Heterogeneous Reactions of SO3 on Ice: An Overlooked Sink for SO3 Depletion

Nitrous Acid (HONO) Dissociation on the Water and Ice Surface: An Ab Initio Molecular Dynamics Study

In the atmosphere, the photodissociation of HONO is a significant source of OH radicals after ozone. In the present study, using Born-Oppenheimer molecular dynamics, we showed that HONO can dissociate on ice and water surfaces without light. In addition, the dissociation time of HONO is found to be much less on the ice surface compared to the same time on the water droplets.

Can Ozone Dissociate at the Surface of Water (Water Droplet and Ice) without Light?

Ozone is a major source of OH radicals in the troposphere. It is well-known that photodissociation of ozone is key for the conversion of ozone into OH radicals. In the present study, using Born-Oppenheimer molecular dynamics simulation, we have shown that on the surface of the droplet and ice, ozone can dissociate without light. In addition, the dissociation time of ozone is found to be much less on the ice surface than the same time on the water droplet. As the dissociation of ozone on the water surface can happen during the day as well as in the night time, we believe this route of forming OH radicals can be even more important than the photodissociation. The present study suggests that the cloud and ice surface can enhance the oxidizing power of the troposphere.

Surface Confinement of Finite-Size Water Droplets for SO3 Hydrolysis Reaction Revealed by Molecular Dynamics Simulations Based on a Machine Learning Force Field

As an important source for sulfuric acid in the atmosphere, hydrolysis of sulfur trioxide (SO₃) takes place with water clusters of sizes from several molecules to several nanometers, resulting in various final products, including neutral (H₂SO₄)-(H₂O) clusters and ionic (HSO₄)^--(H₃O)⁺ clusters. The diverse products may be due to the ability of proton transfer and the formation of hydrated ions for water cluster of finite sizes, especially the sub-micrometer ones. However, the detailed molecular-level mechanism is still unclear due to the lack of available characterization and simulations tools. Here, we developed a quantum chemistry-level machine learning (ML) model to simulate the hydrolysis of SO₃ with water clusters of sizes up to nanometers. The simulation results demonstrate diverse reaction paths taking place between SO₃ and water clusters of different sizes. Generally, neutral (H₂SO₄)-(H₂O) clusters are preferred by water clusters of ultra-small size, and a loop structure-mediated mechanism with SO₃(H₂O)_n≤4 structures and a non-loop structure-mediated mechanism with structure relaxation are observed. As the water cluster size increases to (H₂O)₈, a (HSO₄)^--(H₃O)⁺ ion-pair product emerges; and the Eigen-Zundel ion conversion-like proton transfer mechanism takes place and stabilizes the ion pairs. As the water cluster sizes further increase beyond several nanometers ((H₂O)_n≥32), the (SO₄)^2-[(H₃O)⁺]₂ ion-pair product appears. The reason could be that the surface of these water clusters is large enough to screen Coulomb repulsion between two tri-coordinated ion-pair complexes. These findings would provide new perspectives for understanding SO₃ hydrolysis in the real atmosphere and sulfuric acid chemistry in atmospheric aerosols.

Gas phase acidity of water clusters

In the present work, we have estimated the gas-phase acidity of different water clusters, i.e., (H₂O)_n, n = 1-20, 30, 35, 42, 54, 80, and 100. The present work indicates that the gas-phase acidity of the terminal hydrogen atom increases with the size of water clusters and starts converging at (H₂O)₃₀. Furthermore, the present work also indicates that the gas-phase acidity of a terminal hydrogen atom is higher than that of the corresponding bulk hydrogen atom for the same size of water cluster.

OH + HCl Reaction on the Surface of Ice: An Ab Initio Molecular Dynamics Study

We have investigated the OH + HCl reaction on the surface of ice using Born-Oppenheimer molecular dynamics (BOMD) simulation. The present work revealed that the OH + HCl reaction becomes ∼1 order of magnitude faster on the ice surface compared to the bare reaction. The BOMD simulation also indicates that the Cl radical formed on the ice surface through the title reaction can form two hydrogen bonds at a time with the water molecules present on the ice surface; hence, the Cl radical cannot escape from the ice surface easily.

Heterogeneous Formation of HONO Catalyzed by CO2

Gas-phase nitrous acid (HONO) is a major precursor of hydroxyl radicals that dominate atmospheric oxidizing capacity. Nevertheless, pathways of HONO formation remain to be explored. This study unveiled an important CO₂-catalysis mechanism of HONO formation, using Born-Oppenheimer molecular dynamics simulations and free-energy samplings. In the mechanism, HCO₃^- formed from CO₂ hydrolysis reacts with NO₂ dimers to produce HONO at water surfaces, and simultaneously, itself reconverts back to CO₂ via intermediates OC(O)ONO^- and HOC(O)ONO. A flow system experiment was performed to confirm the new mechanism, which indicated that HONO concentrations with CO₂ injections were increased by 29.4-68.5%. The new mechanism can be extended to other humid surfaces. Therefore, this study unveiled a previously overlooked vital role of CO₂ that catalyzes formation of HONO and affects atmospheric oxidizing capacity.