© 2000 Heron Publishing—Victoria, Canada
Isoprene emission inventory for the BOREAS southern study area
Hal Westberg (1), Brian Lamb (1), Kelly Kempf (1) and Gene Allwine (1)
1. Laboratory for Atmospheric Research, Washington State University, Pullman, WA 99164-2910, USA / Received June 3, 1999
Summary
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.
Keywords:
biogenic emission modeling, boreal forest carbon balance, micrometeorological gradient, Picea mariana, Populus tremuloides.
