Linking Carbon Dioxide Fluxes to Urban Density

Urban areas comprise major carbon sources as this is where anthropogenic activities are concentrated. In order to quantify the role of cities to surface-atmosphere exchanges of carbon dioxide, the eddy covariance (EC) technique has been used to directly measure the net release of CO2. The fluxes of carbon dioxide (FC) observed are used for a wide range of applications: to document anthropogenic injections, to quantify the impact of a specific urban area, to verify urban land surface models and to evaluate emission inventories (for greenhouse gases, pollutants, anthropogenic heat flux) and the role of vegetation in offsetting emissions. While the number of urban sites conducting eddy covariance observations has increased dramatically, relatively few sites have been operational for multiple seasons. The immense range of surface characteristics that are “urban” requires that care is taken in the interpretation and analysis of observed turbulent fluxes. Linking the tower-based EC measurements to their surface source area is particularly difficult as anthropogenic sources are often distributed in the urban canopy (vertically and horizontally). Urban areas are characterized by anthropogenic emissions from buildings and traffic but can also include a significant amount of vegetation and even bare soil (e.g. suburban areas, urban parks). Hence variations in the degree of urbanization (population, traffic volume, building density) can cause strong gradients of carbon fluxes within cities and also between urban agglomerations of different density. Analysis of EC carbon dioxide fluxes collected over a multi-year period at sites in southern England is presented. Fluxes from London's Central Business District are compared to those from a suburban environment (Swindon) and a natural ecosystem. Surface controls on the carbon dioxide exchanges are considered in the context of source area variations at the different sites (as a function of e.g. wind direction, vegetated surface cover fraction, traffic volume etc.). Clear phenological and anthropogenic signals are identified in the fluxes of carbon dioxide at daily, seasonal and annual timescales. They illustrate the differences in FC between city centre and suburban sites in southern England, characterizing the effect of urban density. These results are then put into a broader context using summer time flux data from the Urban Flux Network Database (www.urban-climate.org; www.geog.ubc.ca/urbanflux/). The latter provides details on many of the EC observation sites in urban settings (ca. 60 different cities worldwide; past and present). This multiple-site comparison confirms the linkage between carbon dioxide fluxes and the nature of the urban surface, specifically the built fraction.