Theoretical and Experimental Advances in Atmospheric Photochemistry
Location: CECAM-HQ-EPFL, Lausanne, Switzerland
At first glance, the Earth’s atmosphere appears to be mainly composed of simple, inert gases like O2, N2, or the infamous CO2. However, the reality is somewhat different, and our atmosphere behaves more like a massive chemical reactor due to the presence of reactive molecules like CH4, halocarbons, O3, N2O, NO, or volatile organic compounds (VOCs). The degradation of biogenic and anthropogenic VOCs takes place via a highly complex network of chemical processes and is intimately connected to the concentration of OH, ozone, and secondary organic aerosol (SOA) precursors in the troposphere. The result of this is VOCs are strong contributors to both global warming and air pollution, and a great deal of effort has been devoted to understanding and predicting their atmospheric concentrations using detailed chemical models. As environmental policy decisions are driven by such atmospheric models, it is essential they accurately reflect the different chemical reactions in the atmosphere.
Historically, these chemical mechanisms have mostly neglected reactions involving the interaction of VOC intermediates with sunlight, and the resulting photochemical reactions. Hence, a complete family of chemical reactions is missing in current atmospheric models, and the influence of these reactions on the composition of our atmosphere is largely unknown, partly as photochemical experiments on (transient) VOCs are highly complex to realize.
How can theoretical and computational chemistry help? Simulating the photochemistry of a molecule requires the inclusion of nonadiabatic effects, i.e., the coupling between electronic states and nuclear motion, which is not straightforward as it challenges several approximations commonly used in theoretical chemistry. For example, nonadiabatic effects lead to a breakdown of the Born-Oppenheimer approximation, classical approximations for the nuclear degrees of freedom may be inadequate, and out-of-equilibrium processes can challenge established reaction rate theories.
While numerous methods have been devised to tackle these issues – e.g., MCTDH, trajectory surface hopping, ab initio multiple spawning – their application to study the photochemistry of atmospheric molecules faces numerous challenges. Examples are: the complexity in simulating observables of interest for spectroscopists and atmospheric modelers; the challenging electronic structure of multichromophoric VOCs; the types of excited-state dynamics created by sunlight excitation; the long-time excited-state dynamics associated with VOCs; the importance of intersystem crossings or collisional processes; and the effect of an aqueous environment, such as in atmospheric aerosols and clouds.
Over the last decades, atmospheric chemistry has stimulated the development of new theoretical methods to investigate complex ground-state chemical reactions and their mechanisms. Such a connection between theory and experiment does not currently exist in atmospheric photochemistry involving electronically excited states, despite the importance for current atmospheric models and a strong push from the experimental side to obtain reliable data for modeling the composition of the atmosphere. With this CECAM workshop, we wish to create a bridge between the worlds of computational photochemistry and atmospheric chemistry to bolster a synergistic discussion between these two groups aiming to (i) connect theory with experiment, (ii) define key targets for theory, and (iii) identify current theoretical challenges and their possible solutions.
Basile Curchod (University of Bristol) - Organiser
Andrew Orr-Ewing (University of Bristol) - Organiser