CCl 3F (Freon -11) as an indicator of transport processes in an urban atmosphere—a case study in Melbourne

Abstract

In view of the lack of surface uniformity, and the importance of relatively uniform ground level area sources of pollution in an urban environment, point source diffusion experiments are of limited usefulness in studying transport processes in an urban atmosphere. While it is not feasible to carry out controlled area source diffusion experiments, we argue here that CCl 3F is an existing pollutant which for many cities represents a good ground level area source. For cities such as Melbourne where industrial use of CCl 3F occurs, the effect of a few strong point sources superimposed on the area sources may need to be accounted for. As an example of the use of CCl 3F as an urban atmospheric tracer, vertical concentration profiles and simultaneous profiles of relevant meteorological parameters were obtained at Aspendale during a very stable June night. The profiles show a marked variation of concentration with height which is explained qualitatively by the change in wind direction with height and the variation with direction of the source strength (as determined by a source inventory) around Aspendale. The futility of estimating pollutant concentrations at height from ground level measurements is clear from our results, as is the importance of a knowledge of the non-uniformity of wind (both vertically and horizontally) due to local effects, such as cold air drainage.