On the relationship between tropospheric CO and CO&lt;sub&gt;2&lt;/sub&gt; during KORUS-AQ
and its role in constraining anthropogenic CO&lt;sub&gt;2&lt;/sub&gt;
Authors: Tang Wenfu , Gaubert Benjamin , Emmons Louisa , Choi Yonghoon , DiGangi Joshua P. , Diskin Glenn S. , Xu Xiaomei , He Cenlin , Worden Helen , Tilmes Simone , Buchholz Rebecca , Halliday Hannah S. , Arellano Avelino F. 
Published: August 2020

Abstract. While the complementarity of CO data in monitoring CO2 from fossil-fuel combustion (ffCO2) is widely known, a rigorous demonstration of its use in reducing uncertainties on top-down regional ffCO2 emissions is still warranted. Here, we report a case study investigating the regional covariation of observed and modeled abundances of CO, CO2, and ffCO2 and demonstrating its implication to joint CO:CO2 inversions. We use data from a recent aircraft field campaign (KORUS-AQ) conducted over Korea and neighboring regions on May 2016 for this case study. We use the Community Atmosphere Model with Chemistry (CAM-Chem) to simulate CO, CO2, ffCO2 and associated source tags, using a posteriori fluxes from global CO2 flux inversions and CO emissions independently calibrated against CO data. Among other model-data comparisons, CAM-Chem simulations show an underestimation in CO2 (1 ppm), CO (24 ppb) and ffCO2 (1 ppm) against aircraft measurements. These are all within the range of model and data uncertainties. Although the overall observed enhancement ratio, ΔCO⁄ΔCO2 (~ 13.3 &amp;om; 0.21 ppb/ppm), is well captured by CAM-Chem (~ 13.8 &amp;pm; 0.23 ppb/ppm), we find an overestimation (29 ppb/ppm) for air samples between 2 to 3 km, where East Asian influence is substantial (35 %). The contribution of ffCO2 from Korea and Japan is smaller (30 %) and localized below 3 km, suggesting that regional ffCO2 and background and non-ffCO2 cannot be neglected in interpreting observed enhancements in this region. These spatial variations translate in the joint CO:CO2 inversion to increases in a posteriori ffCO2 estimates from East Asia (27 % &amp;pm; 24 %) and Korea and Japan (9 % &amp;pm; 17 %). This is consistent (albeit larger in 1-sigma uncertainty) with our estimate using 14CO2 data (27 % &amp;pm; 9 % and 10 % &amp;pm; 3 %, respectively). In contrast, the inversion using only CO2 data shows a decrease by ~ 5 % &amp;pm; 27 % in East Asia and ~ 6 % &amp;pm; 19 % in Korea and Japan. Our results show that inversions using both CO2 and CO can be an effective approach in constraining ffCO2 when the regional variations of CO and CO2 relationships are appropriately accounted for. Although this further points to the potential of augmenting current observing system of CO2 with CO for global inverse analyses of ffCO2 from different regions of the globe, we highlight the need to verify the spatiotemporal distribution of the covariation of CO with CO2 in both regional and global models. We caution its use for constraining local ffCO2, unless the spatiotemporal a priori flux distribution and surface processes are reasonably represented, as they may confound the analysis. These have important implications on inversion studies using columnar data from satellite observations, especially for regions lacking necessary verification measurements.