Surface controls on carbon dioxide and water vapour fluxes over a Mediterranean city

Local-scale fluxes of latent heat and carbon dioxide (CO₂) were measured at four heights on a mast above Marseille as part of the ESCOMPTE field campaign in the summer of 2001. The mast site is in the highly urbanized central district characterized by minimal vegetation cover (10 – 20% mean plan area), tall buildings and high vehicle traffic. Here we explore the complex inter-relations between the fluxes of CO₂ and latent heat and the properties of the urban surface. The connection between the two was studied using a turbulent source area model to identify the surface area contributing the bulk of the fluxes for each 30-minute time period. Sources of carbon dioxide and water vapour in Marseille are localized and patchy. Thus a small change in source area, such as that associated with an increase or decrease in measurement height and/or atmospheric conditions, can result in the source area having a significantly different set of surface properties. This was particularly noticeable for the carbon dioxide fluxes. Although the city mainly acted as a source of CO₂, and the fluxes reflected diurnal traffic rhythms, during the late afternoon localized sequestration of carbon was found when source areas coincided with patches of greater vegetation cover. Latent heat fluxes were generally small, reflecting the low fraction of vegetation cover and the absence of open water in the source areas of the turbulent flux measurements.