Reactivity and Photochemistry of Doped Salt Clusters

Sea salt aerosols play an important role in the earths atmosphere. They can serve as condensation nuclei during cloud formation, they absorb and reflect sunlight and contribute to the chemistry of trace gases. Sea salt aerosols are formed by spraying sea water to small droplets and the subsequent evaporation of water. Due to their small size, they float in the atmosphere for an extended period of time and age, changing their chemical composition. The aerosols consist of a complex mixture of rock salt, magnesium sulfate, organic substances, water and a multitude of additional elements, which are present only in traces, like iron or iodine. Due to their unclear composition, it is not possible to know the exact sequence of chemical reactions that take place upon ageing of the aerosols. Here, the current project sets in: we explore small subsections of sea salt aerosols, for which we precisely know their composition, atom by atom. These so-called clusters are generated in a mass spectrometer, which is usually applied for chemical analytics, and stored under vacuum conditions for several seconds. In the mass spectrometer, the clusters get in touch with single molecules of a reaction gas. If a chemical reaction takes place during this contact, the mass of the cluster changes, and we can detect the reaction by measuring this mass change. Besides collisions with a reactive gas, also light can induce chemical reactions in the aerosol through so-called photochemical pathways. In a laboratory experiment, we specifically generate clusters that contain a photochemically active component. One example is pyruvate, which is embedded in a sodium chloride cluster, absorbing a fraction of the sunlight reaching the ground. To understand what precisely happens during the absorption of sunlight, we use a laser system that allows us to freely choose the wavelength of the light. This in turn makes it possible to deliberately excite quantum mechanical states in the molecule, triggering characteristic photochemical processes. These processes are replicated with quantum chemical computer simulations, which provide a complete picture of the photochemistry in the salt cluster. By mixing in trace elements, we can explore their impact on photochemistry, e.g., whether iodine is directly involved in photochemistry or whether a doubly charged magnesium ion has a strong enough influence on an organic ion in its neighborhood to change its photochemistry. With the interplay of these experiments with computer simulations, we obtain a detailed understanding of elementary chemical reactions in sea salt aerosols.