The Aerosol Mass Spectrometer
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A New Instrument for Measuring Aerosol Composition and Size Our group is currently developing and testing a new instrument, the Aerosol Mass Spectrometer (AMS), for measuring the composition of airborne particles as a function of their size. The work is in collaboration with a U.S. research company, Aerodyne Research Inc. (ARI) who designed and constructed the instrument.
Figure 1: The Aerosol Mass Spectrometer operating from the observatory at Calton Hill in Edinburgh. Members of the research group are from left to right Rami Alfarra, Paul Williams and Keith Bower. Photographed by the final member of the group James Allan. The UMIST AMS is only the third of its type in the world and the first outside the U.S.A. Our understanding of aerosol formation, transformation and deposition processes is currently limited by the lack of instrumentation capable of providing size resolved chemical composition information of aerosols at the highest possible temporal and size resolution. The standard method to date has been by collection onto a filter that is later analysed. The AMS has been designed to address these problems and shortfalls and so help to improve our understanding of atmospheric particles.
Why should we bother? Well, aerosol particles are important components of the Earth’s atmosphere. They are either solid or liquid mixtures that have been produced from a wide range of sources and are of small enough size that they remain suspended in the air for hours to days. Particles range in size from a few nanometres diameter, a cluster of only a few molecules, to several microns diameter. Aerosols influence the earth’s energy budget both directly by scattering and absorbing sunlight and indirectly by acting as sites for the formation of cloud droplets. Atmospheric particulate matter can also influence gas phase chemistry by providing a sink for soluble gases and by acting as a surface for important heterogeneous reactions. Aerosols also contribute significantly to the dry deposition of many pollutants to the earth’s surface and quantification of this process for a wide range of aerosol species is required to determine critical loads of pollutant deposition and identify control policies. In urban environments, evidence is accumulating that elevated concentrations of certain aerosols may exacerbate a range of human health disorders. Because of the enormous variety of aerosol species (including crustal components, heavy metals, inorganic salts, elemental carbon (soot) and organic compounds) and the complex way that this material is distributed across aerosol sizes, the aerosol population at any one size will consist of particles with widely differing toxic, hygroscopic, and radiative characteristics.
How does the AMS work?
Figure 2. Aerosol Mass Spectrometer for size and composition analysis of submicron aerosol. Aerosol are sampled at ambient pressure by a unique aerodynamic lens. Aerodynamic particle size is determined from a time-of-flight (TOF) measurement. Size resolved particle composition is obtained by positive ion quadrupole mass spectrometry (QMS) following flash thermal particle vaporization. The instrument, shown schematically in Figure 2, is composed of three main sections: (1) an aerosol sampling chamber, (2) a particle sizing chamber and (3) a particle composition detection chamber. Each chamber is separated by critical apertures and is differentially pumped. The AMS samples particles into high vacuum through an "aerodynamic lens" that contracts and expands the sampled air stream through a series of orifices. The particles have sufficient momentum to leave the streamlines and exit the "lens" in a well-defined beam. The particle beam is directed onto a resistively heated source and the aerosols are vaporised, then ionised by an electron impact ioniser and molecular fragments measured using a quadrupole mass spectrometer. The size of the particles is measured by synchronizing the molecular ions signals from the mass spectrometer with a chopper wheel positioned immediately behind the aerodynamic lens, approximately 50 cm in front of the heated source. The size of the particles can be calculated from the delay time between the chopper wheel allowing a slug of air containing particles to enter the instrument and detection at the heater.
Figure 3. A photograph of the AMS body. The inlet can be seen at the left, with the various chambers shown in figure 2 in the aluminium block, the quadrupole mass spec is on top of the block at the right of the image. The UMIST AMS underwent its first field trials in Edinburgh city centre in early November 2000, the instrument was housed in the observatory house on top of Calton Hill. It sampled successfully for two weeks and has already proven to be a significant advance in field measurement of atmospheric particles. Large variations in the concentrations of nitrate, sulphate and organic species occur on timescales of a few minutes and these can easily be linked to different sources. The different species vary with time in different ways and are associated with particles of different sizes. This type of information has not been possible until now as the standard technique of filter measurements only provides data averaged over several hours and offer poor size discrimination. The AMS has now been used on a number of large field projects, yeilding a considerable amount of data. It has also had a major hardware upgrade as part of the UFAM facility to increase the sensitivity and range of the instrument. Hardware development is still ongoing, and other upgrades are planned for the near future. Laboratory calibrations have been carried out to understand how the instrument responds to a variety of aerosol mixtures. The efficiency of the AMS varies for different types of aerosol when the heater temperature is varied has been quantified. For instance sea salt, an important component of marine aerosol volatilises at a much higher temperature than ammonium nitrate a common continental particle. We have observed from field measurements that the AMS can tell us a great deal about organic material in aerosol particles and a considerable amount of work on the response of the instrument to organic material has been undertaken. The work carried out with the AMS so far has certainly shown that instruments like the Aerosol Mass Spectrometer are the future of aerosol composition measurement for the next decade or more.
Some AMS data from several recent projects
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Comments and Questions about these pages should be directed to Dr. Michael Flynn | ||
Specific questions about the AMS should be directed to Dr. Hugh Coe