Effects of urban density on carbon dioxide exchanges: Observations of dense urban, suburban and woodland areas of southern England

Anthropogenic and biogenic controls on the surface-atmosphere exchange of CO₂ are explored for three different environments. Similarities are seen between suburban and woodland sites during summer, when photosynthesis and respiration determine the diurnal pattern of the CO₂ flux. In winter, emissions from human activities dominate urban and suburban fluxes; building emissions increase during cold weather, while traffic is a major component of CO₂ emissions all year round. Observed CO₂ fluxes reflect diurnal traffic patterns (busy throughout the day (urban); rush-hour peaks (suburban)) and vary between working days and non-working days, except at the woodland site. Suburban vegetation offsets some anthropogenic emissions, but 24-h CO₂ fluxes are usually positive even during summer. Observations are compared to estimated emissions from simple models and inventories. Annual CO₂ exchanges are significantly different between sites, demonstrating the impacts of increasing urban density (and decreasing vegetation fraction) on the CO₂ flux to the atmosphere.