Terpene fluxes from a Norway spruce (Picea abies) forest and an orange orchard (Citrus clementii and Citrus sinensis) were measured by relaxed eddy accumulation (REA) during summer 1997. alpha-pinene and beta-pinene were the most abundant terpenes emitted from Norway spruce and constituted approximately 70% of the flux. A much lower flux was observed for myrcene, limonene and gamma-terpinene and both alpha-terpinene and camphor were only occasionally detected. The average terpene flux was 107.6 ng m(-2) s(-1) which corresponds to 0.73 mu g g(dw)(-1) h(-1) (30 degrees C) when calculated relatively the weight of the dry biomass. The five terpenes which were detected in all samples at the orange orchard were limonene, sabinene, alpha-pinene, trans-ocimene and beta-pinene with an average Aux of 126.3 ng m(-2) s(-1). Cis-ocimene, linalool and myrcene were occasionally detected but no systematic upward or downward flux was observed. The results from a laboratory intercomparison made in Spain deviated by maximum 7%. The flux measured at the two sites exhibited a strong diurnal variation with maximum in the afternoon and minimum in the morning hours and evenings. The applied REA system is new in its design and the major advantage is that the air is sampled at a constant flow rate on the adsorbent tubes. This is realised by having two primary Teflon lines with a relatively large flow rate (61 min(-1)). The air is then sampled from these two Teflon lines on two identical adsorbent tubes at a constant and much lower rate by using two precision pumps operated at approximately 60 mi min(-1). The terpenes collected on the adsorbent tubes were significantly decomposed by ozone during sampling unless ozone scrubbers were applied. (C) 2000 Elsevier Science Ltd. All rights reserved.