Measurement of Halogen Species by Resonance Fluorescence

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.