An atmospheric stability index based on radon progeny measurements for the evaluation of primary urban pollution
Introduction
It is well known that air pollution over urban areas is the result of a complex interaction between chemistry and meteorology and that the atmospheric concentration of pollutants depends on their emission, transformation and deposition rate as well as on their dilution in the planetary boundary layer. The knowledge of the dilution properties of the lower air layers is, thus, an essential tool for understanding the accumulation of pollutants and, in general, the time evolution of all pollution processes. We could obtain useful information about the dilution potential of the planetary boundary layer, which is not directly measured by any standard meteorological procedure, by monitoring a ground-emitted and chemically stable compound whose emission rate can be considered to be constant in the space and time scale of our observations
Radon gas, which is produced in the soil by the radioactive decay of 226Ra, a member of the 238U series, is released from the soil into the surface layer of the atmosphere, where it is dispersed mainly by turbulent diffusion (Jacobi and Andrè, 1963; Porstendorfer et al., 1991). The atmospheric concentration of radon and of its short-lived decay products (radon progeny), which are fixed on atmospheric aerosol particles, is governed by the source term and the dilution factor. Radon emanation rate varies from one place to another according to soil composition, moisture content, porosity and permeability, but the variations can be considered to be negligible in a time scale of some days and a space scale of some kilometres (Pearson and Jones, 1965; Shweikani et al., 1995). It follows that radon concentration in the air mainly depends on the dilution factor and that radon progeny can be considered as a good natural tracer of the mixing properties of the lower boundary layer (Hsu et al., 1980; Leach and Chandler, 1992; Allegrini et al., 1994; Duenas et al., 1996; Febo et al., 1997).
This paper reports the results of a research project aimed to describe the mixing properties of the lower boundary layer in the urban area of the city of Rome by monitoring the air concentration of radon progeny during a 1-yr period. On the basis of radon progeny measurements, an atmospheric stability index able to classify each day of the period under study in terms of pollutants dispersion—and thus in terms of intensity of a potential primary pollution event—has been defined. The index allows us to uncouple the two contributions that determine the atmospheric concentration of a primary non-reactive pollutant, that is the emission factor and the dilution factor.
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Experimental
Natural radioactivity has been measured by means of a stability monitor (SM200, OPSIS AB, Furulund, Sweden) that consists of a particulate matter sampler equipped with a Geiger–Muller counter for determining the total beta activity of the short-lived radon progeny. The instrument is automatic and operates on two filters at the same time: sampling is performed on the first filter for a 2-h sampling duration, then this filter undergoes the beta measurement phase while a second filter undergoes
Interpretation of natural radioactivity trends
Since radon flux emanating from the ground can be considered, for practical purposes, to be horizontally uniform and constant in time, the time variations of natural radioactivity constitute a good tracer of the mixing properties of the lower atmosphere. In particular, in case of advection or convective mixing of the atmosphere the air concentration of radon has no possibility to build up, while in case of stability radon accumulates into the lower layers of the atmosphere and its concentration
Conclusion
The atmospheric stability index, calculated on the basis of natural radioactivity data, allows the characterisation of the period under study in terms of meteorological predisposition of each day to the occurrence of a primary pollution event. The ASI allows the uncoupling of the two main factors determining primary pollution events—the dilution properties of the lower atmosphere and the emission flows. This study has shown that the mixing properties of the boundary layer play a main role in
Acknowledgements
The authors are indebted to S. Pareti, whose reliable technical assistance made this study possible. Special thanks are also extended to M. Giusto and M. Montagnoli for their support. This study (Progetto AUGUSTO) was financed by the Environmental and Agricultural Policy Department of the Town Council of Rome. The Department is also gratefully acknowledged for having provided traffic flow and primary pollution data.
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