The Development of a Highly Sensitive and Selective Technique to Monitor Tracer Dispersion within an Urban Environment

The transport and dispersion of pollutants has important implications for the environment on urban, regional and global scales. At the city level local emissions of pollutants can directly impact the health of the inhabitants, while chemical changes and deposition during transport can affect large regional areas. Various atmospheric dispersion models have been developed to simulate the transport of pollutants over these different spatial scales. Atmospheric tracers play an important role in the testing and validation of these model-simulated meteorological processes. Perfluorocarbons, in particular the perfluorocycloalkanes (PFCAs), are one class of inert, non-toxic and non-depositing tracers, which have been successfully used to directly measure the atmospheric mixing ratios along the trajectory path (Lovelock and Ferber, 1982).

The development and enhancement of the tracer technology necessary to characterise atmospheric dispersion within an urban environment is described. Preliminary results from urban field campaigns in Birmingham (UK), as part of the UK URGENT programme, are presented. These results compare well with those obtained from an advanced operational dispersion model and simple analytical models run at Cambridge University (Britter et al. 2000).

Perfluoromethylcyclohexane was chosen as the initial tracer species, as it has a very low atmospheric background concentration of approximately 4.6 fl/l ±0.8 (Straume et al. 1998). This tracer was released on two occasions within the Birmingham city (UK) conurbation via a heated chimney of height 4.5 m. Two types of automatic sequential samplers have been specifically designed for this project. The first automatically samples air into a series of Tedlarä bags, the second samples onto a series of adsorption tubes, both of which are returned to the laboratory for subsequent analysis. The samplers were placed at various receptor sites which were predetermined using models run by Cambridge University (UK) (Britter et al., 2000).

Analysis is carried out using capillary Gas Chromatography with Negative Ion Chemical Ionisation Mass Spectrometry detection (GC-MS-NICI). This method of detection is both highly selective and ultrasensitive to the strongly electronegative perfluorocarbons and is compound specific. Thus whole air samples can be analysed without the need for the extensive catalytic scrubbers used in previous methods to remove unwanted atmospheric halocarbons. This is particularly important when analysing urban air, due to the high level of indigenous pollutants. The high sensitivity of the mass spectrometer in NICI mode means that lower sample volumes are needed compared to other tracer techniques, thereby shortening the sample times. This allows samples to be taken with the greater temporal resolution needed for fluid dynamic experiments in the urban environment. Although the sample volumes are lower than those used previously, a pre-concentrated step is still required prior to separation by capillary GC. This is achieved by utilising an Adsorption/Desorption System (ADS) developed at Bristol University for monitoring trace amounts of volatile organic compounds (Simmonds et al., 1995).

References

Britter, R., F. Caton, K. Cooke, S. Di Sabatino, P. Simmonds, G. Nickless, (2000), Dispersion of a Passive Tracer Within and Above an Urban Canopy: Birmingham (UK) Experiment, Third Symposium on the Urban Environment, University of California, Davis, California.

Lovelock, J.E. and G.J. Ferber, (1982), Atmospheric Environment, 16 1467-1471

Simmonds, P.G., S. O'Doherty, G. Nickless, G.A. Sturrock, R. Swaby, P. Knight, J. Ricketts, G. Woffendin, and R. Smith, (1995), Analytical Chemistry, 34, 717.

Straume, A., R. N. Dietz, E. N. Koffi and K. Nodop, (1998), Atmospheric Environment, 32, 4109-4122,