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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/amt-2019-505
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/amt-2019-505
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  30 Mar 2020

30 Mar 2020

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A revised version of this preprint was accepted for the journal AMT and is expected to appear here in due course.

Quality controls, bias, and seasonality of CO2 columns in the Boreal Forest with OCO-2, TCCON, and EM27/SUN measurements

Nicole Jacobs1, William R. Simpson1, Debra Wunch2, Christopher W. O'Dell3, Gregory B. Osterman4, Frank Hase5, Thomas Blumenstock5, Qiansi Tu5, Matthias Frey5,6, Manvendra K. Dubey7, Harrison A. Parker7,8, Rigel Kivi9, and Pauli Heikkinen9 Nicole Jacobs et al.
  • 1Department of Chemistry and the Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA
  • 2Department of Physics, University of Toronto, Toronto, Canada
  • 3Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, CO, USA
  • 4Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
  • 5Karlsruhe Institute of Technology, Institute of Meteorology and Climate Research, Karlsruhe, Germany
  • 6National Institute for Environmental Studies, Tsukuba, Japan
  • 7Earth and Environmental Sciences, Los Alamos National Laboratory, Los Alamos, NM, USA
  • 8California Institute of Technology, Pasadena, CA, USA
  • 9Finnish Meteorological Institute, Sodankylä, Finland

Abstract. Seasonal CO2 exchange in the Boreal Forest plays an important role in the global carbon budget and in driving interannual variability in seasonal cycles of atmospheric CO2. Satellite-based observations from polar orbiting satellites like the Orbiting Carbon Observatory-2 (OCO-2) offer an opportunity to characterize Boreal Forest seasonal cycles across longitudes with a spatially and temporally rich dataset, but data quality controls and biases still require vetting at high latitudes. With the objective of improving data availability at northern, terrestrial high latitudes, this study evaluates quality control methods and biases of OCO-2 retrievals of atmospheric column-averaged dry-air mole fractions of CO2 (XCO2) in Boreal Forest regions. In addition to the standard quality control filters recommended for ACOS B8 (B8 QC) and ACOS B9 (B9 QC) OCO-2 retrievals, a third set of quality control filters were specifically tailored to Boreal Forest observations (Boreal QC) with the goal of increasing data availability at high latitudes without sacrificing data quality. Ground-based reference measurements of XCO2 include observations from two sites in the Total Carbon Column Observing Network (TCCON) at East Trout Lake, Saskatchewan, Canada and Sodankylä, Finland. OCO-2 retrievals were also compared to ground-based observations from two Bruker EM27/SUN FTS at Fairbanks, Alaska, United States. EM27/SUN spectrometers that were deployed in Fairbanks were carefully monitored for instrument performance and were bias corrected to TCCON using observations at the Caltech TCCON site. The B9 QC were found to pass approximately twice as many OCO-2 retrievals over land north of 50° N than the B8 QC, and the Boreal QC were found to pass approximately twice as many retrievals in May, August, and September as the B9 QC. While Boreal QC results in a substantial increase in passable retrievals this is accompanied by increases in the standard deviations in biases at Boreal Forest sites from ∼ 1.4 ppm with B9 QC to ∼ 1.6 ppm with Boreal QC. Total average biases for coincident OCO-2 retrievals at the three sites considered did not consistently increase or decrease with different QC methods, and instead responses to changes in QC varied according to site and satellite viewing geometries. Regardless of the quality control method used, seasonal variability in biases was observed, and this variability was more pronounced at the TCCON sites than when comparing to EM27/SUN observations in Fairbanks. Monthly average biases generally varied between −1 ppm and +1 ppm at the three sites considered, with more negative biases in spring (MAM) and autumn (SO), but more positive biases in summer months (JJA). Monthly standard deviations in biases ranged from approximately 1.0 ppm to 2.0 ppm and do not exhibit strong seasonal dependence apart from exceptionally high standard deviation observed with all three QC methods at Sodankylä in June. There was no evidence found to suggest that seasonal variability in bias is a direct result of airmass dependence in ground-based retrievals or of proximity bias from coincidence criteria, but there were a number of retrieval parameters used as quality control filters that exhibit seasonality and could contribute to seasonal dependence in OCO-2 bias. Furthermore, it was found that OCO-2 retrievals of XCO2 without the standard OCO-2 bias correction exhibit almost no perceptible seasonal dependence in average monthly bias at these Boreal Forest sites, suggesting that seasonal variability in bias is introduced by the bias correction. Overall, we found that modified quality controls can allow for significant increases in passable OCO-2 retrievals with only marginal compromises in data quality, but seasonal dependence in biases still warrants further exploration.

Nicole Jacobs et al.

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Nicole Jacobs et al.

Nicole Jacobs et al.

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Latest update: 05 Aug 2020
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Short summary
The Boreal forest is the largest seasonally-varying biospheric carbon dioxide exchange region on Earth. This region is also undergoing amplified climate warming leading to concerns about the potential for altered regional carbon exchange. Satellite missions such as the orbiting carbon observer-2 (OCO-2) project can measure CO2 abundance over the Boreal forest, but need validation for assurance of accuracy. Therefore, we carried out ground-based validation of OCO-2 CO2 data at three locations.
The Boreal forest is the largest seasonally-varying biospheric carbon dioxide exchange region on...
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