Profiles and simulated exchange of H2O, O-3, NO2 between the atmosphere and the HartX Scots pine plantation

Vertical profiles of H2O, CO2, O-3, NO and NO2 were measured during the Hartheim Experiment (HartX) to develop and calibrate a multi-layer resistance model to estimate deposition and emission of the cited gaseous species. The meteorological and gas concentration data were obtained with a 30 m high telescopic mast with 7 gas inlets located at 5 m intervals and meteorological sensors at 5, 15 and 30 m above ground; a complete gas profile was obtained every 9 min 20 s. Measured profiles were influenced by several exchange processes, namely evapotranspiration, dewfall, assimilation of CO2 in the tree crowns, soil respiration, deposition of NO2 and O-3 to the soil and advection of NOx from the nearby highway. Surprisingly, no decrease in O-3 concentration was observed in the crown layer during daytime, probably due to the relatively low density of foliage elements and strong turbulent mixing. The advantage of measuring in-canopy profiles is that turbulent exchange coefficients need not be estimated as a prerequisite to obtaining vertical flux estimates. In recent years, flux-gradient relationships in canopies have been subject to many criticisms. If fluxes are calculated at several heights considering only the transfers between the turbulent air and the interacting surfaces at a certain height, and those fluxes are then integrated vertically in a subsequent step, then exchange estimates (deposition or emission) can be obtained independent of turbulent exchange conditions. Typical estimated deposition velocities calculated for a 3-day period are between 4 and 10 mm/s for NO2 and about 4-9 mm/s for O-3 (day and night values respectively). This leads to deposition rates of about 20-40 ng N/m(2)s for NO2 and about 30-40 mg O-3/m(2) deposited daily under the conditions encountered during HartX. Sensitivity tests done with the best available and most realistic values for model parametrization have shown that sensitivity is large with respect to the soil and cuticula resistances as well as for gas-phase ozone destruction and that more research is required to describe the effectiveness of cuticula and soil in modifying sink characteristics for NO2 and O-3.