A framework for comparing remotely sensed and in-situ CO2 concentrations R. Macatangay1,2, T. Warneke1, C. Gerbig2, S. Körner2, R. Ahmadov2, M. Heimann2, and J. Notholt1 1Institute of Environmental Physics, University of Bremen, Bremen, Germany 2Max Planck Institute for Biogeochemistry, Jena, Germany
Abstract. A framework has been developed that allows validating CO2 column
averaged volume mixing ratios (VMRs) retrieved from ground-based solar
absorption measurements using Fourier transform infrared spectrometry (FTS)
against measurements made in-situ (such as from aircrafts and tall towers).
Since in-situ measurements are done frequently and at high accuracy on the
global calibration scale, linking this scale with FTS total column
retrievals ultimately provides a calibration scale for remote sensing. FTS,
tower and aircraft data were analyzed from measurements during the
CarboEurope Regional Experiment Strategy (CERES) from May to June 2005 in
Biscarrosse, France. Carbon dioxide VMRs from the MetAir Dimona aircraft,
the TM3 global transport model and Observations of the Middle Stratosphere
(OMS) balloon based experiments were combined and integrated to compare with
the FTS measurements. The comparison allows for calibrating the retrieved
carbon dioxide VMRs from the FTS. The Stochastic Time Inverted Lagrangian
Transport (STILT) model was then utilized to identify differences in surface
influence regions or footprints between the FTS and the aircraft CO2
concentrations. Additionally, the STILT model was used to compare carbon
dioxide concentrations from a tall tower situated in close proximity to the
FTS station. The STILT model was then modified to produce column
concentrations of CO2 to facilitate comparison with the FTS data. These
comparisons were additionally verified by using the Weather Research and
Forecasting – Vegetation Photosynthesis and Respiration Model (WRF-VPRM).
The differences between the model-tower and the model-FTS were then used to
calculate an effective bias of approximately −2.5 ppm between the FTS and
the tower. This bias is attributed to the scaling factor used in the FTS
CO2 data, which was to a large extent derived from the aircraft
measurements made within a 50 km distance from the FTS station: spatial
heterogeneity of carbon dioxide in the coastal area caused a low bias in the
FTS calibration. Using STILT for comparing remotely sensed CO2 data
with tower measurements of carbon dioxide and quantifying this comparison by
means of an effective bias, provided a framework or a "transfer standard"
that allowed validating the FTS retrievals versus measurements made in-situ.
Citation: Macatangay, R., Warneke, T., Gerbig, C., Körner, S., Ahmadov, R., Heimann, M., and Notholt, J.: A framework for comparing remotely sensed and in-situ CO2 concentrations, Atmos. Chem. Phys., 8, 2555-2568, doi:10.5194/acp-8-2555-2008, 2008.