MODELED AND FIELD-MEASUREMENTS OF BIOGENIC HYDROCARBON EMISSIONS FROM A CANADIAN DECIDUOUS FOREST

The Biogenic Emission Inventory System (BEIS) used by the United States Environmental Protection Agency (Lamb et al., 1993, Atmospheric Environment 21, 1685-1705; Pierce and Waldruff, 1991, J. Air Waste Man. Ass. 41, 937-941) was tested for its ability to provide realistic microclimate descriptions within a deciduous forest in Canada. The microclimate description within plant canopies is required because isoprene emission rate's from plants are strongly influenced by foliage temperature and photosynthetically active radiation impinging on leaves while monoterpene emissions depend primarily on leaf temperature. Model microclimate results combined with plant emission rates and local biomass distribution were used to derive isoprene and alpha-pinene emissions from the deciduous forest canopy. In addition, modelled isoprene emission estimates were compared to measured emission rates at the leaf level. The current model formulation provides realistic microclimatic conditions for the forest crown where modelled and measured air and foliage temperature are within 3 degrees C. However, the model provides inadequate microclimate characterizations in the lower canopy where estimated and measured foliage temperatures differ by as much as 10 degrees C. This poor agreement may be partly due to improper model characterization of relative humidity and ambient temperature within the canopy. These uncertainties in estimated foliage temperature can lead to underestimates of hydrocarbon emission estimates of two-fold. Moreover, the model overestimates hydrocarbon emissions during the early part of the growing season and underestimates emissions during the middle and latter part of the growing season. These emission uncertainties arise because of the assumed constant biomass distribution of the forest and constant hydrocarbon emission rates throughout the season. The BEIS model, which is presently used in Canada to estimate inventories of hydrocarbon emissions from vegetation, underestimates emission rates by at least two-fold compared to emissions derived from field measurements. The isoprene emission algorithm proposed by Guenther et al. (1993), applied at the leaf level, provides relatively good agreement compared to measurements. Field measurements indicate that isoprene emissions change with leaf ontogeny and differ amongst tree species. Emission rates defined as function of foliage development stage and plant species need to be introduced in the hydrocarbon emission algorithms. Extensive model evaluation and more hydrocarbon emission measurements from different plant species are required to fully assess the appropriateness of this emission calculation approach for Canadian forests.