Eddy covariance measurements of ozone fluxes at 4 levels above and within a forest canopy in the Po valley (Italy)

In June and July 2012, during the intensive field campaign of the ECLAIRE project, ozone fluxes as well as\r\nsensible heat and momentum fluxes were measured at four different levels at the Bosco Fontana site, a 26 m tall\r\nmixed oak-hornbeam forest in the Po valley (I). Each measuring level (41 m, 32 m, 24 m and 16 m) was equipped\r\nwith a sonic anemometer and a fast ozone analyser, absolute ozone concentrations were measured by means of\r\na UV photometer. Additional meteorological parameters were measured on the top and at each level and ozone\r\nconcentrantions were measured at ground level (0.15 m) by another UV photometer.\r\nIn order to compare measurements collected with different instruments, data were preliminary processed and\r\nthe following methodology were applied: despiking, instantaneous rotations, WPL corrections, frequency loss\r\ncorrections and calculation of the random error.\r\nThe main aims of this field campaign were the description of the deposition processes within and above canopy\r\nand the test of the constant flux hypothesis. About this latter interesting features emerged: the conservation of\r\nthe flux was valid for the momentum only outside the canopy and no conservation was observed for the sensible\r\nheat. On the contrary ozone fluxes showed a much more irregular behaviour: for the three upper levels, ozone\r\nfluxes were nearly constant in the first hours of the day while an enhancement of the fluxes was observed at 24\r\nm. This latter fact was strictly linked with the in-canopy dynamics: a greater heating of the canopy was observed\r\nin the afternoon, leading to the formation of an inversion at this level. This inversion divided the in-canopy air\r\nvolume into two layers: the lower one with a stable stratification and the upper one with a turbulent regime; as a\r\nconsequence of this inversion a remarkable reduction of the ozone concentration was observed for the two lowest\r\nlevels in the afternoon, when in the lowest layers ozone is consumed by NO emissions from soil.\r\nThe ozone canopy uptake was estimated in two different ways: with the big-leaf approach and as a difference of\r\nthe above and below canopy fluxes. A comparison of the results of these two approaches will be showed.