On the controlling factors for the variability of carbon dioxide flux in a heterogeneous urban environment

Abstract

Local heterogeneity of CO 2 sources and sinks is a key factor for the variability of carbon dioxide flux (F C ) in urban areas. Information on the urban structure around a site, especially the related emission characteristics, is thus of great importance to the understanding of observed F C . Strong spatially confined sources like major roads inhibit a direct correlation of F C to area-averaged features of the urban structure and may lead to a heavily biased signal.Four years of F C measured at Basel Aeschenplatz, Switzerland, are analysed with respect to the controlling factors and the cause for variability on different time scales. The source area is segregated into equal sectors to address heterogeneous emission patterns. Residential areas to the east are bordered by business areas and major roads to the west, which leads to a fundamental dependence of F C on wind direction. Besides, its diurnal course is explainable with traffic emissions while its annual course follows heating-related combustion emissions. Vegetation fraction is rather considered to be an indicator for urban land use types (residential/business) and the attributable emission characteristics than to be a measure for biological sink effects. Inter-annual variability occurs as a result of anomalies in wind direction patterns or air temperature. Average yearly F C is 16.4 μmol m -2 s -1 with slight variations (±0.55 μmol m -2 s -1 ) over the 4 years. It likely originates from an average of 70% traffic and 30% heating-related emissions with significant sectoral differences.As a continuous measure for the emissions of each sector, the expected CO 2 flux (eF C ) per sector is introduced, leading to an enhanced comparability. Relating sectoral eF C instead of F C to urban surface fractions of buildings and vegetation results in a better agreement (also with data from other studies).

Ten years of eddy covariance measurements in Basel, Switzerland: Seasonal and interannual variabilities of urban CO2 mole fraction and flux

“…Recently, many studies focused on approaches toward a better understanding of the linkage between carbon dioxide (CO 2 ) emissions in urban environments and their controlling factors [ Christen et al , ; Contini et al , ; Lietzke et al , ; Velasco et al , ]. Modeling approaches are an important step forward to transform information from point measurement to larger spatial (e.g., city) scales, which is essential to achieve a comprehensive picture of the entire urban metabolism.…”

“…As the eddy covariance (EC) method for the determination of turbulent fluxes of mass and energy only became a widely applicable method in urban areas since the late nineties, multiyear studies able to catch long‐term trends and interannual variabilities are still rare. A comprehensive overview of recent studies on urban CO 2 fluxes ( F C ) is given in Lietzke et al [], whereas the longest time series reported in literature cover at most 5–6 years [ Crawford et al , , Gioli et al , ]. In contrast, there are numerous long‐term data sets of CO 2 mole fraction ( ρ C ) and F C from natural terrestrial ecosystems (e.g., FLUXNET, http://www.fluxdata.org) or remote background concentration measurements (e.g., Carbon Dioxide Information Analysis Center, http://cdiac.ornl.gov/) and a variety of corresponding studies [ Hofmann et al , ; Liu et al , ].…”

“…The need of adequate monitoring systems to quantify the impact of cities to the total CO 2 budget is obvious, and a large number of studies about measurement and modeling of urban air pollution have been released during the last decade. Current research on local‐scale urban micrometeorology focuses on the link between direct measurements of CO 2 and its controlling factors derived from geographic information system (GIS) data (e.g., land use, traffic count, and local climate zones) [ Lietzke et al , ] or on bottom up modeling of emission processes tuned by experimental data [ Christen et al , ].…”

“…The application of a sectorial analysis allows attributing the flux data to distinct sources or land use classes within the footprint area. Many studies have shown that, e.g., correlations can be found between F C and traffic volume or heating activity indices [ Helfter et al , ; Lietzke and Vogt , ; Lietzke et al , ; Pataki et al , ; Pawlak et al , ; Velasco et al , ]. Despite the theoretical requirements [ Aubinet et al , ] of the EC‐method (e.g., horizontal homogeneity and stationarity) are barely satisfied, EC measurements turned out to deliver reliable results even for the heterogeneous urban environment if an appropriate station setup is chosen as suggested by Feigenwinter et al [].…”

Diurnal Variation of CO₂ Flux in an Urban Area of Tokyo

“…These studies, however, focused on seasonal variations, not diurnal-scale variations. Revealing the controlling factors of diurnal scale variations is still limited (Velasco et al 2009;Ward et al 2013;Ward et al 2015;Lietzke et al 2015).…”