Gradients of column CO<sub>2</sub> across North America from the NOAA Global Greenhouse Gas Reference Network

Abstract

Abstract. This study analyzes seasonal and spatial patterns of column carbon dioxide (CO 2 ) over North America, calculated from aircraft and tall tower measurements from the NOAA Global Greenhouse Gas Reference Network from 2004 to 2014. Consistent with expectations, gradients between the eight regions studied are larger below 2 km than above 5 km. The 11-year mean CO 2 dry mole fraction (XCO 2 ) in the column below ∼ 330 hPa (∼ 8 km above sea level) from NOAA's CO 2 data assimilation model, CarbonTracker (CT2015), demonstrates good agreement with those calculated from calibrated measurements on aircraft and towers. Total column XCO 2 was attained by combining modeled CO 2 above 330 hPa from CT2015 with the measurements. We find large spatial gradients of total column XCO 2 from June to August, with north and northeast regions having ∼ 3 ppm stronger summer drawdown (peak-to-valley amplitude in seasonal cycle) than the south and southwest regions. The long-term averaged spatial gradients of total column XCO 2 across North America show a smooth pattern that mainly reflects the large-scale circulation. We have conducted a CarbonTracker experiment to investigate the impact of Eurasian long-range transport. The result suggests that the large summertime Eurasian boreal flux contributes about half of the north-south column XCO 2 gradient across North America. Our results confirm that continental-scale total column XCO 2 gradients simulated by CarbonTracker are realistic and can be used to evaluate the credibility of some spatial patterns from satellite retrievals, such as the long-term average of growing-season spatial patterns from satellite retrievals reported for Europe which show a larger spatial difference (∼ 6 ppm) and scattered hot spots.

Examining CO2 Model Observation Residuals Using ACT‐America Data

“…biases can be integrated over continental scales, making attribution of flux errors to specific regions and processes difficult. gradients mostly reflecting large-scale circulation (Lan et al, 2017). Therefore, comparatively small free tropospheric biases are in line with our expectations.…”

Regional‐Scale, Sector‐Specific Evaluation of Global CO2 Inversion Models Using Aircraft Data From the ACT‐America Project

“…This implies that the source of the error is either in the upstream boundary conditions, or in regional fluxes far enough upstream to have a Mid‐Atlantic LFT signal. North American uptake signals that progressively increase west‐to‐east have been noted previously (Chen et al., 2019; Lan et al., 2017; Sweeney et al., 2015).…”

“…The CarbonTracker global inversion system, version CT2015 (Peters et al., 2007, with updates documented at http://carbontracker.noaa.gov) was shown by Lan et al. (2017) to have horizontal gradients of column‐averaged CO 2 (XCO 2 ) that compare well with the available data. Stephens et al.…”

“…Tropospheric profiles of GHGs have been obtained for the last two decades at approximately biweekly intervals by the National Oceanographic and Atmospheric Administration (NOAA) light aircraft profiler network at over 22 sites in North America, and have provided independent assessment of the skill of global inversions throughout the troposphere (Sweeney et al, 2015). These data have been used to show evidence of increased CO 2 depletion in the boreal growing season from west-to-east across the North American continent, consistent with a surface biogenic sink and a prevailing eastward motion of air masses, though some of the depletion was attributed to Eurasian sinks upstream (Lan et al, 2017;Sweeney et al, 2015). The Car-bonTracker global inversion system, version CT2015 (Peters et al, 2007, with updates documented at http:// carbontracker.noaa.gov) was shown by Lan et al (2017) to have horizontal gradients of column-averaged CO 2 (XCO 2 ) that compare well with the available data.…”

Evaluating consistency between total column CO<sub>2</sub> retrievals from OCO-2 and the in-situ network over North America: Implications for carbon flux estimation

“…The pressure-weighted partial column (from the surface to 8 [km] a.s.l.) mean bias (µ sim−obs ) ranges from −0.12 in autumn to 0.18 [ppm] in the spring and is comparable to the typical measurement uncertainty within the in-situ Global Greenhouse Gasses Research Network of ∼ 0.15 [ppm] as derived from long term comparisons of differences between different within-network sampling and analysis approaches for CO 2 (e.g., Andrews et al, 2014;Lan et al, 2017;Sweeney et al, 2015). Low partial column bias relative to independent vertical profile CO 2 data show that errors in WRF-STLT transport contribute very minimally to bias in X sim CO2 .…”