Quantification of CH$_{4}$ emissions from waste disposal sites near the city of Madrid using ground- and space-based observations of COCCON, TROPOMI and IASI

Abstract

The objective of this study is to derive methane (CH$_{4}$) emissions from three landfills, which are found to be the most significant CH$_{4}$ sources in the metropolitan area of Madrid in Spain. We derive CH$_{4}$ emissions from the CH$_{4}$ 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 CH$_{4}$ mixing ratios (XCH$_{4}$). Considering the prevalent wind regimes, we calculate the wind-assigned XCH$_{4}$ anomalies for two opposite wind directions. Pronounced bipolar plumes are found when applying the method to NO$_{2}$, 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 CH$_{4}$ emissions, the wind-assigned plume method is applied to the TROPOMI XCH$_{4}$ and to the lower tropospheric CH$_{4}$  dry-air column ratio (TXCH$_{4}$) of the combined TROPOMI+IASI product. As CH$_{4}$ emission strength we estimate 7.4 × 10$^{25}$ ± 6.4 × 10$^{24}$ molec. s$^{-1}$ from the TROPOMI XCH$_{4}$ data and 7.1 × 10$^{25}$ ± 1.0 × 10$^{25}$ molec. s$^{-1}$ from the TROPOMI+IASI merged TXCH$_{4}$ data. We use COCCON observations to estimate the local source strength as an independent method. COCCON observations indicate a weaker CH$_{4}$ 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