One year of ozone flux measurements and O3, NOx and CO2 profiles at the micrometeorological flux tower of Bosco Fontana (Mantua, Italy)

In the framework of the ÉCLAIRE Project of the 7th EU framework programme an intensive joint measuring campaign and a subsequent long-term campaign have been conducted at a oak-hornbeam mixed mature forest in the Po Valley (Bosco Fontana, Mantova, Italy). Within the forest a 42 m tall micrometeorological tower was installed in order to assess the mass and energy exchange between the ecosystem and the surrounding air, with the focus of the campaign measurements having been the in-canopy chemistry and exchange of aerosols and the focus of the long-term measurements being the ozone and carbon emission/uptake. During the joint measuring campaign a profile of ozone fluxes was obtained at four levels above and within the canopy, using the EC technique, with the ozone fluxes at the top level continuing during the long-term measurements, together with fluxes of water and CO2. The forest represented a relevant sink of tropospheric ozone, both in summer and in winter, with an average monthly ozone removal of 4.4 kg ha-1 and a total annual deposition of more than 50 kg ha-1.. The amount of ozone removed by the forest after entering the leaves stomates was estimated by deriving the bulk stomatal resistance through the inversion of the Monteith equation and then using it to calculate the ozone stomatal fluxes. The stomatal sink resulted only a minor part of the ozone removed by the forest but it is meaningful for trees since ozone is a phototoxic agent. The maximum stomatal fraction was derived for June (40% of the total ozone flux) and the minimum for October (13%). The remaining part of the ozone fluxes, obtained as a residual among the total and the stomatal fluxes, is due to a complex of removal processes – either of physical and chemical nature- collectively named as “non-stomatal” deposition. The non-stomatal ozone deposition, not causing any relevant phototoxic effect, could be regarded a net ecosystem service offered by the forest to the urban community. On the contrary the net carbon sequestration by the forests was almost negligible on an annual basis, as it could be expected for a mature ecosystem. The main results on ozone flux profiles, ozone flux partition and storage will be presented. This kind of study highlights the importance of making continuous direct measurements in order to raise the knowledge and the awareness of the ecosystem services offered to the cities by the urban and periurban vegetation.