Mid-Atlantic US observations of radiocarbon in CO₂: Fossil and biogenic source partitioning and model evaluation

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 Atmospheric Carbon and Transport-America (ACT-America) airborne mission between 2016 and 2019 aimed to improve the accuracy of regional greenhouse gas flux estimates, through refining our 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 USA. Observed variability in boundary layer CO2 at timescales ranging from intra-day to seasonal was largely driven by biogenic CO2 (CO2bio) variability that ranged from-19.7 ppm in summer to 16.2 ppm in fall, while fossil fuel CO2 (CO2ff) variability remained at 3.3±2.0 ppm. Carbonyl sulfide uptake was well-correlated with CO2bio uptake, and examining this relationship, as well as that between CO2 and CO2bio 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 CO2ff and evaluate the magnitude and diurnal variability of modeled CO2ff, deducing likely transport errors in an example flight. Although ACT-America CO2ff signals were attenuated due to the broad source regions sampled, results illustrate the value of Δ14CO2 sampling and observation-based methodologies for regional CO2 flux attribution, evaluation and improvement of modeled CO2.