Estimation of Volatile Organic Compound Fluxes Using the Forest–Land–Atmosphere Model (FLAME)

Abstract

The prognostic one-dimensional Forest?Land?Atmosphere Model (FLAME) has been further extended to simulate diurnal cycles of volatile organic compound (VOC) fluxes inside and above an idealized mixed forest mainly composed of oaks and pines. The tree height is 12 m and the leaf area index is 4. The canopy crown is divided into five layers of equal leaf area increment. The algorithms developed by Guenther et al. are applied to predict the monoterpene emission from leaves in each canopy layer. The value of photosynthetic active radiation (PAR) and foliage temperature (Tleaf) required by these algorithms are provided by FLAME. The modeled PAR and Tleaf are used for different leaf angle classes i from sun to shaded leaves within each layer j in the canopy. In the present study the authors examine the VOC fluxes modeled at a reference level above the forest for two cases, A and B, for which the modeled canopy temperature Tc reaches a maximum of approximately 21° and 31°C, respectively. It is supposed that VOC fluxes above the canopy are related to Tc by a function of the form F(Tc) = Fs exp[?c(Tc ? TS)]. The authors intend to study the temperature exponent at canopy level ?c by deriving best-fit slopes. The resulting mean values of ?c = 0.125 ± 0.002 K?1 for the morning VOC fluxes, and ?c = 0.267 ± 0.004 K?1 for the afternoon, are larger than those used to calculate the emission on leaf scale from cuvette data (?1 = 0.124 K?1 and ?2 = 0.09 K?1 for oaks and pines, respectively). A sensitivity study was carried out using the modeled mean values of Tleaf and PAR in a canopy layer instead of the angle dependence to simulate measurements by an infrared radiation thermometer. The authors also tested the importance of the basal emission rate Es on the total VOC fluxes and ?c above the canopy.