and NH₃, respectively, can significantly enhance the nucleation ability of SA-HNO₃-NH₃. In addition, hydrated hydrogen ion (H₃O⁺) as well as sulfate ions (SO₄^2-) ionized by SA can also actively participate in the process of ion-induced nucleation. The results reveal that the enhancement effect of five ions on the SA-HNO₃-NH₃ nucleation can be ordered as follows: SO₄^2- > H₃O⁺ > HSO₄^- > NO₃^- > NH₄⁺. Many ion-induced nucleation pathways of SA-HNO₃-NH₃ with the Gibbs free energies of formation (ΔG) lower than -100 kcal mol^-1 were energetically favorable. HNO₃ and NH₃ can promote the nucleation of SA-HNO₃-NH₃ and water (W) molecules are also beneficial to promote the new particle formation (NPF) of SA-HNO₃-NH₃. Under the action of H-bonds and electrostatic interaction, ion-induced nucleation could lead to the rapid nucleation of H₂SO₄-HNO₃-NH₃ in the UT.