Emission fluxes of VOC by orange trees determined by both relaxed eddy accumulation and vertical gradient approaches

Vertical fluxes of a series of monoterpenes emitted by orange trees along the Spanish Mediterranean coast have been measured in the atmospheric surface layer in June 1997 by the relaxed eddy accumulation (REA) and vertical gradient (VG) methods. In both approaches, the products to be analysed were trapped on Tenax-TA coated steel tubes and subsequently analysed by GC–MS. The concentrations obtained were treated in combination with micrometeorological observations made by sonic anemometers. The fluxes obtained by the two methods are weak but show similar behaviour. The influence of chemical destruction on the observed monoterpene fluxes can be investigated by calculating the Damköhler number, i.e. the ratio of the dynamic and chemical characteristic timescale by considering that the chemical destruction pathways for monoterpenes occurred through reactions with ozone and OH, the values obtained for the Damköhler number were all lower than 0.05, confirming the low influence of chemical destruction on the flux measurements.