Probing organic composition of the atmosphere with Time of Flight Mass Spectrometry

There are a huge number of organic compounds present in the atmosphere. They are emitted from a wide range of sources that are both natural and man-made. Once in the atmosphere the compounds play a significant role in the oxidation chemistry of the atmosphere and may, in the presence of oxides of nitrogen produce significant amounts of ozone. Many of the products of this oxidation have lower volatility and are more polar and soluble and so may be deposited to the surface or into cloud droplets, may condense onto and grow existing atmospheric particulate matter or nucleate new particles. Many of these compounds are important tracers, are important indicators of particular sources, or are key components in the atmospheric cycle of nitrogen or sulphur. Some molecules, particularly from made sources may be toxic to humans. Considerable uncertainty surrounds many of these properties and processes and it is therefore very important to understand and quantify both the atmospheric burden of these and their emission or removal rates to and from the atmosphere.

Chemical ionization with high-resolution time-of-flight mass spectrometry can now provide sensitive, real-time identification and quantification of a large number of gas-phase compounds in sampled air. Chemical ionisation provides selective detection based on the employed reagent ion chemistry and new orthogonal extraction time of flight mass spectrometers provide very sensitive detection systems. A number of these instruments are now in operation and are being used to sample atmospheric concentrations of a wide range of compounds. An instrument at the forefront of such developments is the Aerodyne Inc. CI-ToFMS which is being developed and used by a number of research groups world-wide, including Manchester. Our instrument is coupled to an inlet that can sample gas directly from the atmosphere while at the same time collecting particulate which is subsequently analysed by thermal ramping and backflushing the evolved gas into the mass spectrometer. We are widely using this instrument in laboratory, ground based field, and airborne studies.

In addition, we are seeking to expand our mass spectrometric capability with the addition of an extractive electrospray ionization source to our ToF-MS. This source offers molecular ion formation for a wide range of organic compounds in particulate with very little fragmentation.

The successful PhD student will utilise these instrumental techniques. They will develop further calibration and identification methodologies and will be involved in chamber studies, field experiments and airborne measurements. The focus will be to develop our understanding of organic molecules in the atmosphere through these combined approaches.