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

Local heterogeneity of CO2 sources and sinks is a key factor for the variability of carbon dioxide flux (FC) 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 FC. Strong spatially confined sources like major roads inhibit a direct correlation of FC to area-averaged features of the urban structure and may lead to a heavily biased signal. Four years of FC 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 FC 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 FC is 16.4 µmol m–2s–1 with slight variations (±0.55 µmol m–2s–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 CO2 flux (eFC) per sector is introduced, leading to an enhanced comparability. Relating sectoral eFC instead of FC to urban surface fractions of buildings and vegetation results in a better agreement (also with data from other studies).