What was done
As the world's second largest city, the capital of Mexico is an urban center with a wide range of direct and indirect CO2 emissions from a mix of commercial, industrial, residential and mobile sources. In the present study, the authors installed an eddy covariance flux system to a height of 37 meters above the ground in a densely populated section of the city to investigate the diurnal flux of CO2 over the urban landscape. CO2 flux data were obtained over 23 days (7-29 April 2003) during the warm dry season, including a period of significantly reduced traffic during the Holy Week national holiday (14-20 April).

What was learned
Urban CO2 concentrations showed a clear diurnal pattern over the period of study with the highest concentrations (398 to 444 ppm, average of 421 ppm) occurring during the morning hours and the lowest concentrations (average of 375 ppm) occurring in the afternoon. The average CO2 flux over the 23-day period was predominately positive, indicating that the surrounding urban surface was a net source of CO2 to the atmosphere, and that CO2 uptake by urban vegetation was not sufficient to offset the CO2 release from anthropogenic sources. The authors also note that urban CO2 concentrations were an average of 20 ppm higher during the morning rush hour of the higher-traffic week prior to Holy Week and 14 ppm higher during the following week. CO2 concentrations were additionally found to be higher on weekdays versus weekends, which effect, in the words of Velasco et al., is "directly related to vehicular traffic," since the transportation sector accounts for approximately 60% of the urban CO2 emissions burden.

What it means
The results of this study add to the growing body of evidence that identifies the urban CO2 dome as a major, although localized, human-induced perturbation of the background rural environment.