Uncertainties in Isoprene Emissions from a Mixed Deciduous Forest Estimated Using a Canopy Microclimate Model

Abstract

Utilizing the concepts of localized near-field diffusion, a modeling system was developed to estimate isoprene emissions from foliage of a mixed deciduous forest. The model determined radiation disposition and foliage temperature inside the canopy using as input forcing variables measured above the forest. Such model outputs were then combined with an isoprene emission algorithm to estimate emissions from the forest ecosystem and thus assess uncertainties in inventory emissions. Under conditions of atmospheric stability, modeled temperature in the bottom half of the forest stand was overestimated by as much as 4°C. For unstable conditions modeled and measured temperatures agreed more closely, the model overestimating temperature by less than 2°C. Based on an uncertainty analysis, parameters exerting the greatest influence on calculated leaf temperature and light levels inside the forest included radiation extinction coefficients and the partitioning between sunlit and shaded leaves. The uncertainties associated with the estimation of isoprene forcing variables gave overestimated isoprene emissions of 30% compared to measurements during the middle of the growing season. To estimate seasonal isoprene emissions, two emission rates were considered. Using a constant emission rate representing middle of the growing season conditions, modeled isoprene flux densities were greater by a factor of 2 compared to gradient?diffusion-derived fluxes during the early part of the growing season. Once applied to the entire growing season, the estimated isoprene emissions were within 20% of measured quantities when seasonally adjusted emission rates were included in the flux calculations.