Measurement of Halogen Species by Resonance Fluorescence

Lead Research Organisation: University of Birmingham
Department Name: Sch of Geography, Earth & Env Sciences

Abstract

Atmospheric ozone levels are of critical importance to human health and the environment for a number of reasons: In the stratosphere, the ozone layer absorbs harmful solar UV radiation. In the troposphere, ozone is a pollutant, harmful to human health, vegetation and materials, and is a greenhouse gas, but is also the major source of the key atmospheric oxidant OH, which initiates the removal of most compounds emitted to the atmosphere, including pollutants and greenhouse gases such as methane. Understanding the processes which may affect the atmospheric abundance of ozone and OH is therefore of considerable societal importance. Compounds containing the halogen atoms iodine and bromine, are known to be released to the atmosphere in marine environments through biological and physical processes. Once in the gas phase, the constituent halogen atoms may be released through photolysis, leading to a number of effects upon tropospheric composition: Catalytic ozone destruction, alteration of HOx and NOx ratios and abundance, and in the case of iodine compounds the formation of new particles which may grow and contribute to CCN concentrations, hence potentially affecting radiative balance and climate. It is therefore important to understand the sources and distribution of halogen species in the lower atmosphere, and to quantify their potential effects upon atmospheric composition. The main aim of this project is to develop a new instrument for the measurement of halogen (iodine and bromine) content in the lower atmosphere. The technique of resonance fluorescence will be used to detect halogen atoms; this then permits the detection of ambient I and Br atoms, photolabile iodine- and bromine-containing species (detected following broadband UV-visible photolysis leading to the release of their constituent halogen atoms), and BrO radicals (detected through chemical conversion to Br by reaction with added NO). A prototype instrument for the detection of iodine species, funded through a previous NERC award, was developed and a trial field deployment successfully completed in summer 2007, demonstrating the viability of the concept and the basic measurement capability. This project, a continuation of the previous award, will develop the prototype into a practical field system, improving the performance and extending the detection capability to bromine species. A second aim of the project is to apply the system to measure iodine compounds on a forthcoming research cruise in the Eastern Pacific Ocean, and to perform point measurements of bromine species at a coastal site in the North-East Atlantic. The former measurements will assess the boundary layer iodine activity in an area identified by satellite observations to be of high halogen activity, and will constrain the total gas-phase iodine content of the lower atmosphere, hence determining the concentration of condensable iodine species which are potentially available to form new particles. The latter experiment, which will include the first measurements of photolabile bromine content in the atmosphere, will determine the spatial distribution (coastal vs. open ocean) of bromine activity at Mace Head (previously only measured using long-path absorption instruments which average spatial heterogeneity), and also provide a field test / demonstration of the full, optimised instrument capability.

Publications

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Description Atmospheric ozone levels are of critical importance to human health and the environment for a number of reasons: In the stratosphere, the ozone layer absorbs harmful solar UV radiation. In the troposphere, ozone is a pollutant, harmful to human health, vegetation and materials, and is a greenhouse gas, but is also the major source of the key atmospheric oxidant OH, which initiates the removal of most compounds emitted to the atmosphere, including pollutants and greenhouse gases such as methane. Understanding the processes which may affect the atmospheric abundance of ozone and OH is therefore of considerable societal importance.



Compounds containing the halogen atoms iodine and bromine, are known to be released to the atmosphere in marine environments through biological and physical processes. Once in the gas phase, the constituent halogen atoms may be released through photolysis, leading to a number of effects upon tropospheric composition: Catalytic ozone destruction, alteration of HOx and NOx ratios and abundance, and in the case of iodine compounds the formation of new particles which may grow and contribute to CCN concentrations, hence potentially affecting radiative balance and climate. It is therefore important to understand the sources and distribution of halogen species in the lower atmosphere, and to quantify their potential effects upon atmospheric composition.



Within this project, a new approach for the measurement of atmospheric halogen species, based upon the technique of resonance fluorescence to detect halogen atoms, has been developed and refined, building upon an initial prototype instrument. Following the instrument development work, the system was applied through two sets of actual experiments: measurements of iodine activity at Mace Head, Ireland, to probe iodine sources in the coastal North Atlantic, and in chamber experiments at the University of Bayreuth, Germany, to study aspects of the photochemistry and cycling of gaseous iodine compounds. The former set of measurements found a strong anticorrelation between iodine levels and local tide height, supporting the hypothesis of a local coastal (littoral or intertidal) source dominating halogen activity at this location. The Bayreuth chamber experiments, performed in collaboration with UEA (atmospheric modelling) and the Universities of Heidelberg (cavity absorption measurements of IO) and Bayreuth (chamber hosts), measured the ozone destruction rate for given iodine source strengths, and found IONO2 to be a less stable reservoir than previously thought, implying that modest amounts of pollution (NOx) in coastal areas will not in fact substantially suppress iodine activity. These results will be incorporated in future models of atmospheric iodine cycling, improving their accuracy.
Exploitation Route Improved instrumental performance for monitoring iodine and related species in a range of industrial contexts Principal exploitation route is to improve the accuracy of models of atmospheric composition (and hence air pollution / climate change)
Sectors Environment

 
Description The main users of this research are scientists and researchers working in the fields of tropospheric chemistry and composition, and atmospheric halogen cycling. The contribution to higher level beneficiaries is achieved indirectly, through improved fundamental understanding. The academic and research communities have been involved, through the wider participation in the Bayreuth experiments (groups from UEA, Heidelberg, Bayreuth; the wider DFG HALOPROC consortium). Results have been communicated through presentations at conferences and meetings.
First Year Of Impact 2012
Sector Environment
 
Description FP7: CHICA: Chamber Studies of Iodine Chemistry in the Atmosphere
Amount € 25,000 (EUR)
Organisation European Commission 
Department Seventh Framework Programme (FP7)
Sector Public
Country European Union (EU)
Start 01/2012 
End 12/2012
 
Description Chamber Studies of Iodine Chemistry in the Atmosphere 
Organisation University of Bayreuth
Country Germany 
Sector Academic/University 
PI Contribution Experiments at the University of Bayreuth simulation chamber, to probe the relationship between atmospheric iodine source strength, IO abundance, and ozone loss rate. Supported by funding from the EU FP7 EUROCHAMP-2 Programme.
Start Year 2012