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

Abstract. 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.