Decadal trends in global CO emissions as seen by MOPITT

Abstract. Negative trends of carbon monoxide (CO) concentrations are observed in the recent decade by both surface measurements and satellite retrievals over many regions of the globe, but they are not well explained by current emission inventories. Here, we analyse the observed CO concentration decline with an atmospheric inversion that simultaneously optimizes the two main CO sources (surface emissions and atmospheric hydrocarbon oxidations) and the main CO sink (atmospheric hydroxyl radical OH oxidation). Satellite CO column retrievals from Measurements of Pollution in the Troposphere (MOPITT), version 6, and surface observations of methane and methyl chloroform mole fractions are assimilated jointly for the period covering 2002–2011. Compared to the model simulation prescribed with prior emission inventories, trends in the optimized CO concentrations show better agreement with that of independent surface in situ measurements. At the global scale, the atmospheric inversion primarily interprets the CO concentration decline as a decrease in the CO emissions (−2.3 % yr⁻¹), more than twice the negative trend estimated by the prior emission inventories (−1.0 % yr⁻¹). The spatial distribution of the inferred decrease in CO emissions indicates contributions from western Europe (−4.0 % yr⁻¹), the United States (−4.6 % yr⁻¹) and East Asia (−1.2 % yr⁻¹), where anthropogenic fuel combustion generally dominates the overall CO emissions, and also from Australia (−5.3 % yr⁻¹), the Indo-China Peninsula (−5.6 % yr⁻¹), Indonesia (−6.7 % y⁻¹), and South America (−3 % yr⁻¹), where CO emissions are mostly due to biomass burning. In contradiction with the bottom-up inventories that report an increase of 2 % yr⁻¹ over China during the study period, a significant emission decrease of 1.1 % yr⁻¹ is inferred by the inversion. A large decrease in CO emission factors due to technology improvements would outweigh the increase in carbon fuel combustions and may explain this decrease. Independent satellite formaldehyde (CH₂O) column retrievals confirm the absence of large-scale trends in the atmospheric source of CO. However, it should be noted that the CH₂O retrievals are not assimilated and OH concentrations are optimized at a very large scale in this study.