Ice catalysed stratospheric chemistry

A key step in stratospheric ozone depletion is the dissociation of hydrochloric acid on large ice clusters, i.e.,

                                   HCl --> H⁺ + Cl^-                                                    (R1)

The Cl^- product reacts with stratospheric ClONO₂ or HOCl to form Cl₂, which photodissociates to Cl radicals.  These radicals react with and destroy stratospheric ozone, hence forming ozone holes and allowing  harmful ultraviolet radiation to reach the earth's surface.

We have performed molecular-level studies on Reaction (R1) to gain a deeper fundamental understanding of the reaction and to obtain kinetic data for modelling of stratospheric chemistry.  For example, our QM/MM studies showed, for the first time, that HCl dissociation on ice surfaces is barrierless, and that surface relaxation is crucial for dissociation. 

The pictures below show the top and side views of the HCl dissociation mechanism (click on the pictures to see the dynamics). The ice temperature is kept at 0 K to clarify the reaction mechanism. Only a part of the ice surface used in the computer simulations is shown in the figures. The Cl atom (ion) is shown in green, the H atoms in dark blue, the oxygen (O) atoms that form part of the reaction center are in red (and treated with quantum mechanical forces) and the other O atoms are in light blue. Note that there are two 'dangling' hydrogens initially on the ice surface that solvate the final Cl ion, that the hexagonal ice structure is very distorted after HCl dissociation and that the product Cl ion lies above the ice surface. This facilitates its reaction with other molecules and hence destruction of the ozone.

The two figures below are for the same reactions but at an ice temperature of 150 K. Note that the reaction mechanism is very similar at elevated temperatures (fewer frames are shown in these movies than for the 0 K ice surface.)

The next two figures are for the same reaction at an ice temperature of 0 K. However, in this case there is only a single dangling surface hydrogen in the central hexagon. The solvation provided by this danlging hydrogen is not sufficient to lead to ionization of the HCl, which remains molecularly adsorpbed on the surface.

The remaining figures show adsorption of HCl at ice surface defects (the ice temperature is 0 K). The first defect is a heptagon that can be found on ice surfaces at elevated temperatures relevant to the stratosphere. The other defects are 'ledge' defects that can be formed during growth or evaporation of ice surfaces (where crystalline islands are found on the ice surface). As with the perfect crystalline surface shown above, the HCl can either by molecularly or dissociatively adsorbed depending on the number of surface danlging hydrogens and their proximity to the adsorbed HCl. It can also be seen that the initial position of the HCl above the defect structure is important to the adsorption mechanism.

 

More information can be found in our publications.  These calculations were done in close collaboration with experimental studies at the University of Gotheburg.