What was done
The authors measured CO2 concentrations and fluxes - via the eddy covariance approach - at heights of approximately 40 meters in the Melbourne (Australia) suburbs of Preston (from February 2004 to June 2005) and Surrey Hills (from February 2004 to July 2004).

What was learned
Because of the large height at which measurements were made in this study (40 meters above the ground), both diurnal and seasonal atmospheric CO2 concentration variations (on the order of 15 to 20 ppm) were much less than those observed in cities where concentrations were measured within the first one to two meters above the ground, such as Phoenix, Arizona, USA, Baltimore, Maryland, USA, Cotonou, Benin, West Africa, and Paris, France, where diurnal variations of one to several hundred ppm have been observed. As a result, Coutts et al. report that only after the sun had risen and turbulent mixing had begun did the CO2 begin to disperse from the surface throughout the lower boundary layer and develop the morning peak concentration they observed at the 40-meter elevation, which phenomenon they refer to as "a flushing of the urban canopy dominated by anthropogenic activities."

As has been observed in other cities, the Australian researchers also found that "fluxes of CO2 were influenced predominantly by traffic volumes, but were moderated by suburban vegetation, which can help reduce the daytime flux." More specifically, they determined that "diurnal patterns of fluxes were largely influenced by traffic volumes, with two distinct peaks occurring at the morning and evening traffic peak hours," but that "the vegetative sinks were not sufficient to offset the net sources of CO2 to the atmosphere."

What it means
The results of this study help to better define the origin and nature of the urban CO2 dome, which may in turn provide new insights into how to best revitalize urban areas in terms of residential living density and space for vegetation, particularly within the context of planning efforts such as Melbourne's long-range goal to minimize greenhouse gas emissions as its population continues to grow.