Turbulent transport, characteristic length and time scales above and within the BEMA forest site at Castelporziano

Turbulent transport characteristics were determined above and inside the BEMA-forest to estimate the time and length scales for the diffusive transport of trace constituents like VOCs between the canopy and the atmosphere. The Eulerian (τT) and Lagrangian (τL) time scales for turbulent transport in the crown region during neutral to unstable conditions are found to be less than 102 s. Inside the canopy for heights below z/hc = 0.9 during free convective conditions with small u* (and u* < w*), τT and τL increased up to 2 × 102–6 × 102 s. When mechanical turbulence caused more intensive mixing (u* > w*) also down to the trunk space atmosphere, values of about τT 2 × 102 were determined for z/hc < 0.9. Time scales τcrt for chemical reactions of VOC with radicals (OH, NO3) mostly fulfil the relation τcrt > τT, τL for compounds emitted from the crown. Only for τT > 2 × 102 the relation τcrt ≈ τT holds for some compounds in the lower canopy and an influence of chemical reactions on the net VOC flux to the atmosphere can be expected.