What was done
The authors studied the urban CO2 dome of Copenhagen, Denmark, via (1) eddy covariance measurements made atop a 40-m mast located in the center of the city that logged data for more than 14,500 half-hourly periods that were evenly distributed over an entire year, (2) a second eddy covariance system (located atop a 9-m telescopic mast mounted on a vehicle giving an effective height of 10 m) that was operated periodically in different urban settings, including a major entrance road and residential and industrial areas, and (3) mobile transects made during the winter.

What was learned
In the words of the authors, "traffic is the largest single CO2 source in the city," with the mobile measurements demonstrating that "emission rates range from less than 0.8 g CO2 m^-2 h^-1 in the residential areas up to a maximum of 16 g CO2 m^-2 h^-1 along the major entrance roads in the city center."  In the summer, in fact, they report that "86% of the variance in the CO2-flux is explained by the traffic intensity."

In the summer, Soegaard and Moller-Jensen report that "traffic accounts for 51% of the total CO2 emission, whereas in winter the contribution from traffic declines to 39% of the total emission due to the increase in local heating."  In addition, their mobile transects in the winter revealed that city center CO2 concentrations were "50% higher than in the surroundings."

What it means
This paper demonstrates an amazing similarity between both the magnitude and primary source of the urban CO2 dome of Copenhagen, Denmark, and that of Phoenix, Arizona, USA [see Urban CO2 Dome in our Subject Index].