BIOGENIC EMISSIONS OF VOLATILE ORGANIC-COMPOUNDS FROM GORSE (ULEX-EUROPAEUS) - DIURNAL EMISSION FLUXES AT KELLING HEATH, ENGLAND

Volatile organic compound (VOC) emission fluxes from Gorse (Ulex europ aeus) were measured during May 30-31, 1995 at Kelling Heath in eastern England by using bag enclosure and gradient methods simultaneously. T he 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 it s emission rates fluctuated from 6 ng (g dwt)(-1) h(-1) in the early m orning to about 9700 ng (g dwt)(-1) h(-1) at midday. Averaged emission rates standardized to 20 degrees C were 1625, 2120, and 3700 ng (g dw t)(-1) h(-1) for the new grown, ''mixed'', and flowering branch, respe ctively. Trans-ocimene and alpha-pinene were the main monoterpenes emi tted and represented, on average, 47.6% and 36.9% of the total monoter penes. Other monoterpenes, camphene, sabinene, beta-pinene, myrcene, l imonene and gamma-terpinene, were positively identified but together r epresented less than 1.5% of the total VOC emissions from gorse. Maxim um isoprene concentrations in air at the site were measured around mid day at 2 m (174 parts per trillion by volume, or pptv) and 6 m (149 pp tv), and minimum concentrations were measured during the night (8 pptv at both heights). Mean daytime alpha-pinene air concentrations of 141 and 60 pptv at 2 and 6 m height were determined, but trans-ocimene co ncentrations were less than the analytical detection limit (4 pptv), s uggesting rapid chemical removal of this compound from air. The isopre ne 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 mu g 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 emissi on fluxes from the enclosure measurements. When averaged over the 2 da y experiment, isoprene fluxes of 29.8 and 27.8 mu g m(-2) h(-1) were o btained from the gradient and the enclosure extrapolation respectively . These isoprene fluxes to the atmosphere represented between 0.12% an d 0.35% of the net assimilated carbon (as CO2) uptake rate for gorse.