AMBIENT BIOGENIC HYDROCARBONS AND ISOPRENE EMISSIONS FROM A MIXED DECIDUOUS FOREST

Experiments were conducted during the growing season of 1993 at a mixe d deciduous forest in southern Ontario, Canada to investigate the atmo spheric abundance of hydrocarbons from phytogenic origins, and to meas ure emission rates from foliage of deciduous trees. The most abundant phytogenic chemical species found in the ambient air were isoprene and the monoterpenes cr-pinene and P-pinene. Prior to leaf-bud break duri ng spring, ambient hydrocarbon mixing ratios above the forest remained barely above instrument detection limit (similar to 20 parts per tril lion), but they became abundant during the latter part of the growing season. Peak isoprene mixing ratios reached nearly 10 parts per billio n (ppbv) during mid-growing season while maximum monoterpene mixing ra tios were close to 2 ppbv. Both isoprene and monoterpene mixing ratios exhibited marked diurnal variations. Typical isoprene mixing ratios w ere highest during mid-afternoon and were lowest during nighttime. Pea k isoprene mixing ratios coincided with maximum canopy temperature. Th e diurnal pattern of ambient isoprene mixing ratio was closely linked to the local emissions from foliage. Isoprene emission rates from foli age were measured by enclosing branches of trees inside environment-co ntrolled cuvette systems and measuring the gas mixing ratio difference between cuvette inlet and outlet airstream. Isoprene emissions depend ed on tree species, foliage ontogeny, and environmental factors such a s foliage temperature and intercepted photosynthetically active radiat ion (PAR). For instance, young (< 1 month old) aspen leaves released a pproximately 80 times less isoprene than mature (> 3 months old) leave s. During the latter part of the growing season the amount of carbon r eleased back to the atmosphere as isoprene by big-tooth and trembling aspen leaves accounted for approximately 2% of the photosynthetically fixed carbon. Significant isoprene mixing ratio gradients existed betw een the forest crown and at twice canopy height above the ground. The gradient diffusion approach coupled with similarity theory was used to estimate canopy isoprene flux densities. These canopy fluxes compared favorably with values obtained from a multilayered canopy model that utilized locally measured plant microclimate, biomass distribution and leaf isoprene emission rate data. Modeled isoprene fluxes were approx imately 30% higher compared to measured fluxes. Further comparisons be tween measured and modeled canopy biogenic hydrocarbon flux densities are required to assess uncertainties in modeling systems that provide inventories of biogenic hydrocarbons.