the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 31 Aug 2026
Understanding the sources and variability of methane emissions in Madrid with Sentinel-5P TROPOMI, GHGSat, and aircraft observations
Abstract. Routine, passive monitoring using satellite sensors offers an effective means of understanding the location, frequency and intensity of industrial methane emissions. Satellites have been used largely to identify fugitive point source emissions in sectors such as oil and gas, but their efficacy for more diffuse sources such as landfills is less established, and formal integration into official reporting and compliance frameworks has yet to be achieved. A persistent methane hotspot originating from Madrid was identified using TROPOMI, where it remains one of the largest signals in Europe. Two landfills, Las Dehesas and Pinto, were identified as significant contributors based on GHGSat observations.
This study presents a comprehensive analysis of the TROPOMI and GHGSat observational record up to 2024 for Madrid, alongside aircraft observations, all contextualised against previous short-term studies, facility-scale reporting, and gridded inventories. Plume-based city-scale emissions averaged 13 ± 9 t/hr from TROPOMI compared to ~8 t/hr from the gridded inventories. Facility-scale emissions ranged from 2.5 ± 1.3 t/hr (Las Dehesas) and 3.8 ± 2.0 t/hr (Pinto) from GHGSat, and 3.0 ± 1.8 to 4.6 ± 2.7 t/hr respectively from aircraft, compared to reported averages of 0.4 and 1.0 t/hr between 2018–2024. Measurement-derived rates exceeded reported estimates by a factor of 0.5–6, highlighting a severe gap between bottom-up inventory and top-down observational approaches. We discuss challenges associated with satellite interpretation and meteorology, connect emissions to on-site management practices, and make recommendations towards operational use of these datasets.
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Status: open (until 01 Nov 2026)
- RC1: 'Comment on egusphere-2026-3797', Anonymous Referee #1, 06 Oct 2026 reply
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- 1
This manuscript describes a research study using government databases, regional- and facility-scale satellite methane observations, and aircraft methane imaging to investigate methane emissions from landfills around Madrid. I have seen presentations on this project previously, and it is of considerable interest to the waste methane community, both because of the magnitude of the emissions observed and because of the multi-tiered measurement approach. In particular, the progression from regional satellite observations to higher-resolution aircraft measurements provides substantially more source-level information than many previous studies relying primarily on satellite observations.
Overall, I found the manuscript excellent to read and very detailed. The authors have clearly taken considerable care in refining the analysis and examining the data from multiple perspectives, including, for example, re-quantifying emissions estimates using alternative wind speeds. The paper is technically strong and generally very well presented. I recommend publication after only minor revisions, which is not a recommendation I make often.
Although this manuscript reports only the first portion of the broader investigation undertaken at these landfills, I think it ends at an appropriate point and presents a coherent story. My comments below are relatively limited.
First, I am not convinced that Figure 3 is necessary in the main manuscript. It could potentially be moved to the Supplement. The broader inventory information is useful context, but the manuscript does not subsequently discuss many of the other emitters represented in the figure. To me, the regional satellite measurements provide a more natural starting point for the main story, particularly since those observations appear to have motivated the subsequent investigation. The regional inventory values could still be discussed textually for context without requiring a dedicated figure.
In contrast, I found the facility-level government-reported estimates particularly striking. Here I am referring to the reported inventory values rather than the contemporary satellite measurements. The very large reported decrease in emissions at one facility between 2021 and 2022 is especially notable.
Related to this, I would appreciate additional explanation of the "electrochemical cell measurements." Do the authors mean measurements using a handheld electrochemical instrument? What specific field procedure was used - walking surveys, something analogous to EPA Method 21, or another approach? I do not currently understand how these measurements were performed or how they were translated into facility-scale emissions estimates. This methodological detail would be useful, particularly because the resulting estimates appear difficult to reconcile with any other form of measurement.
The magnitude of the reported decrease also raises questions about the inventory review process. I don’t understand how such a large change in reported emissions could occur without apparently triggering further scrutiny. The authors have handled the PRTR comparison appropriately and cautiously, but the discrepancy highlights an increasingly important issue: large emission sources can now be independently observed using satellite and airborne systems, and reported inventories will increasingly be evaluated against those measurements. Governments and operators need to pay attention.
Overall, the figures are very strong and contain a great deal of useful information. One possible exception is Figure 7. I am not sure that combining the regional- and facility-scale estimates on the same plot is the clearest presentation, since some of the smaller values become difficult to distinguish. It may be worth considering a two-panel or stacked presentation, with regional estimates shown separately from facility-scale estimates. At present, there is perhaps a little too much information competing within a single panel.
Figure 8 is particularly interesting. I appreciate the use of change detection and think this type of analysis could be applied more broadly in satellite studies of emission trends.
I was also intrigued by the finding that emissions derived on "flip" days were lower on average than those derived on non-flip days. Could this be related to the meteorological conditions associated with the wind-direction transition? For example, changing pressure fields or periods of stagnation? Alternatively, could the effect arise partly from the IME methodology itself, if a wind shift shortens or compacts the observable plume and therefore affects the inferred plume length or integrated mass enhancement? I am not sure I fully understand the mechanism, and perhaps the authors do not yet either, but the difference appears large enough that it deserves some additional discussion.
Finally, the Conclusions section is relatively long, while there is no formal Discussion section. This may simply reflect the journal's preferred format, in which case I have no objection. However, some interpretive material appears quite late in the manuscript, after the conclusions have effectively begun. It may improve the flow to move some of this discussion earlier, leaving the Conclusions section more focused on the principal findings and implications.
Overall, these are relatively minor comments. I found the manuscript technically strong, carefully developed, and highly relevant to current work on landfill methane emissions. I recommend minor revision.