TURBULENT TRANSPORT, CHARACTERISTIC LENGTH AND TIME SCALES ABOVE AND WITHIN THE BEMA FOREST SITE AT CASTELPORZIANO

Turbulent transport characteristics were determined above and inside t he BEMA-forest to estimate the time and length scales for the diffusiv e transport of trace constituents like VOCs between the canopy and the atmosphere. The Eulerian (tau(T)) and Lagrangian (tau(L)) time scales for turbulent transport in the crown region during neutral to unstabl e conditions are found to be less than 10(2) s. Inside the canopy for heights below z/h(c) = 0.9 during free convective conditions with smal l u (and u* < w*), tau(T) and tau(L) increased up to 2 x 10(2)-6 x 10 (2) s. When mechanical turbulence caused more intensive mixing (u >w* ) also down to the trunk space atmosphere, values of about tau(T) less than or equal to 2 x 10(2) were determined for z/h(c) < 0.9. Time sca les tau(crt) for chemical reactions of VOC with radicals (OH, NO3) mos tly fulfil the relation tau(crt) > tau(T), tau(L) for compounds emitte d From the crown. Only for tau(T) > 2 x 10(2) the relation tau(crt) ap proximate to tau(T) holds for some compounds in the lower canopy and a n influence of chemical reactions on the net VOC flux to the atmospher e can be expected. (C) 1997 Elsevier Science Ltd.