Biogenic emissions of volatile organic compounds from gorse (Ulex europaeus): Diurnal emission fluxes at Kelling Heath, England

摘要

Volatile organic compound (VOC) emission fluxes from Gorse (Ulex europaeus) were measured during May 30-31, 1995 at Kelling Heath in eastern England by using bag enclosure and gradient methods simultaneously. The enclosure measurements were made from branches at different stages of physiological development (flowering, after flowering, and mixed). Isoprene was found to represent 90% of the total VOC emissions, and its emission rates fluctuated from 6 ng (g dwt)-1 h-1 in the early morning to about 9700 ng(g dwt)-1 h-1 at midday. Averaged emission rates standardized to 20°C were 1625, 2120, and 3700 ng (g dwt)-1 h-1 for the new grown, "mixed," and flowering branch, respectively. Trans-ocimene and α-pinene were the main monoterpenes emitted and represented, on average, 47.6% and 36.9% of the total monoterpenes. Other monoterpenes, camphene, sabinene, β-pinene, myrcene, limonene and γ-terpinene, were positively identified but together represented less than 1.5% of the total VOC emissions from gorse. Maximum isoprene concentrations in air at the site were measured around midday at 2 m (174 parts per trillion by volume, or pptv) and 6 m (149 pptv), and minimum concentrations were measured during the night (8 pptv at both heights). Mean daytime α-pinene air concentrations of 141 and 60 pptv at 2 and 6 m height were determined, but trans-ocimene concentrations were less than the analytical detection limit (4 pptv), suggesting rapid chemical removal of this compound from air. The isoprene fluxes calculated by the micrometeorological gradient method showed a pattern similar to that of those calculated by the enclosure method, with isoprene emission rates maximum at midday (100 μg m-2 h-1) and not detectable during the nighttime. Assessment of the fraction of the site covered by gorse plants enabled an extrapolation of emission fluxes from the enclosure measurements. When averaged over the 2 day experiment, isoprene fluxes of 29.8 and 27.8 μg m-2 h-1 were obtained from the gradient and the enclosure extrapolation respectively. These isoprene fluxes to the atmosphere represented between 0.12% and 0.35% of the net assimilated carbon (as CO2) uptake rate for gorse.