Isoprene emission inventory for the BOREAS southern study area

Abstract

An isoprene emission inventory for a section of boreal forest in central Saskatchewan was developed based on measured emission rates from the two dominant isoprene-emitting species, black spruce (Picea mariana (Mill.) BSP) and aspen (Populus tremuloides Michx.). The micrometeorological gradient technique was used to determine isoprene emission factors for establishing the inventory. Isoprene fluxes were measured during each of the three BOREAS intensive field campaigns (IFCs) during the 1994 growing season. Measured isoprene fluxes varied from 0.04 to 3.3 mg C m-2 h-1 over the black spruce canopy, and from 0.05 to 7.3 mg C m-2 h-1 above the aspen forest. Midsummer standard isoprene emission fluxes were 1.2 mg C m-2 h-1 and 2.3 mg C m-2 h-1 (at 20 °C and photosynthetically active radiation (PAR) of 1000 μmol m-2 s-1) for black spruce and aspen, respectively. With light and temperature differences accounted for, there was an apparent seasonal effect on emissions with the highest rates in the summer months. The total amount of isoprene emitted from this section of the boreal forest was estimated to be 8.6 Gg C year-1, which is about 1% of the net ecosystem carbon exchange for the study area. Aspen was the largest contributor, accounting for approximately 70% of the total. Branch enclosure and relaxed eddy accumulation measurements made at the black spruce site were used to define the uncertainty associated with flux measurements. Emission rates obtained by the gradient, enclosure and relaxed eddy accumulation methods showed good agreement when normalized to standard light and temperature conditions. The coefficient of variance between the three techniques was 12% for summer (IFC-2) measurements. The sensitivity of the annual isoprene emission total to the assignment of mean irradiance and temperature was also examined. If the hourly mean temperatures were increased by 1 °C throughout the growing season, annual carbon loss due to isoprene emission would increase by 14% from 8.6 to 9.8 Gg C.