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 importan t chemistry in the stratosphere. Likely sources of halogens in the troposph ere include reactions of gas phase pollutants with particles containing alk ali halide salts such as sea salt particles. In this article we describe ho w modern ultra-high vacuum surface science experiments can provide signific ant new insight into the detailed mechanisms of reactions that are importan t 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 desc ription of the experimental techniques. Studies from our laboratory that ar e 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 t o a recrystallization of the nitrate layer and phase separation from the Na Cl. This results in open areas of clean NaCl that are then available for fu rther reaction. The reaction of water with NaCl surfaces which have been pr eviously 'corroded' by reaction with nitric acid and water leads to the dis sociative adsorption of water to form surface OH- species. This is in stark contrast to the completely reversible adsorption of water on defect free N aCl surfaces. A model is also presented that is consistent with the informa tion provided by the detailed ultra-high vacuum surface science experiments and explains the dependence of the reaction of nitric acid with NaCl on th e nitric acid pressure.