Improved Constraints on Northern Extratropical CO₂ Fluxes Obtained by Combining Surface‐Based and Space‐Based Atmospheric CO₂ Measurements

Abstract

Top‐down estimates of CO2 fluxes are typically constrained by either surface‐based or space‐based CO2 observations. Both of these measurement types have spatial and temporal gaps in observational coverage that can lead to differences in inferred fluxes. Assimilating both surface‐based and space‐based measurements concurrently in a flux inversion framework improves observational coverage and reduces sampling related artifacts. This study examines the consistency of flux constraints provided by these different observations and the potential to combine them by performing a series of 6‐year (2010–2015) CO2 flux inversions. Flux inversions are performed assimilating surface‐based measurements from the in situ and flask network, measurements from the Total Carbon Column Observing Network (TCCON), and space‐based measurements from the Greenhouse Gases Observing Satellite (GOSAT), or all three data sets combined. Combining the data sets results in more precise flux estimates for subcontinental regions relative to any of the data sets alone. Combining the data sets also improves the accuracy of the posterior fluxes, based on reduced root‐mean‐square differences between posterior flux‐simulated CO2 and aircraft‐based CO2 over midlatitude regions (0.33–0.56 ppm) in comparison to GOSAT (0.37–0.61 ppm), TCCON (0.50–0.68 ppm), or in situ and flask measurements (0.46–0.56 ppm) alone. These results suggest that surface‐based and GOSAT measurements give complementary constraints on CO2 fluxes in the northern extratropics and can be combined in flux inversions to improve constraints on regional fluxes. This stands in contrast with many earlier attempts to combine these data sets and suggests that improvements in the NASA Atmospheric CO2 Observations from Space (ACOS) retrieval algorithm have significantly improved the consistency of space‐based and surface‐based flux constraints.

Contrasting Regional Carbon Cycle Responses to Seasonal Climate Anomalies Across the East‐West Divide of Temperate North America

“…The FluxSat GPP product (Joiner et al, 2018) is based on an MODIS remote sensing calibrated against global eddy covariance flux measurements and has been found to produce more realistic IAV in GPP when compared to FLUXNET sites relative to other upscaled GPP products (Joiner et al, 2018). The flux inversion NEE product used here is reported in Byrne et al (2020). This product is derived from a global CO 2 flux inversions and is unique in that it assimilates both surfaceand space-based CO 2 measurements, providing increased observational constraints relative to single data set NEE flux inversion products.…”

“…To examine IAV in NEE, we employ the combined "GOSAT + surface + TCCON" product of Byrne et al (2020). This product is unique in that it assimilates both surface-and space-based CO 2 measurements, providing increased observational constraints relative to other top-down NEE flux inversion products.…”

“…The NEE fluxes of Byrne et al (2020) are produced from a flux inversion analyses spanning 2010-2015. The flux inversions assimilate CO 2 measurements from the Greenhouse Gases Observing Satellite (GOSAT), Total Carbon Column Observing Network (TCCON), and the surface in situ and flask measurements network concurrently.…”

“…Posterior NEE fluxes are aggregated to monthly mean values for this analysis. A detailed description of the experimental setup and evaluation of the fluxes can be found in Byrne et al (2020). We also contrast the posterior IAV of the "GOSAT + surface + TCCON" ensemble of inversions with the flux inversions assimilating only surface-based flask and in situ meansurements, referred to as "surface only".…”

“…This type of inversion is temporally limited due the fact that GOSAT was launched in 2009. Byrne et al (2020) argue that this combined flux inversion (referred to as "GOSAT + surface + TCCON") provides improved CO 2 flux estimates relative to flux inversions that only assimilate flask and in situ measurements (referred to as "surface only"). Therefore, we may expect that flask and in situ CO 2 flux inversions may not separate IAV between eastern and western temperate North America as distinctly.…”