Synergistic Effect of Ammonia and Methylamine on Nucleation in the Earth's Atmosphere. A Theoretical Study.

Ammonia and amines are important common trace atmospheric species that can enhance new particle formation (NPF) in the Earth's atmosphere. However, the synergistic effect of these two bases involving nucleation is still lacking. We studied the most stable geometric structures and thermodynamics of quaternary (NH)(CHNH)(HSO) (HO) ( m = 1-3, n = 0-4) clusters at the PW91PW91/6-311++G(3df,3pd) level of theory for the first time. We find that the proton transfer from HSO molecule to CHNH molecule is easier than to NH molecule in the free or hydrated HSO-base clusters, and thus leads to the stability. The energetically favorable formation of the (NH)(CHNH)(HSO) (HO) ( n = 0-4) clusters, by hydration or attachment of base or substitution of ammonia by methylamine at 298.15 K, indicate that ammonia and methylamine together could enhance the stabilization of small binary clusters. At low RH and an ambient temperature of 298.15 K, the concentration of total hydrated (NH)(CHNH)(HSO) clusters could reach that of total hydrated (NH)(HSO) clusters, which is the most stable ammonia-containing cluster. These results indicate that the synergistic effect of NH and CHNH might be important in forming the initial cluster with sulfuric acid and subsequently growth process. In addition, the evaporation rates of (NH)(CHNH)(HSO)(HO), (NH)(CHNH)(HSO) and (NH)(CHNH)(HSO) clusters, three relative abundant clusters in (NH)(CHNH)(HSO) (HO) system, were calculated. We find the stability increases with the increasing number of HSO molecules.