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 (NH3 )(CH3 NH2 )(H2 SO4 ) m (H2 O) n ( 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 H2 SO4 molecule to CH3 NH2 molecule is easier than to NH3 molecule in the free or hydrated H2 SO4 -base clusters, and thus leads to the stability. The energetically favorable formation of the (NH3 )(CH3 NH2 )(H2 SO4 ) m (H2 O) n ( 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 (NH3 )(CH3 NH2 )(H2 SO4 )2 clusters could reach that of total hydrated (NH3 )(H2 SO4 )2 clusters, which is the most stable ammonia-containing cluster. These results indicate that the synergistic effect of NH3 and CH3 NH2 might be important in forming the initial cluster with sulfuric acid and subsequently growth process. In addition, the evaporation rates of (NH3 )(CH3 NH2 )(H2 SO4 )(H2 O), (NH3 )(CH3 NH2 )(H2 SO4 )2 and (NH3 )(CH3 NH2 )(H2 SO4 )3 clusters, three relative abundant clusters in (NH3 )(CH3 NH2 )(H2 SO4 ) m (H2 O) n system, were calculated. We find the stability increases with the increasing number of H2 SO4 molecules.