Isoprene and monoterpene fluxes measured above Amazonian rainforest and their dependence on light and temperature

Canopy scale emissions of isoprene and monoterpenes from Amazonian rainforest were measured by eddy covariance and eddy accumulation techniques. The peak mixing ratios at about 10 in above the canopy occurred in the afternoon and were typically about 90 ppt(v) of alpha-pinene and 4-5ppb(v) of isoprene. alpha-pinene was the most abundant monoterpene in the air above the canopy comprising 50% of the total monoterpene mixing ratio. Measured isoprene fluxes were almost 10 times higher than alpha-pinene fluxes. Normalized conditions of 30 degreesC and 1000 mumol m(-2) s(-1) were associated with an isoprene flux of 2.4 mg m(-2) h(-1) and a beta-pinene flux of 0.26 mg m(-2) h(-1). Both fluxes were lower than values that have been specified for Amazon rainforests in global emission models. Isoprene flux correlated with a light- and temperature-dependent emission activity factor, and even better with measured sensible heat flux. The variation in the measured a-pinene fluxes, as well as the diurnal cycle of mixing ratio, suggest emissions that are dependent oil both light and temperature. The light and temperature dependence can have a significant effect on the modeled diurnal cycle of monoterpene emission as well as on the total monoterpene emission. (C) 2002 Elsevier Science Ltd. All rights reserved.