The CAPITOUL (Canopy and Aerosol Particles Interaction in TOulouse Urban Layer) field observation program takes place over the city of Toulouse, France, from February 2004 to March 2005. There are five primary objectives: (i) to study the response of the urban surface and atmosphere to a wide range of atmospheric conditions with a focus on the urban surface energy balance (SEB) (ii) to quantify the response of the boundary layer to the characteristics of the urban surface, e.g., urban breezes, urban (thermodynamical) plumes, urban fog formation and dissipation, and to continuously observe trends in the urban heat island (UHI). (iii) to measure the transformation of the properties (physical, chemical and radiative) of urban aerosols, including urban emission characteristics (and turbulent fluxes of aerosols and their transport to rural areas up to 50 km downwind of the city. (iv) to study the dispersion of passive tracer in the atmosphere in suburban environment. (v) to measure the thermal signature of the urban surface. Instrumentation includes a 30 m tower located in city center for the measurement of turbulent fluxes and the surface energy balance, 20 stations for observing the canopy layer UHI, surface radiative temperature measurements, scintillometers for turbulent heat fluxes, rural stations, including flux and aerosol measurement equipment, wind profilers, radiosonde launches and an aerosol profiler. Specialized aircraft flights were also conducted to sample the boundary layer and surface characteristics using flight paths at various altitudes that include both urban and rural boundary layers.