In atmospheric science the definition of a heterogeneous process is broader than the surface catalysis example above. For instance, liquid phase reactions occuring within cloud droplets are characterized as heterogeneous even though the reactions do not require the presence of a surface to occur. This is because the constituent concentrations in the air in which the droplet is suspended are modified by the liquid-phase reactions occurring within the droplet, and the presence of the liquid phase is critical to the changes in the gas phase. Heterogeneous reactions occuring on and in polar stratospheric cloud particles combine with abnormally high levels of atmospheric chlorine to produce the Antarctic ozone hole. Also, in the lower stratosphere a haze of sulfuric acid droplets blankets the earth and significantly modifies the trace constituent chemistry in the lower stratosphere.
Heterogeneous processes are very important to tropospheric chemistry as well.
The role played by heterogeneous processes in the troposphere is not as well understood as in the stratosphere, and this is currently a subject of intense interest. A number of natural and anthropogenic processes can influence stratospheric photochemistry. An important part of the work I do is to use the models we have developed to evaluate the significance of each of these perturbations. Chemical Partitioning Chemical species in the atmosphere can be broken into more or less distinct families. Reactions or processes which move Cl x into its reservoir species reduce ozone destruction, while processes which convert HCl and ClONO 2 to Cl x increase ozone destruction.