Mid-Atlantic U.S. observations of radiocarbon in CO₂: fossil and biogenic source partitioning and model evaluation

Abstract. Accurately quantifying regional anthropogenic CO₂ fluxes is fundamental to improving our understanding of the carbon cycle and for creating effective carbon mitigation policies, and the radiocarbon to total carbon ratio in atmospheric CO₂ (Δ¹⁴CO₂) is a robust tracer of fossil fuel CO₂ that can discriminate between biogenic and fossil fuel CO₂ sources. NASA’s ACT-America airborne mission between 2016 and 2019 aimed to improve the accuracy of regional greenhouse gas flux estimates, through refining understanding and characterization of fluxes and flux uncertainties in models. Δ¹⁴CO₂ observations from 26 flights are presented for examining seasonal CO₂ source partitioning in the Mid-Atlantic U.S. Observed variability in boundary layer CO₂ at time scales ranging from intra-day to seasonal was largely driven by biogenic CO₂ (CO_2bio) variability that ranged from -19.7 ppm in summer to 16.2 ppm in fall, while fossil fuel CO₂ (CO_2ff) variability remained at 3.3 ± 2.0 ppm. Carbonyl sulfide uptake was well-correlated with CO_2bio uptake, and examining this relationship, and that between CO₂ and CO_2bio variability reinforces the seasonal extent of gross primary productivity response throughout ACT-America. We use airborne Δ¹⁴CO₂ flask sampling alongside in situ carbon monoxide measurements to calculate high-frequency CO_2ff and evaluate the magnitude and diurnal variability of modeled CO_2ff, deducing likely transport errors in an example flight. Although ACT-America CO_2ff signals were attenuated due to broad source regions sampled, results illustrate the value of D¹⁴CO₂ sampling and observation-based methodologies for regional CO₂ flux attribution and evaluation and improvement of modeled CO₂.