Using remote sensing methane data to track superemitting coal mines
Background
In this study, a research team from the SRON Netherlands Institute for Space Research aims to quantify methane emissions from coal mines in Queensland, Australia using methane column data from the TROPOMI satellite instrument. Remember, “methane columns” represent the concentration of methane for the entire atmosphere over the measurement point. TROPOMI also provides measurements for portions of the atmosphere, such as the tropospheric column concentrations.
The coal mines in this study are referred to as “super emitters” and contribute a significant portion of methane emissions across Australia. After calculating methane emission rates for these mines, the research team compared the results to reported emission rates at the national and international levels.
Here’s a link to the paper: https://doi.org/10.1021/acs.est.1c03976
Methods and Data
Let’s review the data used. The methane column data came from TROPOMI. They used an extensive quality assurance process for the TROPOMI data. Firstly, they set the pixel quality assurance value to 1 (the recommended value is 0.75 for most studies). Then, adjusted the data for striping (this is when you see “stripes” in the satellite images) and elevation as this could be a sign of calibration inaccuracies. They also removed orbits that had a high correlation with surface albedo or aerosol optical thickness to remove any potential bias in the data.
To derive emission rates from these satellite data, they applied a methodology called the “cross-sectional flux method” to generate emission data. Refer to the paper for an in-depth breakdown of this methodology. At a high level, this method uses transects (think of this as a straight line from the emission source) along the x-axis and y-axis of the plume to model methane column enhancements (i.e. concentrations above background values). There are additional steps that we will not review here in detail, but once the methane emission rates were calculated, the researchers had to remove methane column contributions from surrounding coal mines from the methane enhancement measurements.
These calculated emission rates were compared to two emission inventory datasets:
Australian national inventory
EDGARv4.3.25 global inventory
Results
The results were similar to other studies that compare satellite measurements with national emission inventory in the sense that they were significantly higher.
The TROPOMI estimates are seven times higher than the global inventory emission rates, and two times higher than the nationally reported values, making the case for remote sensing applications in greenhouse gas tracking.
The differences between TROPOMI methods and nationally reported methods are significant enough to spur revisions to the numbers reported to the United Nations Framework Convention on Climate Change (UNFCCC).
In this study, a single coal mine (called the Hail Creek mine) emitted 20% of national methane emissions, but only provided 1% of the country’s coal production.
Emissions from this mine were also compared to reconstructed bottom-up estimates from the national inventory and the results were 35 times higher than the reported values!
Studies like this demonstrate the value of remote sensing data when tracking global greenhouse gas emissions. Although many parts of this study were limited by TROPOMI’s coarse resolution, we can assume that these will be less problematic as remote sensing instruments continue to improve in both spatial and temporal resolution, studies like this should continue to happen for improved reporting and mitigation.
Reference:
Sadavarte, P., Pandey, S., Maasakkers, J. D., Lorente, A., Borsdorff, T., Denier van der Gon, H., Houweling, S., & Aben, I. (2021). Methane Emissions from Superemitting Coal Mines in Australia Quantified Using TROPOMI Satellite Observations. Environmental Science & Technology, 55(24), 16573–16580. https://doi.org/10.1021/acs.est.1c03976
Thanks for reading!