Measurements of CO2 fluxes in the Mexico City megacity

Urban areas are acknowledged to be major sources of anthropogenic CO2, however, there are few direct measurements of CO2 emissions in urban areas. This is particularly true for megacities, such as Mexico City, where there is rapid growth with a wide range of sources. In this account, we deployed an eddy covariance flux system on a tall urban tower within a densely populated section of Mexico City (Iztapalapa, 12,000 inhabitants per km2) to obtain direct measurements of CO2 emissions in an urban environment. The applicability of micrometeorological techniques to a potentially inhomogeneous area, such as a city with diverse emission sources, is confined to conditions that meet stationarity criteria, such that the tower height exceeds the blending height at which the small scale heterogeneity merge into a net flux above the city landscape. In this paper, we demonstrate that the underlying boundary conditions for flux measurements are satisfied through stationarity, spectral and co-spectral analyses. Results from a field experiment conducted during April, 2003 show a diurnal pattern with a peak of 2.60 g m-2 h-1 during the morning rush hour. After the rush hour, the CO2 flux stays relatively constant around 1.60 g m-2 h-1 during the day and decreases at night to relatively low levels.