Isoprene emission inventory for the BOREAS southern study area

An isoprene emission inventory for a section of boreal forest in central Sa skatchewan was developed based on measured emission rates from the two domi nant isoprene-emitting species, black spruce (Picea mariana (Mill.) BSP) an d aspen (Populus tremuloides Michx.). The micrometeorological gradient tech nique was used to determine isoprene emission factors for establishing the inventory. Isoprene fluxes were measured during each of the three BOREAS in tensive field campaigns (IFCs) during the 1994 growing season. Measured iso prene 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. Mids ummer standard isoprene emission fluxes were 1.2 mg C m(-2) h(-1) and 2.3 m g C m(-2) h(-1) (at 20 degrees C and photosynthetically active radiation (P AR) of 1000 mu mol m(-2) s(-1)) for black spruce and aspen, respectively. W ith light and temperature differences accounted for, there was an apparent seasonal effect on emissions with the highest rates in the summer months. T he 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 ecosys tem carbon exchange for the study area. Aspen was the largest contributor a ccounting for approximately 70% of the total. Branch enclosure and relaxed eddy accumulation measurements made at the bla ck spruce site were used to define the uncertainty associated with flux mea surements. Emission rates obtained by the gradient, enclosure and relaxed e ddy accumulation methods showed good agreement when normalized to standard light and temperature conditions. The coefficient of variance between the t hree techniques was 12% for summer (IFC-2) measurements. The sensitivity of the annual isoprene emission total to the assignment of mean irradiance an d temperature was also examined. If the hourly mean temperatures were incre ased by 1 degrees C throughout the growing season, annual carbon loss due t o isoprene emission would increase by 14% from 8.6 to 9.8 Gg C.