Fluxes of atmospheric carbon dioxide over a suburban area of Vancouver

Atmospheric carbon dioxide concentration is increasing globally. Most studies on this topic have focused on surface-atmosphere CO₂ exchanges over relatively simple and homogeneous environments typically resolved by large-scale climate and general circulation models (e.g., forest, tundra, ocean, etc.), while the carbon budget associated with urban environments remains largely ignored. Although cities comprise only a small fraction of the global surface area, they are the location of many of the primary sources of anthropogenic CO₂ through vehicular, industrial and building emissions. Since urban areas are often characterized by microscale complexity and variable terrain, measurements of CO₂ fluxes from cities are rarely attempted. However, with the development of finer scale models that can increasingly resolve spatial variability and complexity, atmospheric CO₂ exchanges in urban environments are an increasingly pertinent area of study.

To address this void, a relatively long-term (August 2001 - December 2002) measurement study of atmospheric carbon dioxide fluxes was conducted in a fairly homogeneous suburb of Vancouver, British Columbia. Similar urban structure and cover extends at least 1.5 kilometers in all directions from the measurement tower and consists primarily of one and two-storey detached houses with vegetation (trees, shrubs, and grass) surrounding the buildings. Standard eddy covariance techniques were adopted: a Li-cor 7500 open-path infrared gas analyzer measured the relative densities of carbon dioxide and water vapour, and a Gill sonic anemometer measured the vertical, horizontal and crosswind components of wind at about 25 m above mean ground level. Covariance of the vertical velocity of wind with the concentration of carbon dioxide yields the flux of carbon dioxide between the surface and constant flux layer. In order to characterize the pattern of atmospheric carbon dioxide fluxes from a local-scale suburban area, trends illustrating seasonal variations of carbon dioxide fluxes are discussed. In addition, diurnal flux signatures are demonstrated, with CO₂ fluxes related to prevailing wind direction and surface source areas.