A brief release of 'Halogens in the Troposphere (HitT)' Task Team White paper is available. click here
Terms of Reference
1) Background and Introduction
While halogen chemistry has been recognised as an important factor influencing the composition of the stratosphere only recently it became clear that halogens can also have a decisive influence on trace gas cycles in the troposphere. For instance in polar spring boundary layer ozone can be completely destroyed on areas amounting to several percent of the Earth's surface (the 'tropospheric ozone holes'). In particular emission of reactive halogen compounds (molecular Cl, Br and I, halogen oxides and acids) from the ocean surface, sea salt particles, salt pans, and sea ice is known to influence the tropospheric ozone budget, the oxidation capacity, and perhaps the particle formation in the MBL. In addition, and potentially even more importantly there is accumulating evidence for the existence of a widespread – if not global – abundance of reactive bromine (largely BrO) in the free troposphere. This 'bromine background' with concentrations of a few times 107 molecules per cm3 could have a large impact on the tropospheric ozone budget.
2) Scientific Objectives of the Task
Investigate the role of halogen species in tropospheric chemistry and assess the consequences for climate, oxidation capacity, and possibly other properties of the atmosphere. At present in particular the following questions appear to be of interest:
- Liberation mechanisms of reactive halogen species from ice, salt surfaces (including aerosol), and sea water.
- Reaction of hydrogen halides and other halogen species at aerosol surfaces leading to the reactivation of reactive halogen species like Cl atoms or halogen oxides.
- Quantification of biotic and abiotic sources of organo-halogen species to the atmosphere.
- Transport mechanisms of halogen species to the free troposphere and the abundance of reactive halogen species in the free troposphere (e.g. by aircraft measurements).
- Degradation processes of organo-halogen species in the atmosphere.
- (Laboratory) Studies of halogen reactions in the gas and liquid phase and at surfaces.
- The role of halogen species in coastal regions.
- Particle formation from halogen (iodine) precursors.
- Consequences of tropospheric halogen chemistry on the regional and global tropospheric ozone budget.
- Influence of tropospheric halogen chemistry on the oxidation capacity of the atmosphere and its impact on climate.
3) Objectives of the Task Team
Define the scientific questions related to halogen chemistry in the troposphere. Identify interested research groups and scientists. Plan coordinated research activities in the area of halogens in the troposphere. Provide forum for exchange of information, knowledge, and expertise on the subject.
Provide link to other projects, in particular to IGAC, SOLAS, AICI, OCEANS, WCRP-CliC, ILEAPS, identify areas where synergies are to be expected. Resolve possible overlap in research topics.
4) Proposed First Steps
Meeting of the Task Team to discuss most promising approaches, which could be:
- Discuss scientific issues related to halogen chemistry in the troposphere. Identify key questions and priorities for research.
- Identify further individuals and organisations who might be interested.
- Write research plan of task.
- Identify promising approaches for research
- Identify and approach projects and tasks with related scientific topics (e.g. IGAC, SOLAS, AICI, …)
- Decide on submission of a formal task proposal