Dr Stephen Ball

I welcome scientific enquiries from colleagues and students about any aspect of my research.

Optical absorption spectroscopy is widely applied in the atmospheric sciences to measure trace gases. The results of such measurements are used in environmental monitoring and for validation of results from computer models of the atmosphere. Optical methods have the distinct advantages that the target species is identified unambiguously and its concentration is obtained directly. But many of the most interesting species - the radicals that drive chemical reactions in the atmosphere - are typically present in such small concentrations that it is necessary to perform absorption measurements over very long optical paths. Unfortunately, the use of extended optical paths necessarily introduces the issue of spatial averaging, which can mask important local variations in the chemistry of short-lived species. Another method is required.

Our work incorporates the recently developed technique of cavity ringdown spectroscopy (CRDS) into instruments for atmospheric field measurements. In CRDS, direct measurements of an absorber's concentration are provided by monitoring the temporal decay of laser light trapped inside a cavity composed of two or more highly reflective mirrors. With the very high quality of optics now available, optical paths of many kilometres are accessible in a compact instrument. Consequently, CRDS instruments are portable and can be deployed in a variety of applications to explore the local atmospheric sources, sinks and variability of short-lived reactive trace species.

One of the species we have chosen to study is the nitrate radical (NO3). This species dominates the oxidation of a range of natural and anthropogenic source gases in the night-time troposphere. The figure shows the absorption spectrum of a laboratory sample of NO3 recorded by our CRDS instrument, with spectra of other important trace gases below. Our instruments are deployed to measure ambient levels of NO3 radicals, iodine compounds and other short-live gases during field experiments, for example, during the North Atlantic Marine Boundary Layer Experiment (Mace Head, Ireland).

• Modelling molecular iodine emissions in the coastal marine environment: the link to new particle formation, A. Saiz-Lopez, J.M.C. Plane, G. McFiggans, S.M. Ball, M. Bitter, R.L. Jones, C. Hongwei, T. Hoffman, Atmos. Chem. Phys. Discuss. 2005, 5, 5405-5439.
• Small scale structure in the atmosphere: implications for chemical structure and observational methods, R.L. Jones, S.M. Ball, D.E. Shallcross, Faraday Discussions 2005 130, 165-179.
• A broadband cavity ringdown spectrometer for in-situ measurements of atmospheric trace gases, M. Bitter, S.M. Ball, I.M. Povey, R.L. Jones, Atmos. Chem. Phys. Discuss. 2005, 5, 3491-3532.
• Broadband cavity enhanced absorption spectroscopy using light emitting diodes, S.M. Ball, J.M. Langridge, R.L. Jones, Chemical Physics Letters 2004, 398, 68-74.
• Broadband cavity ringdown spectroscopy, S.M. Ball, R.L. Jones, Chemical Reviews 2003, 103, 5239-5262.
• Broadband cavity ringdown spectroscopy of the NO₃ radical, S.M. Ball, I.M. Povey, E.G. Norton, R.L. Jones, Chemical Physics Letters 2001, 342, 113-120.

Fellow of the Higher Education Academy.

• BSc (Oxford)
• DPhil (Oxford)