Abstract

ur knowledge of the composition of the atmosphere and our understanding of its chemistry relies on the synergistic efforts of field measurements, atmospheric modelling and laboratory studies: any attempt to replicate field observations by means of modelling requires accurate characterisation of the kinetics and photochemistry of atmospheric processes. This seminar focuses on the measurement aspects of atmospheric science by presenting laboratory studies of halogen oxide reactions implicated in ozone depletion along with the development of a sensor for ambient ammonia underpinned by novel primary gas standards.

The self- and cross-reactions of halogen oxide radicals are complex multichannel processes, with both bimolecular and termolecular components contributing to the overall reaction; as some product channels do not contribute to the catalytic destruction of ozone, an accurate determination of the product branching is of utmost importance for a comprehensive understanding of the atmospheric ozone budget. Kinetic results for the ClO and BrO self-reactions and the BrO + ClO cross-reaction are presented. These were studied by means of laser photolysis coupled with UV absorption spectroscopy with charge-coupled device (CCD) detection which allowed broadband time-resolved acquisition of spectra, leading to the unequivocal identification of multiple species and to the accurate quantification of their concentrations. The findings are compared with previous studies and their implications are discussed.

It is estimated that by 2020 ammonia will be main source of acidification, eutrophication and secondary aerosol formation in Europe as a result of intensive farming and the use of nitrogen-based fertilisers. Accurate measurements of ambient ammonia are needed to verify the impact of environmental policies, as well as to minimise uncertainties in emissions inventories and to provide independent verification of atmospheric model predictions. However state-of-the-art methods currently used in the field suffer from a number of drawbacks, including low accuracy and poor temporal resolution and also require complex post-exposure analysis. The further development of an optical sensor based on cavity ring-down spectroscopy (CRDS), underpinned by primary ammonia reference standards, is described, with a view to extending this rapid on-line technique for exposure chamber validation tests under controlled conditions and ambient monitoring in the field.