Emissions from Mining-related Activities in Africa using TROPOMI Satellite Observations

We have analyzed TROPOMI NO₂ data over the Copperbelt, a mining region which straddles the Democratic Republic of Congo and Zambia. While the ore mined there is primarily copper, this region is currently of great strategic interest because it is the world’s biggest producer of cobalt. Demand for cobalt, key to clean energy technologies (e.g., electric car batteries), is increasing worldwide and cobalt control is becoming a matter of national and global energy security. The impact of increasing mining-related activities on local air quality (high NO_x is harmful to respiratory systems and crops) is unknown.

TROPOMI, onboard ESA’s Sentinel-5 Precursor, is an imaging spectrometer in a sun-synchronous orbit at 824 km of altitude which measures concentrations of relevant atmospheric species (trace gases, aerosols, cloud) with quasi-global daily coverage and at high spatial resolution (~ 3.5 x 5.5 km² in the case of NO₂).

We show that mining-related activities (such as extraction, smelting, and refining) can be remotely detected based on their TROPOMI NO₂ signature, even in the presence of high background NO₂ from biomass burning. Annual TROPOMI NO₂ means for 2019, 2020, and 2021 show local enrichments consistent with point sources spatially collocated with both mines and large cities where mining-related activities take place. We have identified temporal trends in NO₂ from these point sources and, when possible, we have compared those to production figures from the mining companies involved. We have quantified top-down annual NO_x (NO+NO₂) emissions for each of the point sources identified by applying the divergence method to the TROPOMI retrievals, using ancillary ERA5 meteorological data. Because in situ NO_x measurements are not available, we contrast our emission results with emissions from the CAMS-GLOB-ANT v5.1 inventory.

Our results show that NO_x emissions from mining-related activities can be quantified remotely, which is important in the absence of local air quality monitoring. They also demonstrate that NO₂ trend analysis can be a good indicator of mine production. This is particularly relevant for non-publicly traded mining companies, which are not required to publish their production figures. Lack of TROPOMI SO₂ enhancements colocated with our NO₂ point sources is consistent with SO₂ capture and transformation into H₂SO₄, which is then used in mining-related processes or commercialized.