Heterogeneous chemistry in the troposphere: a modern surface chemistry approach to the study of fundamental processes

There is an increasing amount of evidence that points to the importance of halogen chemistry in the troposphere in addition to the well known important chemistry in the stratosphere. Likely sources of halogens in the troposphere include reactions of gas phase pollutants with particles containing alkali halide salts such as sea salt particles. In this article we describe how modern ultra-high vacuum surface science experiments can provide significant new insight into the detailed mechanisms of reactions that are important to the chemistry of particles in the atmosphere. An introduction to what is known about the surface chemistry of NaCl is provided followed by a description of the experimental techniques. Studies from our laboratory that are described here have shown that the reaction of dry nitric acid with NaCl is self-limiting leading to a chemically inert surface that is covered with a layer of sodium nitrate. Our experiments also show that very low vapour pressures of water provide enhanced ionic mobility on the surface leading to a recrystallization of the nitrate layer and phase separation from the NaCl. This results in open areas of clean NaCl that are then available for further reaction. The reaction of water with NaCl surfaces which have been previously 'corroded' by reaction with nitric acid and water leads to the dissociative adsorption of water to form surface OH- species. This is in stark contrast to the completely reversible adsorption of water on defect free NaCl surfaces. A model is also presented that is consistent with the information provided by the detailed ultra-high vacuum surface science experiments and explains the dependence of the reaction of nitric acid with NaCl on the nitric acid pressure.