Abstract
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.
Highlights
Carbon monoxide (CO) is an air pollutant that leads to the formation of tropospheric ozone (O3) and carbon dioxide (CO2)
carbon monoxide (CO) concentrations observed by both MOPITTv6 satellite XCO retrievals and surface in situ measurements show significant negative trends over most of the world from 2002 to 2011
The CO concentration trends in the forward CTM simulations prescribed with CO emission inventories show considerable inconsistency with the observed Measurements of Pollution in the Troposphere (MOPITT) XCO from 2002 to 2011
Summary
Carbon monoxide (CO) is an air pollutant that leads to the formation of tropospheric ozone (O3) and carbon dioxide (CO2). Atmospheric chemistry transport models (ACTMs) prescribed with emission inventories are commonly used to analyse the role of emissions in the atmospheric concentration Most of these simulations tend to underestimate CO concentrations in the middle to high latitudes of the Northern Hemisphere (NH), whereas they overestimate them over emission hotspots (Shindell et al, 2006; Duncan et al, 2007; Naik et al, 2013; Stein et al, 2014; Yoon and Pozzer, 2014). This bias reveals an incorrect balance between CO sources, at the surface and in the atmosphere, and CO sinks (Naik et al, 2013).
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