Four-year (2006-2009) eddy covariance measurements of CO2 flux over an urban area in Beijing

Long-term measurements of carbon dioxide flux (F-c) and the latent and sensible heat fluxes were performed using the eddy covariance (EC) method in Beijing, China over a 4-yr period in 2006-2009. The EC setup was installed at a height of 47 m on the Beijing 325-m meteorological tower in the northwest part of the city. Latent heat flux dominated the energy exchange between the urban surface and the atmosphere in summer, while sensible heat flux was the main component in the spring. Winter and autumn were two transition periods of the turbulent fluxes. The source area of F-c was highly heterogeneous, which consisted of buildings, parks, and highways. It was of interest to study of the temporal and spatial variability of F-c in this urban environment of a developing country. Both on diurnal and monthly scale, the urban surface acted as a net source for CO2 and downward fluxes were only occasionally observed. The diurnal pattern of F-c showed dependence on traffic and the typical two peak traffic patterns appeared in the diurnal cycle. Also F-c was higher on weekdays than on weekends due to the higher traffic volumes on weekdays. On seasonal scale, F-c was generally higher in winter than during other seasons likely due to domestic heating during colder months. Total annual average CO2 emissions from the neighborhood of the tower were estimated to be 4.90 kg C m(-2) yr(-1) over the 4-yr period. Total vehicle population was the most important factor controlling the inter-annual variability of F-c in this urban area.