Evaluation of Regional CO 2 Mole Fractions in the ECMWF CAMS Real‐Time Atmospheric Analysis and NOAA CarbonTracker Near‐Real‐Time Reanalysis With Airborne Observations From ACT‐America Field Campaigns

This study systematically examines the regional uncertainties and biases in carbon dioxide (CO 2 ) mole fractions from two of the state‐of‐the‐art global CO 2 analysis products, namely, the Copernicus Atmosphere Monitoring Service (CAMS) real‐time atmospheric analysis from the European Centre for Medium‐Range Weather Forecasts (ECMWF) and the CarbonTracker Near‐Real‐Time (CT‐NRT) reanalysis from the National Oceanic and Atmospheric Administration (NOAA), by evaluation against hundreds of hours of airborne in situ measurements from the summer 2016 and winter 2017 Atmospheric Carbon and Transport (ACT)‐America field campaigns. Both the CAMS and CT‐NRT analyses agree reasonably well with the independent ACT‐America airborne CO 2 measurements in the free troposphere, with root‐mean‐square deviations (RMSDs) between analyses and observations generally between 1 and 2 ppm but show considerably larger uncertainties in the atmospheric boundary layer where the RMSDs exceed 8 ppm in the lowermost 1 km of the troposphere in summer. There are strong variations in accuracy and bias between seasons, and across three different subregions in the United States (Mid‐Atlantic, Midwest, and South), with the largest uncertainties in the Mid‐Atlantic region in summer. Overall, the RMSDs of the CAMS and CT‐NRT analyses against airborne data are comparable to each other and largely consistent with the differences between the two analyses. The current study provides uncertainty estimates for both analysis products over North America and suggests that these two independent estimates can be used to approximate regional CO 2 analysis uncertainties. Both statistics are important in future studies in quantifying the uncertainties in regional CO 2 mole fraction and flux estimates. Two global CO 2 analysis products are compared with airborne in situ data collected during the first two ACT‐America field campaigns Both analyses agree reasonably well with observations but show considerable biases in CO 2 in the Mid‐Atlantic region during summer 2016 The two independent analysis products can be used to quantify the overall analysis uncertainties in estimated CO 2 mole fractions