Heterogeneous processes in the atmosphere

Heterogeneous processes play an important role in atmospheric chemistry. Both heterogeneous chemistry on polar stratospheric clouds (PSCs) and on sulfuric acid aerosols are of central importance for the depletion of stratospheric ozone. On these surfaces inactive halogen reservoir species, especially ClONO2, HCl, HOCl, BrONO2, HBr are converted into their active forms, Cl and/or Br species, which subsequently catalyse the loss of ozone. An overview is given on the types of heterogeneous reactions that occur on various surfaces (liquid or solid) in the stratosphere. Basic definitions on heterogeneous processes, relating reaction probabilities, solubilities and reactivity are described. Laboratory techniques for the measurements of the uptake coefficients are presented and the uptake coefficients for the major Cl- and Br-containing species reacting on NAT, water-ice and sulfuric acid aerosols are reviewed. Finally, heterogeneous reactions on NaCl-salts are summarized.