Evidence is accumulating that reactive halogen species can have an important influence on the chemistry of the lower and the free troposphere. Chlorine atoms react faster with most hydrocarbons than OH does. Inorganic bromine and iodine can catalytically destroy tropospheric ozone and oxidise mercury. These reactions are most obvious in the polar troposphere during the springtime ozone depletion
events, over salt lakes and in volcanic plumes. However there are also indications that halogens play a spatially wider role in the marine boundary layer and free troposphere for ozone destruction, changes in the ratios of OH/HO2 and NO/NO2, destruction of methane and in the
oxidation of mercury.

We invite contributions in the following areas:

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

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

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

This session is a contribution to the SOLAS/IGAC task "Halogens in the troposphere" (HitT).