Engsvang Morten, Elm Jonas
Department of Chemistry, iClimate, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark.
ACS Omega. 2022 Feb 24;7(9):8077-8083. doi: 10.1021/acsomega.1c07303. eCollection 2022 Mar 8.
Sulfuric acid and ammonia are believed to account for a large fraction of new-particle formation in the atmosphere. However, it remains unclear how small clusters grow to larger sizes, eventually ending up as stable aerosol particles. Here we present the largest sulfuric acid-ammonia clusters studied to date using quantum chemical methods by calculating the binding free energies of (SA) (A) clusters, with up to 20. Based on benchmark calculations, we apply the B97-3c//GFN1-xTB level of theory to calculate the cluster structures and thermochemical parameters. We find that the cluster structures drastically evolve at larger sizes. We identify that an ammonium ion is fully coordinated in the core of the cluster at = 7, and at = 13 we see the emergence of the first fully coordinated bisulfate ion. We identify multiple ammonium and bisulfate ions that are embedded in the core of the cluster structure at = 19. The binding free energy per acid-base pair levels out around = 8-10, indicating that at a certain point the thermochemistry of the clusters converges toward a constant value.
硫酸和氨被认为在大气中新粒子形成过程中占很大比例。然而，目前尚不清楚小团簇如何长大到更大尺寸，最终形成稳定的气溶胶颗粒。在此，我们通过计算高达20个（SA）ₓ（A）ₙ团簇的结合自由能，展示了迄今为止使用量子化学方法研究的最大硫酸 - 氨团簇。基于基准计算，我们应用B97 - 3c//GFN1 - xTB理论水平来计算团簇结构和热化学参数。我们发现，在更大尺寸时团簇结构会急剧演变。我们确定在n = 7时，铵离子在团簇核心中完全配位，在n = 13时，我们看到第一个完全配位的硫酸氢根离子出现。我们确定在n = 19时，多个铵离子和硫酸氢根离子嵌入团簇结构的核心。每个酸碱对的结合自由能在n = 8 - 10左右趋于平稳，这表明在某个点上团簇的热化学向一个恒定值收敛。
