Computational study on hydration motifs of atmospheric sulphuric acid-ammonia clusters Adam Ee Xian Loh 1 , Jia Yang Lim 1 , Hui Ying Gao 2 , Zachary Lam 2 , Wai Kit Tang 1 , Chi-Kit Siu 2 1 Department of Chemistry, Faculty of Science, Universiti Malaya, Malaysia, 2 Department of Chemistry, City University of Hong Kong, SAR, China Understanding the formation of cloud condensation nuclei (CCN) is essential for predicting cloud precipitation pathway, which in turn determines the atmospheric radiation budget. Up to half of CNN is originated from cluster formation and stabilization via gas-to-particle transition. 1 In general, atmospheric cluster consists of sulphuric acid- ammonia pair (H 2 SO 4 ) x (NH 3 ) y . 2 Owing to the large abundance of water molecules in our atmosphere, the details of hydration of acids and bases has attracted a lot of interest in the last decades. However, the role of water toward the acid-base cluster growth remains elusive. Here, we investigated the energetic stability of stepwise hydration in (H 2 SO 4 ) x (NH 3 ) y clusters using density functional theory based molecular dynamics (DFT-MD). The stepwise hydration begins with (H 2 SO 4 ) 3 (NH 3 ) 3 (H 2 O) 4 (abbreviated as 3-3-4) which is obtained from The Atmospheric Cluster Database (ACDB). 3 Input of hydrated structures (3-3-n) are generated by ABCluster using force field method. MD simulations were performed at 200K canonical ensemble for 5 ps. Successive hydration indicates favourable binding energy until a minimum (-65 kJ/mol) at n=9, suggesting slower hydration after n=9 due to evaporation. Notably, simultaneous addition of one H 2 SO 4 and one NH 3 induces large exothermicity and further trigger rapid water evaporation. Therefore, we expect that the acid-base cluster growth will induce shrinkage of cluster. Besides neutral clusters, hydration towards charged clusters, [3-3-4+H] + and [3-3-4-H] - were explored. As a whole, our result shows saturation of hydration and significant evaporation in acid-base cluster growth. Hydration motifs of acid-base cluster and substantial water rearrangement may account for entropic contribution in the formation of CCN. References 1. Kulmala, M.; Kontkanen, J.; Junninen, H.; Lehtipalo, K.; Manninen, H. E.; Nieminen, T.; Petäjä, T.; Sipilä, M.; Schobesberger, S.; Rantala, P.; et al. Direct Observations of Atmospheric Aerosol Nucleation. Science 2013 , 339 (6122), 943-946. 2. Bzdek, B. R.; DePalma, J. W.; Johnston, M. V. Mechanisms of Atmospherically Relevant Cluster Growth. Accounts of Chemical Research 2017 , 50 (8), 1965-1975. 3. Elm, J. An Atmospheric Cluster Database Consisting of Sulfuric Acid, Bases, Organics, and Water. ACS Omega 2019 , 4 (6), 10965-10974.
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