Non-equilibrium quantum states in atmospheric chemistry

O2 “intercepting” an atmospheric pollutant molecule. Hot orange and cool blue colors show molecules in high and low energy quantum states, respectively.

Today’s issue of Science magazine includes a report I wrote with an international team based across Leeds, Cambridge, and Chicago.

Earth’s atmosphere is a huge chemical reactor where sunlight (rather than heat) starts off chemical chain reactions that ultimately control the fate of greenhouse gases and atmospheric pollutants.

Within the atmosphere, one of the most important classes of chemical reactions are so-called “association reactions”, where one molecule (call it A) reacts with another molecule (call it B). Chemical physicists have known for a long time that molecules can exist in both low energy and high energy quantum states, oftentimes referred to as “equilibrium” and “non-equilibrium” states, respectively. For arbitrary A + B reactions taking place in the Earth’s atmosphere, the nearly universal assumption is that, prior to reaction, both A and B are in their equilibrium states.

Earth’s atmosphere is composed of lots of O2, meaning that O2 participates in most atmospheric reaction sequences. Contrary to the assumption that atmospheric association reactions always involve reactants in equilibrium states, our paper shows that, for association reactions of the type O2 + B (leading to peroxy radicals), there is a high probability that O­2 “intercepts” B before its non-equilibrium quantum states have relaxed to equilibrium. Our paper shows that this occurs during the atmospheric degradation of acetylene, which is an important tracer of atmospheric pollution and also participates in formation of atmospheric particulates. Interestingly, the products produced when O2 intercepts another molecule’s non-equilibrium quantum states are different from those produced when the states are in equilibrium.

Using a detailed mathematical model to calculate how fast non-equilibrium quantum states relax to equilibrium, our paper speculates that the interception of non-equilibrium quantum states by O2 is likely important for a range of chemical reactions in Earth’s atmosphere, with possibly unexpected chemical reaction outcomes.

Ultimately, this work improves our fundamental understanding of the microscopic chemical physics driving peroxy radical formation – amongst the most textbook reactions in atmospheric chemistry. It also paves the wave for further studies of how nature harnesses non-equilibrium effects.

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