Spatial and Temporal Distribution of CO₂ and Its Spectrum Characteristic in Beijing Urban Area

Using the turbulence data measured by the open path eddy covariance system deployed at the Beijing 325-m meteorological tower, the concentration and flux and spectrum of CO₂ at seven different height levels are calculated for the period from December 2014 to November 2015. Analysis of the results indicate that the CO₂ concentration displays diurnal variation with double peaks except in the winter. Human activities must be considered in the winter since they reduce the diurnal variation of CO₂ concentration and lead to a relatively flat pattern. At all observational heights, the CO₂ concentration and flux show obvious seasonal variations with the maximum values appearing in the winter and the minimum values occurring at the end of the spring and summer. The CO₂ concentration decreases with height as a whole. Beijing is a carbon dioxide source. Daily changes in the CO₂ flux are not as obvious as daily changes in the CO₂ concentration. The CO₂ flux is negative below 47 m and positive above 47 m. The CO₂ flux decreases with height below 140 m and increases with height above 140 m. According to the analysis of spatial and temporal distribution of CO₂, it is found that the CO₂ concentration and flux in urban boundary layer are strongly affected by surface carbon emission source, underlying surface vegetation, atmospheric stability, environment temperature and weather process and so on. The results of turbulence spectra in the present study are very close to the results of Kaimal, i.e., the normalized velocity spectrum and the CO₂ spectrum have a slope of -2/3 in the inertial subregion and they both have relationships with the stability parameter (Z/L) in the low frequency zone. This indicates that the turbulence spectra of the city with complex topography are not quite different from those with flat topography.