THE AVEXPO CAMPAIGN:An Observational Study of Air-Vegetation Exchange processes in the Po Plain (Italy)

A field campaign took place in late May 1989 at S. Pietro Capofiume nearBologna, Italy, with the aim of gaining knowledge on air-vegetation exchangeprocesses. The main parameters measured were vertical fluxes of turbulent heatand of some chemical species through the atmospheric surface layer. Directdeterminations of stomatal resistances in the wheat canopy and complementarymeteorological measurements were also carried out.The fluxes measured were those of the following physical quantities: sensibleand latent heat, momentum, concentrations of nitrogen dioxide, ozone, andaerosol particles. The fluxes were determined using both the eddy-correlationand vertical gradient techniques, using two 7-m high instrumented towers. Thefirst one was equipped with a sonic anemometer, a Krypton hygrometer, andfast-response chemical sensors for eddy-correlation measurements. The secondwas equipped with temperature probes and particle and ozone sensors at 3 levelsfor gradient measurements.A tethered balloon equipped with a turbulence probe was flown in order toobtain heat fluxes and other turbulence parameters for higher altitudes withinthe boundary layer. Direct measurements of stomatal resistances were done inthe wheat canopy.The results show that, within the experimental conditions (dry soil, strong solarirradiation, wheat in a later maturation stage), fluxes of nitrogen dioxide andparticles can be both upward and downward; the abolute values of theexchange velocity were generally below 0.5 cm/s. The ozone fluxes were mostlydownward with absolute values around 2 cm/s. The heat fluxes had the classicalbehaviour, following the solar irradiation curve.