Global CO emissions and drivers of atmospheric CO trends constrained by MOPITT satellite measurements

Carbon monoxide (CO), an important atmospheric pollutant produced by incomplete combustion and hydrocarbon oxidation, significantly affects atmospheric oxidation capacity and air quality. Accurate quantification of its global emissions and the underlying driver behind its atmospheric trends is essential for understanding changes in global atmospheric environment. Using 20 years (2003–2022) of data from the Measurement of Pollution in the Troposphere (MOPITT) instrument, we analyze changes in global CO emissions and atmospheric concentrations by applying a four-dimensional variational (4D-Var) assimilation framework within the GEOS-Chem adjoint model. A posteriori simulations show good agreement with independent surface and aircraft measurements compared to a priori simulations. Sensitivity analyses further confirm that inferred emissions remain robust against uncertainties associated with satellite vertical sensitivity and variations in hydroxyl radical (OH) concentrations. Our results indicate a substantial decline in global anthropogenic CO emissions of 14 %–17 % (approximately 85–110 Tg^yr-1) over the two-decade period, largely driven by emission reductions in the United States, Europe, and eastern China. Biomass burning emissions exhibited strong interannual variability, with recent increases in Northern Hemisphere high-latitude forests; in particular, the intense 2021 wildfires substantially offset the anthropogenic emission-driven decline in atmospheric CO over the Northern Hemisphere. This study provides a comprehensive assessment of global CO emissions and the mechanisms governing atmospheric CO trends, offering a scientific basis for integrated policies addressing both air pollution and climate change.