Satellite observations African cities: carbon monoxide emissions deviate from inventories

Carbon monoxide (CO) is released during incomplete combustion and plays an important role in air pollution. It can also serve as a tracer for CO₂, which is difficult to measure directly. Researchers from SRON and TNO are now using satellite data to map local CO emissions from African cities. These turn out to differ from commonly used emission inventories. It means that combustion efficiency estimates should be adjusted.

Independent measurements tracking where greenhouse gas emissions occur are essential to support compliance with the Paris Agreement. Satellites offer a unique opportunity to scan the entire globe with one instrument under the same conditions. One of the gases measured by the Dutch space instrument TROPOMI is carbon monoxide (CO), which is released during incomplete combustion of fossil fuels and in forest fires. In cities, it is mainly emitted by road traffic and residential combustion. CO contributes to smog formation and hinders the breakdown of the greenhouse gas methane in the atmosphere. In addition, CO can be used as a tracer for the main greenhouse gas, carbon dioxide (CO₂), of which combustion plumes are difficult to measure because CO₂ is already abundantly present in the atmosphere. Emission of CO₂ is accompanied by CO emission because there is always some degree of incomplete combustion.

Figure: (left) Urban CO emissions as measured by TROPOMI in 2019-2021. (right) CO plume from Cairo observed with TROPOMI on April 7 2019

Researchers from SRON and TNO have now measured urban CO emissions with TROPOMI and compared them with a widely used emission inventory. They focus on African cities because their CO emissions have not been studied in detail. The satellite data provide a more complete picture of the emissions from these cities, which also supports prioritizing air quality policy. The air quality of these cities is particularly important as they are home to many millions of people and will grow strongly in the coming years. North African cities turn out to emit more CO than previously estimated in emission inventories. Meanwhile, the researchers find lower emissions for cities in South Africa and Ivory Coast.

The higher emissions in North African cities indicate less efficient combustion than assumed. The opposite is true for countries with lower emissions. PhD candidate Gijs Leguijt explains: ‘With TROPOMI we obtain independent estimates of the CO emissions from large cities. It appears that the amount of fossil fuel combustion in the inventories is correct, but that the assumed efficiency often deviates from our results.’ The researchers are collaborating with the developers of the emission inventory to incorporate these new insights.

Publication: Gijs Leguijt, Joannes D. Maasakkers, Hugo A.C. Denier van der Gon, Arjo Segers, Tobias Borsdorff, and Ilse Aben, ‘Quantification of carbon monoxide emissions from African cities using TROPOMI’, Atmospheric Chemistry and Physics