In Silico Photochemistry of Atmospheric Molecules in Aqueous Aerosols

Our atmosphere is not only composed of simple di- or triatomic molecules, but also of volatile organic compounds (VOCs) that can undergo chemical or photochemical reactions following sunlight absorption to produce even more complex molecules responsible for the appearance of secondary pollutants. Yet, this ‘gas-phase’ picture of atmospheric chemistry has recently been challenged by the realization that aqueous aerosols – water microdroplets or other hygroscopic atmospheric aerosols with a water layer – act as true miniature chemical reactors and alter the sunlight-induced photochemical reactivity of VOCs. Aqueous aerosols are also suspected to catalyze the formation of secondary organic aerosols, strong contributors to radiative forcing and air pollution. Given that the total surface area of aerosols in the atmosphere is greater than the combined surface area of all bodies of water on Earth, the influence of aerosol photochemistry can be tremendous on the composition of our atmosphere. Still, the impact of aqueous aerosols on photochemical processes is completely neglected in most chemical models used by atmospheric modelers to predict the evolution and composition of the atmosphere, as well as inform political decisions on pollution management. This lack of knowledge is rooted in the challenge of performing photochemical experiments in aqueous aerosols and the absence of a theoretical framework to model such photochemical processes in complex environments.This project, ISPAMIA, launches the field of in silico aerosol photochemistry and will develop a global understanding of photochemical reactions in aqueous aerosols by exploiting our recent breakthroughs in theoretical/computational chemistry. ISPAMIA goes beyond pure theoretical curiosity as it aims to calculate actual photochemical observables and determine simple rules that can be included in atmospheric models in collaboration with atmospheric scientists to achieve, ultimately, a direct social impact.