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 near Bologna, Italy, with the aim of gaining knowledge on air-vegetation exchange processes. The main parameters measured were vertical fluxes of turbulent heat and of some chemical species through the atmospheric surface layer. Direct determinations of stomatal resistances in the wheat canopy and complementary meteorological measurements were also carried out. The fluxes measured were those of the following physical quantities: sensible and latent heat, momentum, concentrations of nitrogen dioxide, ozone, and aerosol particles. The fluxes were determined using both the eddy-correlation and vertical gradient techniques, using two 7-m high instrumented towers. The first one was equipped with a sonic anemometer, a Krypton hygrometer, and fast-response chemical sensors for eddy-correlation measurements. The second was equipped with temperature probes and particle and ozone sensors at 3 levels for gradient measurements. A tethered balloon equipped with a turbulence probe was flown in order to obtain heat fluxes and other turbulence parameters for higher altitudes within the boundary layer. Direct measurements of stomatal resistances were done in the wheat canopy. The results show that, within the experimental conditions (dry soil, strong solar irradiation, wheat in a later maturation stage), fluxes of nitrogen dioxide and particles can be both upward and downward; the abolute values of the exchange velocity were generally below 0.5 cm/s. The ozone fluxes were mostly downward with absolute values around 2 cm/s. The heat fluxes had the classical behaviour, following the solar irradiation curve.