George Shields of Armstrong Atlantic State University is supported by an RUI award from the Theoretical and Computational Chemistry program of the Division of Chemistry to determine the minimum energy structures and the thermodynamics for formation of small aerosols that will serve as neutral and ionic cloud condensation nuclei. His methods allow him to determine the relevant abundance of all the potentially relevant complexes, thus allowing his research group, consisting mainly of undergraduates, to assess which ones are of atmospheric importance.
The project is having a broad impact through its model of a deep, sustained and productive undergraduate research experience involving the same students, including minorities and women, throughout their undergraduate years.
Our research focuses on the application of computational chemistry methods to model the formation of water clusters and hydrates of atmospheric relevance. The end goal is to explain the growth of small gas phase clusters to large aerosols and cloud droplets. At the moment, there is no theory that can adequately explain the formation of aerosol particles of sizes from 1-100 micrometers starting from molecular clusters that are less than 1 nanometer in diameter. Understanding this process will provide valuable information about the size and distribution of aerosols in the atmosphere. That will refine global climate change models and minimize the large uncertainty associated with the role of aerosols in global warming. Since the project is amenable for the involvement of undergraduates, seventeen students have contributed to the work. Our work on small water clusters identified the global minimum structure for (H2O)7, (H2O)9 and (H2O)10 and a large number of low-energy conformers that are atmospherically relevant. Our method uncovered many stable structures that were not discovered by conventional energy minimizations starting from a limited set of structures. The most stable structures we identified were later observed using high-resolution rotational spectroscopy by our collaborators. The thermodynamic data we generated is being used to calculate the rate of formation of pure water droplets under different conditions in the atmosphere. We have investigated the thermodynamics of sulfuric acid (H2SO4) hydration and the formation of large H2SO4-H2O clusters. Since sulfuric acid plays a key role in the formation of aerosol particles in the atmosphere, understanding the initial stage called nucleation is very important. For (H2SO4)m=1-2(H2O)n=1-6, we located a vast number of stable clusters and calculated their structures and energies accurately. We found that ionic pair (HSO4-•H3O+)(H2O)n-1 clusters are as stable as neutral (H2SO4)(H2O)n clusters for n ≥ 3 and are more stable than neutral clusters for n ≥ 4 depending on the temperature. The Gibbs free energies for the formation of (H2SO4)m=1-2(H2O)n clusters were favorable at colder regions of the troposphere (T = 216.65 – 273.15 K) for n=1-6, but clusters with n ≥ 5 are not favorable at higher (T > 273.15K) temperatures. Our results suggest the critical cluster of a binary H2SO4-H2O system must contain more than two H2SO4 molecules and are in concert with recent experimental findings that the role of binary nucleation is small at ambient conditions, but significant at colder regions of the troposphere. Our calculated equilibrium hydrate distribution predicts that sulfuric acid does not overwhelmingly form hydrates as predicted by other models and experiments. Rather, as much as 50−60% of the acid remains unhydrated at 20% RH and 10−15% at 100% RH in the troposphere. A proper treatment of hydrate formation can help decrease the discrepancy between theoretical and observed nucleation rates of sulfuric acid in the atmosphere. On the basis of the thermodynamics of H2SO4(H2O)n=1−6, it is unlikely that binary homogeneous nucleation (BHN) contributes significantly to new particle formation at ambient conditions in the continental boundary layer. Perhaps in the colder regions of the troposphere, BHN can account for the observed nucleation rates. To evaluate the role of ionic and ternary homogeneous nucleation (THN), we investigated the formation of HSO4-(H2O) m, H2SO4(NH3)(H2O)m and H2SO4(NH2CH3)(H2O)n. In each case, the thermodynamics of growing these clusters by adding water incrementally is very similar to that of the binary (H2SO4)m(H2O)n system. We plan to compute binary (H2SO4-H2O) and ternary (H2SO4-H2O-NH3/amines) nucleation rates and comparing them with experimental observations by our collaborator.
