A New NASA Capability to Quantify Regional CO2 and CH4 Surface Exchange and Improve Flux Model Performance

Piers Sellers was a firm believer in the value of eddy covariance flux measurements to elucidate basic vegetation physiological processes and provide ground-truth for model parameterizations. He spent many hours in the cockpit gathering airborne flux data. Direct flux observations from aircraft provide a unique tool for probing greenhouse gas (GHG) sources and sinks on a regional scale. Airborne eddy covariance, which relies on high-frequency, simultaneous measurements of fluctuations in concentration and vertical wind speed, is a robust method for quantifying surface-atmosphere exchange. Thanks in large part to the enthusiastic support of Piers, we have assembled and flown an instrument payload onboard the NASA C-23 Sherpa aircraft capable of measuring CO₂, CH₄, H₂O, and heat fluxes. Flights for the Carbon Airborne Flux Experiment (CARAFE) took place during September 2016 and May 2017 based out of Wallops Flight Facility, VA. Flight tracks covered a variety of ecosystems and land-use types in the Mid-Atlantic, including forests, croplands, and wetlands. Carbon fluxes are derived using eddy covariance and wavelet analysis. Our results show a strong drawdown of CO₂ and near-zero CH₄ emissions from crops and dry-land forest, but seasonally strong CH₄ flux from wetland forest. CARAFE flux data are also compared with observations from several flux towers along the flight path to complement the airborne measurements. We will further assess the effects of land surface type and seasonal variability in carbon exchange. Regional-scale flux observations from CARAFE supply a valuable constraint for improving top-down and bottom up estimates of carbon sources and sinks, and provide another lasting example of Piers’ many contributions to climate and carbon cycle science.