Abstract
Abstract. Eddy-covariance measurements of carbon dioxide fluxes were taken continuously between October 2006 and May 2008 at 190 m height in central London (UK) to quantify emissions and study their controls. Inner London, with a population of 8.2 million (~5000 inhabitants per km2) is heavily built up with 8% vegetation cover within the central boroughs. CO2 emissions were found to be mainly controlled by fossil fuel combustion (e.g. traffic, commercial and domestic heating). The measurement period allowed investigation of both diurnal patterns and seasonal trends. Diurnal averages of CO2 fluxes were found to be correlated with traffic but also exhibited an inverse dependency on atmospheric stability in the near-neutral range, with higher fluxes coinciding with unstable stratification during most seasons and perhaps reflecting how changes in heating-related natural gas consumption and, to a lesser extent, photosynthetic activity controlled the seasonal variability. Despite measurements being taken at ca. 22 times the mean building height, coupling with street level was adequate, especially during daytime. Night-time saw a higher occurrence of stable or neutral stratification, especially in autumn and winter, which resulted in data loss in post-processing and caused the tower to become decoupled from street level. CO2 fluxes observed at night were not always correlated with traffic counts, probably reflecting this decoupling, but also the fact that at night heating was always a larger source than traffic. No significant difference was found between the annual estimate of net exchange of CO2 for the expected measurement footprint and the values derived from the National Atmospheric Emissions Inventory (NAEI), with daytime fluxes differing by only 3%. This agreement with NAEI data also supported the use of the simple flux footprint model which was applied to the London site; this also suggests that individual roughness elements did not significantly affect the measurements due to the large ratio of measurement height to mean building height.
Highlights
In recent years, monitoring of carbon dioxide (CO2) exchange with natural and semi-natural environments has benefited from communication through international flux networks (FLUXNET and its regional components such as CarboEurope, AmeriFlux, AsiaFlux, etc.)
We examine the dynamics of CO2 emissions above central London (UK) based on eddy-covariance measurements taken atop a 190 m telecommunication tower
Data coverage was high in winter and summer (76% and 84% respectively) whilst less than 50% of seasonal data was available in spring and autumn (45% and 44% of data available, respectively)
Summary
In recent years, monitoring of carbon dioxide (CO2) exchange with natural and semi-natural environments has benefited from communication through international flux networks (FLUXNET and its regional components such as CarboEurope, AmeriFlux, AsiaFlux, etc.). Relatively few measurements of CO2 exchange have been performed in urban environments, where over 50% of the world’s population are estimated to live (United Nations, 2007) 1, and the majority of studies were conducted in temperate areas of the Northern hemisphere, e.g. Basel, Switzerland (Vogt et al, 2006); Chicago, USA (Grimmond et al, 2002); Edinburgh, UK (Nemitz et al, 2002); Tokyo, Japan (Moriwaki and Kanda, 2004), as compiled in the database of the International Association for Urban Climate 2. Summer fluxes in Tokyo were comparable to those measured in Chicago
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