Abstract
The exchange of carbon between the Earth’s atmosphere and biosphere influences the atmospheric abundances of carbon dioxide (CO2) and methane (CH4). Airborne eddy covariance (EC) can quantify surface-atmosphere exchange from landscape-to-regional scales, offering a unique perspective on carbon cycle dynamics. We use extensive airborne measurements to quantify fluxes of sensible heat, latent heat, CO2, and CH4 across multiple ecosystems in the Mid-Atlantic region during September 2016 and May 2017. In conjunction with footprint analysis and land cover information, we use the airborne dataset to explore the effects of landscape heterogeneity on measured fluxes. Our results demonstrate large variability in CO2 uptake over mixed agricultural and forested sites, with fluxes ranging from −3.4 ± 0.7 to −11.5 ± 1.6 μmol m−2 s−1 for croplands and −9.1 ± 1.5 to −22.7 ± 3.2 μmol m−2 s−1 for forests. We also report substantial CH4 emissions of 32.3 ± 17.0 to 76.1 ± 29.4 nmol m−2 s−1 from a brackish herbaceous wetland and 58.4 ± 12.0 to 181.2 ± 36.8 nmol m−2 s−1 from a freshwater forested wetland. Comparison of ecosystem-specific aircraft observations with measurements from EC flux towers along the flight path demonstrate that towers capture ∼30%–75% of the regional variability in ecosystem fluxes. Diel patterns measured at the tower sites suggest that peak, midday flux measurements from aircraft accurately predict net daily CO2 exchange. We discuss next steps in applying airborne observations to evaluate bottom-up flux models and improve understanding of the biophysical processes that drive carbon exchange from landscape-to-regional scales.
Highlights
The terrestrial biosphere plays a dynamic role in the global carbon cycle, removing an estimated 25%–30% of the carbon dioxide (CO2) emitted from human activity (Ciais et al 2013, Le Quéré et al 2018).the prognosis for this sink remains poorly constrained due to uncertain climate feedbacks on the atmosphere-biosphere cycling of CO2 (Cox et al 2013, Wenzel et al 2016, Bond-Lamberty et al 2018)
Our results demonstrate large variability in CO2 uptake over mixed agricultural and forested sites, with fluxes ranging from −3.4 ± 0.7 to −11.5 ± 1.6 μmol m−2 s−1 for croplands and −9.1 ± 1.5 to −22.7 ± 3.2 μmol m−2 s−1 for forests
We demonstrate that airborne fluxes, when combined with footprint and land cover information, resolve spatial heterogeneity in landscape flux
Summary
The terrestrial biosphere plays a dynamic role in the global carbon cycle, removing an estimated 25%–30% of the carbon dioxide (CO2) emitted from human activity (Ciais et al 2013, Le Quéré et al 2018).the prognosis for this sink remains poorly constrained due to uncertain climate feedbacks on the atmosphere-biosphere cycling of CO2 (Cox et al 2013, Wenzel et al 2016, Bond-Lamberty et al 2018). Top-down methods use a combination of observed atmospheric mixing ratios, transport models, and prior emissions estimates to infer fluxes of CO2 (Houweling et al 2015, Wang et al 2018) and CH4 (Bousquet et al 2011) on regional to global scales. These atmospheric inversion models provide a useful constraint on flux but offer limited attribution information on the underlying biophysical factors driving the carbon cycle. Inventory-based estimates have large associated uncertainties of up to 75% (Hayes et al 2018), and discrepancies persist between different modeling approaches (Huntzinger et al 2012, Melton et al 2013) and model-tower data comparisons (Schwalm et al 2010, Schaefer et al 2012). Tower-based flux observations can provide benchmark information and a basis for validation, but their spatial representativeness is very limited at regional to continental scales (Villarreal et al 2018)
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