Tracking ozone molecules inside water

Ozone molecules reacting with solutes in water play a major role in atmospheric chemistry. Much of the interaction takes place very close to the water’s surface, which can show up in many forms, from oceans to fog to sea spray. Being able to account for the different types of water is important for creating realistic atmospheric models.

“A surprising amount of atmospheric chemistry happens right at the boundary between air and water, or just beneath it,” said Dai-Bei Yang, author of a study working to improve models of the interfacial chemistry between water droplets and ozone-nitrite. “These interfaces are everywhere: in clouds, aerosols, and humid air.”

Yang et al. used a particle-based reactive diffusion (PBRD) framework which, in contrast to other approaches, represents each molecule as its own particle. Ozone exists in the atmosphere in low concentrations, so its behavior is governed by individual molecules rather than bulk averaged chemistry.

“Each molecule is thus trackable,” said Yang. “It diffuses according to real, measured properties, so its full ‘life story’ is known — where it goes, how long it stays near a surface, and where and which reactions actually happen.”

These include competing — and sometimes simultaneous — processes of colliding with a water surface, then either staying there for a while, dissolving into the water, reacting chemically, or escaping back into the surrounding air. PBRD allowed the researchers to study the effects of these processes, demonstrating that when the droplet size shrinks beyond a specific threshold, the chemistry of the interaction changes, switching from being limited by the rate of the molecules’ random motion through the droplet to limited by the speed of the chemical reaction between the two.

Because ozone’s reactions with water are relatively simple, the authors say it serves as an ideal test case for understanding these competing processes more generally.

Source: “Explicit particle kinetics simulations of reactive diffusion at air-water interfaces,” by Dai-Bei Yang, Xiangyu Chen, and Joseph S. Francisco, Journal of Chemical Physics (2026). The article can be accessed at https://doi.org/10.1063/5.0311877 .