Molecular Dynamics Studies of Halogen Bonding on the Freezing in Clouds

The retention coefficients for halogen gases upon freezing of water are currently not estimated. Estimating their retention coefficients is an important metric, enabling atmospheric chemists and modelers to better approximate the respective influences and lifetimes of trace gasses in the atmosphere. Using molecular dynamic simulations that simulate the freezing of cloud water droplets containing halogen gasses in the upper troposphere (220°K), we have revealed that Cl₂, Br₂, and I₂ are entirely retained when water droplets are frozen during deep convection. The results of the simulations show that the presence of halogen gasses in cloud droplets retards their freezing rates by approximately 35% to 62%.  Enumeration of hydrogen and halogen bonds formed between gas species and water in cloud droplets shows that substantially more halogen bonding than hydrogen bonding occurs among halogen gasses. Additionally, data indicates the size of halogen species within cloud droplets may affect their freezing rates. Our results provide a theoretical framework to make the first estimates regarding halogen gas retention coefficients, helping to elucidate impacts halogens may have on the gas transportation dynamics and chemistries of the upper troposphere and lower stratosphere (UTLS). As halogen gas emissions continue to increase globally, this study calls upon the emergent need to assess the role of halogen gasses in UTLS chemistry, especially in the deep convective regions.