Professor Alastair Lewis
01904 322522
Email: ally.lewis@york.ac.uk
Atmospheric Chemistry
A vast range of chemicals are emitted and processed in the atmosphere of which volatile organic compounds (VOCs) are a critical subset. They have influence on local, regional and continental scales, and impact on radiative forcing and climate as precursors to both ozone and aerosols. VOCs are also of particular interest since their typical atmospheric lifetimes, controlled mainly by the rate of reaction with the hydroxyl radical, are similar to the timescales of many meteorological and transport phenomena. In recent years it has become possible to probe the dynamics of the atmosphere using chemical tracers that act as 'clocks' for the time since emission. This has lead to a greater appreciation of regional and continental scale transport of pollution, and the challenges that this will bring in controlling local and regional air quality which may be dominated by upwind nation emitters.
Observing the transport and transformation of organic pollutants requires detailed field measurements of atmospheric composition, and this in itself produces numerous analytical challenges, be it in the remote inter-annual measurement of ultra trace species in locations such as Antarctica, or fast measurements from research aircraft in flight.
Professor. Lewis is currently Director for Composition Research within the National Centre for Atmospheric Science (NCAS) and NERC Theme Leader for Technology
View details on NCAS and Composition Research
For all correspondence relating to the NERC Technology Theme please use the following email address: ALTL@NERC.AC.UK. Enquiries can also be made via the Theme Leader Assistants at thlassistants@nerc.ac.uk
Current research interests:
- Gas phase oxidant chemistry
The oxidation pathways of organic compounds and the generation of tropospheric ozone from precursors. We are in particular interested in the production and losses of carbonyl compounds such as acetone and methanol in the troposphere and their impact on radical chemistry. This is an area of significant field research, and we operate one of the NERC UFAM (Universities Facilities for Atmospheric Measurements) mobile laboratories which contains significant analytical instrumentation for use in remote locations. To complement the field programme we are also actively developing in the lab, multidimensional chromatographic techniques that allow for a full characterisation of organic compounds in air.
- Transport of Pollution by Weather Systems.
Of particular interest are the transatlantic flow of polluted air from the USA to Europe, the influence of frontal systems in transporting reactive carbon into the free troposphere and the chemical transformations that occur during these dynamical processes. We are involved in joint UK / European / USA Intercontinental Transport Experiments which use large research aircraft to determine composition and dynamics from the surface to 11km altitude, and are involved in developing instrumentation and sensors for use on aircraft. View details of the new joint Universities / NERC / Met Office Bae146 research aircraft. We are also interested in the transport of chemicals in tropical convection and are currently making field observations in Darwin, Australia, of this phenomenon using high altitude aircraft.
- Organic Aerosol Composition.
The uptake of organic material onto atmospheric particles and the coagulation of material to form new fine aerosol is an area of significant current interest, since organic aerosol make a significant contribution to total aerosol number as well as influencing hygroscopic properties and subsequent ability to act as cloud condensation nuclei. Toxicologically there is also evidence that the very fine fraction of aerosol poses the most significant health risk, and that much of this fraction is made up of organic compounds. Determining the exact chemical composition of organics in aerosol is extremely difficult since many tens of thousands of compounds are present, varying widely in chemical properties. Speciation is critical however if we are to make direct links between gas phase emissions (for example from petrochemical evaporation) and aerosol formation and modification. Developing GCxGC-MS methods to resolve such a complex mixture is an important area of our research producing highly detailed but data intensive analysis of aerosol composition. The image below shows a proportion of a multidimensional GCxGC-TOFMS analysis of sub 400nm particles. Every spot is an individual organic compound - in this portion of the analysis over 3000 components are identified.
A significant proportion of organics in aerosol are not amenable to GC however and we are also developing LC-ion trapMS methods to determine the structure and possible VOC precursors to oligomers-like organics. This work uses a combination of liquid chromatography and size exclusion chromatography to determine an initial molecular weight distribution followed by multiple fragmentations of isolated species using a Bruker HCT+ ion trap mass spectrometer.
Details of some of our current field experiments can be found on the Cape Verde Atmospheric Observatory web pages.
Lab-on-a-chip for field observations
Most observations of organic compounds in the environment are made using standard laboratory analytical instrumentation. Whist this is sensitive and very reliable, it is also large, heavy and very power hungry. Making observations in remote environments and where electricity is limited remains a major challenge. Research into new analytical methods using lab-on-a-chip approaches is being undertaken. This work investigates the potential for monolith GC devices that encapsulate all working components needed to make field VOC measurements - injectors, columns, interfaces and detectors in a single microfabricated unit.
The figure above shows a section of microfabricated GC column, part of a planar field device made from wet etching glass. The column channels are circular - like traditional fused silica - but are laid out on a planar surface such that they may be heated and cooled by thermoelectric devices with high efficiency.
Selected Publications:
- Extensive halogen-mediated ozone destruction over the tropical Atlantic Ocean.
K A Read et al., Nature, 2008, 453, 1232-1236. - Chemical Composition Observed Over The Mid-Atlantic And The Detection Of Pollution Signatures Far From Source Regions.
A C Lewis et al., J. Geophys. Res., 2007, 112, D10S39. - OH And Halogen Atom Influence On The Variability Of Non-Methane Hydrocarbons In The Antarctic Boundary Layer
K A Read et al., Tellus B: Chemical and Physical Meteorology, 2007, 59B, 22-38. - A Larger Pool of Ozone-Forming Carbon Compounds in Urban Atmospheres.
A C Lewis et al., Nature, 2000, 405, 778-781. - Microfabricated planar glass gas chromatography with photoionization detection.
A C Lewis, J F Hamilton, C N Rhodes, et al., Journal of Chromatography A, 2010, 1217, 768-774.
