253 posters, 14 videos, 27 audios, 14 topics, 392 authors, 132 institutions

4th Commonwealth Chemistry Posters

October 4 - 5, 2023 | Online, United Kingdom

P93

Computational Study on Hydration Motifs of Atmospheric Sulphuric Acid-Ammonia Clusters

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Poster Presenter

Jia Yang Lim

Authors

Jia Yang Lim, Adam Loh, Hui Ying Gao, Zachary Lam, Wai Kit Tang, Chi-Kit Siu

Affiliations

Department of Chemistry, Faculty of Science, Universiti Malaya, 50603 Kuala Lumpur, Malaysia, Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong SAR, China

Part of Topic

Climate action

Computational Study on Hydration Motifs of Atmospheric Sulphuric Acid-Ammonia Clusters

Introduction

Understanding the formation of cloud condensation nuclei (CCN) is essential for predicting cloud precipitation pathway.¹

CNN originates from cluster of sulphuric acid-ammonia pair.² The role of water toward the growing cluster remains elusive.

Stepwise hydration of (H₂SO₄)₃(NH₃)₃(H2O)_n (abbreviated as 3-3-n) shows minima in enthalpy profile. Hydration of cluster has a saturation point. Simultaneous addition of one H2SO4 and one NH3 into 3-3-n induces large exothermicity and further trigger rapid water evaporation.

Methods

Energetic stability of stepwise hydration in (H₂SO₄)_x(NH₃)_y(H₂O)_n clusters was evaluated using density functional theory based molecular dynamics (DFT-MD). The stepwise hydration begins with 3-3-4 which is obtained from the Atmospheric Cluster Database (ACDB).³ Input of hydrated structures (3-3-n) from n=4 to n=12 are generated by ABCluster using force field method. MD simulations were performed at 200K canonical ensemble for 5 ps.

Cluster Growth via Stepwise Hydration

1. Successive hydration of cluster shows favourable water binding. Average hydration energy liberated upon binding heats up the cluster and triggers evaporation of at least 1 water molecule.

2. Both homogeneous (3-3-n) and heterogeneous (3-4-n & 4-3-n) clusters show minima which indicates saturation of hydration.

3-3-n averages -48 kJ/mol (minimum -65 kJ/mol at n=9)

3-4-n averages -47 kJ/mol (minimum -80 kJ/mol at n=7)

4-3-n averages -53 kJ/mol (minimum -72 kJ/mol at n=8)

Cluster Growth via Acid-Base Pair

1. Acid-base cluster growth liberates large amount of heat.

3-3-4 + 1 acid + 1 base -> 4-4-4 ΔH₁ = −208 kJ mol^-1

3-3-16 + 1 acid + 1 base -> 4-4-16 ΔH₃ = −201 kJ mol^-1

2. Heat liberated induces evaporation of water.

4-4-0 + 1 water -> 4-4-1 ΔH_a = −79 kJ mol^_-1

4-4-1 + 1 water -> 4-4-2 ΔH_b = −18 kJ mol^-1

4-4-2 + 1 water -> 4-4-3 ΔH_c = −50 kJ mol^-1

4-4-3 + 1 water -> 4-4-4 ΔH_d = −53 kJ mol^-1

3. Addition of 1 acid and 1 base induces evaporation of 4 water molecules from the cluster.

3-3-4 : ΔH₁ ≈ ΔH_a + ΔH_b + ΔH_c + ΔH_d

3-3-16 : ΔH₃ ≈ ΔH_a + ΔH_b + ΔH_c + ΔH_d

Cluster Growth of Charged 3-3-n

References

1. Kulmala, M., et al., Direct Observations of Atmospheric Aerosol Nucleation. Science, 2013. 339(6122): p. 943-946.

2. Bzdek, B.R., J.W. DePalma, and M.V. Johnston, Mechanisms of Atmospherically Relevant Cluster Growth. Accounts of Chemical Research, 2017. 50(8): p. 1965-1975.

3. Elm, J., An Atmospheric Cluster Database Consisting of Sulfuric Acid, Bases, Organics, and Water. ACS Omega, 2019. 4(6): p. 10965-10974.