Measurement of Isoprene Emissions over a Black Spruce Stand Using a Tower-Based Relaxed Eddy-Accumulation System

Abstract

Daytime isoprene emissions were measured over a black spruce forest in Saskatchewan (Canada) during the Boreal Ecosystem Atmosphere Study of 1994. The relaxed eddy-accumulation (REA) technique was used to measure isoprene fluxes in parallel with gradient measurements, which are required for using the 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 September. The senescent needles and lower ambient air temperature were most likely the cause of the lower isoprene emissions measured in September. A relationship of isoprene flux with air temperature was derived at the canopy scale because canopy temperature is not readily available. High isoprene emissions were observed at temperatures above 25°C. These were most likely in relation to thermoprotection of photosynthesis. The diurnal trends measured by GT and REA were similar. Isoprene fluxes measured using GT were 63% lower than those using REA. The underestimation resulted from having the lower GT inlet in the roughness sublayer, in which the flux?gradient relationships are not valid. Measuring the gradient at about two canopy height, 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 differences by five to six times compared to those obtained with the REA technique. Previously, black spruce has been assigned to the highest emitting class of spruce forest for inventory purposes;however, the authors? results suggest that the boreal zone black spruce should be assigned a standard emission rate of 6 rather than 18 ?g C g?1 h?1.