Tropospheric halogen chemistry: Reaction mechanisms, processes and global impacts

Around two decades ago reactive halogen compounds (iodine, chlorine and bromine) were found to cause sudden ozone loss in the lowest part of the troposphere in the Arctic. In the meantime reactive halogens were also found in many other parts of the troposphere, mainly in the marine boundary layer but also over salt lakes, in the plumes of volcanoes, in the free troposphere and even in the middle of the continents. The sources for reactive halogens in the troposphere appear to be mainly natural, mostly linked to halides contained in sea water or salt deposits.

The scientific community has made great progress in the measurement of these compounds and also in the understanding of the underlying release and transformation processes. Very detailed process models have been successful in reproducing the intricate chemistry which involves reactions in the gas phase, in and on aerosol particles as well as cloud droplets, which is why we refer to this as multiphase chemistry. Comparisons with field data show that the contribution of reactive halogens to ozone destruction is often on the order of 30-50% (e.g. at the Cape Verde observatory). However very few global models include reactive halogens in the troposphere. The models that do usually have to make crude assumptions regarding the sources and have to employ a reduced reaction mechanism to make it computationally feasible to perform global model runs. Another recent discovery is that chlorine atoms can contribute up to 15% to the chemical loss of methane in the tropics; this loss is not included in any of the climate models. In many continental settings several hundred parts per trillion (ppt) of chlorine have been found indicating that chlorine chemistry can be relevant there as well. It is important to stress that methane and tropospheric ozone are strong greenhouse gases.

In this project we aim to strengthen the theoretical foundation for global models by thoroughly revisiting the reaction mechanisms, providing reduced reaction mechanisms that have been tested in process models for a variety of scenarios encountered in the global troposphere and by developing parameterisations for the release of reactive halogens. The outcomes from this work will be included in a state-of-the-art global chemistry-aerosol model in order to quantify the global impacts of reactive halogen chemistry on ozone destruction and production, methane destruction as well as the formation and growth of aerosol particles. Furthermore, we will compare current day scenarios with preindustrial scenarios in order to establish the importance of anthropogenic pollutants for the release of reactive halogens. This is motivated by the fact that many halogen release mechanism involve acidity and some are linked to nitrogen oxides. Anthropogenic activity has increased both atmospheric acidity and nitrogen oxide concentrations.

This project brings together the UEA group with a long-standing experience in tropospheric halogen chemistry in virtually all tropospherically relevant areas and the Leeds group with a very strong track record in global modelling including halogen chemistry. This project is very timely as in the last few years several data sets have become available and more are being collected that allow us to test our model predictions on a much larger scale than possible just a few years ago. Given the potentially large impacts on tropospheric chemistry and climate the relevance of this project is significant.

In addition to the academic community (see above) this project will benefit the following
communities:

1) Model developers. Our reduced halogen schemes can be used in large 'Earth system' models such as the Met Office / NERC HadGEM-3 model. The code modules that we use for our chemistry and aerosol schemes are similar to the UKCA family of models making knowledge transfer simple. By the end of the project these tested reduced schemes will be ready for use in climate models and will be available electronically.

2) Industrial groups who manufacture and release short-lived organic halogen compounds will be interested in the atmospheric impact of these species on breakdown. This can only be determined through a thorough knowledge of tropospheric chlorine, bromine and iodine chemistry. Industrial users will be invited to our workshop in late 2014.

3) Policy makers at DEFRA interested in regional air quality.
The role of halogens in the oxidising capacity of the troposphere also needs to be understood for accurate assessment of long-range pollution transport and estimates of air quality. DEFRA and Met Office staff will be invited to our workshop.

4) International assessment communities such as IPCC and WMO. The role of halogens needs to be included in assessments of our understanding of the current atmosphere and how it will change.

5) Scientific highlights will be communicated to the wider public through our dedicated project web pages, with links from our main departmental pages, and through press releases at UEA and Leeds.