To current knowledge the atmosphere contains a number of trace gases with concentrations below one part per billion. In contrast to their low concentrations these gases can have large environmental effects. Notably halocarbons are known to be very effective in global warming. For instance, in 2005 the two halocarbons CFC-11 and CFC-12 alone contributed about 14% of the effect of CO2 to the anthropogenic greenhouse effect, while CO2 is about 500,000 times more abundant (IPCC, 2007). Although most of the CFCs are decreasing in the atmosphere due to regulation by the Montreal Protocol their fluorinated substitutes also have high global warming potentials and are increasing quickly.
In addition, halocarbons can enhance the destruction of ozone in the stratosphere. This occurs if chlorine or bromine is released from these molecules. In particular bromine is a very effective ozone "killer" and the object of an ongoing scientific debate. The known organic bromine compounds do not provide enough bromine to account for the observed inorganic bromine in the stratosphere.
Only a limited number of known halocarbons are continuously monitored worldwide and at times new ones are identified. There are strong indications for other fluorinated, chlorinated and brominated substances to be present in the troposphere and stratosphere. The aim of the project is to perform a systematic screening of various "layers" in the Earth's atmosphere (boundary layer, free troposphere, tropical tropopause layer and stratosphere) for unknown halocarbons. After establishing a capable analytical system, the search for substances will start with samples from clean-air ground-based stations around the globe, then proceeding with aircraft observations in the upper troposphere/lower stratosphere and finally with balloon samples from the stratosphere. For the detected novel substances the radiative forcing and atmospheric lifetimes will be estimated in order to calculate their global warming potentials. If necessary for this purpose new infrared cross section measurements will be conducted, and radiative transfer modelling carried out. Archived air samples from remote observatories and firn air samples from deep polar snow dating back about 50 years will provide access to long term time trends. Also the influence of the new halocarbons on stratospheric ozone depletion will be determined. Not only will this search for previously unknown gases be undertaken, but also, importantly, extensive measurements will be made of the global distribution of those gases of radiative or stratospheric importance for which very few measurements presently exist; for example short-lived brominated, chlorinated and iodinated organic gases of stratospheric significance. The project will be carried at the University of East Anglia which has has extensive experience and expertise in this area.