Evidence is accumulating that reactive halogen species exert an important influence on the chemistry and physics of the troposphere and lower stratosphere. Chlorine atoms react faster with most hydrocarbons than OH does. Inorganic bromine and iodine catalytically destroy ozone, and iodine in the marine boundary layer can lead to the formation of new particles. Observational evidence for halogen activation in the troposphere is most obvious in the polar regions, but there are also indications for active halogen chemistry near volcanoes, salt lakes and in the marine boundary layer. These processes can be initiated by various mechanisms including release of halides from sea salt (aerosol, deposits, or seawater) and biogenic emissions of halogenated organics. In the UTLS, halogens play an important role for the ozone balance. While for chlorine there are open issues concerning activation on cirrus clouds, for bromine total amounts and partitioning are still quite uncertain. For TTL iodine little is presently known up on its abundance and hence significance for ozone.

We invite contributions in the following areas:

- Model studies: Investigations of the chemical mechanisms leading to release, transformation and removal of reactive halogen species. Studies of consequences of the presence of reactive halogen species in the troposphere and lower stratosphere.

- Laboratory studies: Determination of gas- and aqueous-phase rate constants, Henry's law and uptake coefficients, UV/VIS spectra, and other physiochemical properties of halogenated species.

- Field experiments: Measurements of inorganic (X, X2, XO, HOX, XONO2, ..., X = Cl, Br, I) reactive halogen species and of organic (CH3Br, CHBr3, CH3I, RX, ...) source gases in the troposphere and lower stratosphere.

Studies that are part of the International Polar Year (IPY), SOLAS and other field and related programmes are especially encouraged to contribute to this session.