Quantification of CH4 emissions from waste disposal sites near the city of Madrid using ground- and space-based observations of COCCON, TROPOMI and IASI

The objective of this study is to derive methane (CH₄) emissions from three landfills, which are found to be the most significant CH₄ sources in the metropolitan area of Madrid in Spain. We derive CH₄ emissions from the CH₄ enhancements observed by spaceborne and ground-based instruments. We apply satellite-based measurements from the TROPOspheric Monitoring Instrument (TROPOMI) and the Infrared Atmospheric Sounding Interferometer (IASI) together with measurements from the ground-based COllaborative Carbon Column Observing Network (COCCON) instruments.

In 2018, a 2-week field campaign for measuring the atmospheric concentrations of greenhouse gases was performed in Madrid in the framework of Monitoring of the Greenhouse Gases Concentrations in Madrid (MEGEI-MAD) project. Five COCCON instruments were deployed at different locations around the Madrid city center, enabling the observation of total column-averaged CH₄ mixing ratios (XCH₄). Considering the prevalent wind regimes, we calculate the wind-assigned XCH₄ anomalies for two opposite wind directions. Pronounced bipolar plumes are found when applying the method to NO₂, which implies that our method of wind-assigned anomaly is suitable to estimate enhancements of trace gases at the urban level from satellite-based measurements. For quantifying the CH₄ emissions, the wind-assigned plume method is applied to the TROPOMI XCH₄ and to the lower tropospheric CH₄ / dry-air column ratio (TXCH₄) of the combined TROPOMI+IASI product.

As CH₄ emission strength we estimate 7.4 × 10²⁵ ± 6.4 × 10²⁴ molec. s⁻¹ from the TROPOMI XCH₄ data and 7.1 × 10²⁵ ± 1.0 × 10²⁵ molec. s⁻¹ from the TROPOMI+IASI merged TXCH₄ data. We use COCCON observations to estimate the local source strength as an independent method. COCCON observations indicate a weaker CH₄ emission strength of 3.7 × 10²⁵ molec. s⁻¹ from a local source (the Valdemingómez waste plant) based on observations from a single day. This strength is lower than the one derived from the satellite observations, and it is a plausible result. This is because the analysis of the satellite data refers to a larger area, covering further emission sources in the study region, whereas the signal observed by COCCON is generated by a nearby local source. All emission rates estimated from the different observations are significantly larger than the emission rates provided via the official Spanish Register of Emissions and Pollutant Sources.