Measurement of isoprene emissions over a black spruce stand using a tower-based relaxed eddy-accumulation system

Daytime isoprene emissions were measured over a black spruce forest in Sask atchewan (Canada) during the Boreal Ecosystem Atmosphere Study of 1994. The relaxed eddy-accumulation (REA) technique was used to measure isoprene flu xes in parallel with gradient measurements, which are required for using th e gradient transport (GT) theory. The average isoprene flux was 2.29 mg C m (-2) h(-1) in late July and decreased to 0.54 mg C m(-2) h(-1) in early Sep tember. The senescent needles and lower ambient air temperature were most l ikely the cause of the lower isoprene emissions measured in September. A re lationship of isoprene flux with air temperature was derived at the canopy scale because canopy temperature is not readily available. High isoprene em issions were observed at temperatures above 25 degrees C. These were most l ikely in relation to thermoprotection of photosynthesis. The diurnal trends measured by GT and REA were similar. Isoprene fluxes measured using GT wer e 63% lower than those using REA. The underestimation resulted from having the lower GT inlet in the roughness sublayer, in which the flux-gradient re lationships are not valid. Measuring the gradient at about two canopy heigh t, with a reduced spacing from 10 to 3 m, would reduce the underestimation of the scalar flux, but it would also reduce the isoprene concentration dif ferences by five to six times compared to those obtained with the REA techn ique. Previously, black spruce has been assigned to the highest emitting cl ass of spruce forest for inventory purposes, however, the authors' results suggest that the boreal zone black spruce should be assigned a standard emi ssion rate of 6 rather than 18 mu g C g(-1) h(-1).