You are here

Home

Harvard Forest >

Harvard Forest Symposium Abstract 2010

  • Title: Modeling regional carbon dioxide fluxes using midday mixing ratio measurements in the canopy surface layer and by aircraft above Harvard Forest, MA, USA
  • Primary Author: ChunTa Lai (San Diego State University)
  • Additional Authors: Jim Ehleringer (University of Utah); xuerui dang (Harvard University)
  • Abstract:

    We modeled regional carbon dioxide (CO2) fluxes by using midday mixing ratios measured in the canopy surface layer and by an aircraft above Harvard Forest, MA, USA over 6 years (2002-2007). Surface CO2 mixing ratios were measured using flask air samples collected at a height 2m above the canopy by an automated sampling system. Aircraft measurements were available from the Global Monitoring Division, Earth System Research Laboratory (http://www.esrl.noaa.gov/gmd/dv/ftpdata.html). Applying an equilibrium boundary layer approach to focus on processes that influence mean CO2 balance in the atmospheric boundary layer (ABL), we estimated monthly average CO2 fluxes by inverting the difference between CO2 concentrations in the ABL and those in the free troposphere. The equilibrium boundary layer approach assumes that CO2 measured under well-mixed conditions suitably capture the mean effect of the diurnal processes when the measurements were averaged longer than one day. We used CO2 mixing ratios derived from the marine boundary layer (MBL) (Globalview-CO2, 2009) as a proxy to represent those in the free troposphere. This proxy was compared to aircraft CO2 concentrations measured at the height 2000-3000m asl in the region. The mean monthly rate of air entrainment into the ABL from the free troposphere was estimated by interpolating NCAR/NCEP Reanalysis data. We scaled the entrainment rate to account for seasonal variations in the ABL height (980-1780m asl) for the region, estimated by a long-term average from the European Center for Medium-Range Weather Forecasts (ECMWF) model data. Modeled regional CO2 fluxes were compared with tower-based eddy covariance NEE measurements. Modeled net CO2 flux appeared to successfully capture the seasonal variation of measured NEE during each of the 6 years. However, modeled net CO2 fluxes were smaller in summertime uptake and greater in wintertime emission than the local NEE in all years. This discrepancy was apparent irrespective of which CO2 mixing ratio data were used in the calculation. We suggest that this discrepancy can be explained by the difference in the spatial scales represented by the tower flux and CO2 mixing ratio measurements. Whereas the tower flux likely only covers a footprint within the productive forest, the ABL inversion likely covers a footprint beyond the forest and includes CO2 fluxes from the surrounding areas that include less productive forests, agricultural fields and other human developments

  • Research Category: Forest-Atmosphere Exchange

  • Figures:
    • HV_NEE_com1.pdf