Current potential of CH4 emission estimates using TROPOMI in the Middle East

Methane (CH₄) is the second most important greenhouse gas after CO₂. The TROPOspheric Monitoring Instrument (TROPOMI) on board the Sentinel 5 Precursor (S5-P) satellite measures CH₄ at a high horizontal resolution of 7 × 7 km², showing the capability on identifying and quantifying the sources at a local to regional scale. The Middle East is one of the strong CH4-emitting regions in the world. However, it is difficult to estimate the emissions here because of the uncertainties caused by bright surfaces and high aerosol loadings. Furthermore, several sources are located near the coast or in places with complex topography, where satellite observations are often of reduced quality. We use the product from the University of Bremen, WMF-DOAS XCH₄ v1.8 product, which has good spatial coverage over the ocean and mountains, to estimate the emissions in the Middle East. The Aerosol Optical Depth (AOD) data from the MODIS/Aqua satellite instrument, which has a similar overpass time as TROPOMI, was adopted to filter potential unreliable XCH₄ in the product.

For the inversion, we use the divergence method of Liu et al., (2021), which has been proven to be a fast and efficient way to estimate CH₄ emissions from satellite observations. We have improved our method by comparing the fluxes in different directions for better background corrections over areas with complicated topographies. The temporal filter was established to further filter false emissions caused by surface albedos. We derived CH₄ emissions on a grid of 0.2° from 2018 to 2021 and compared them to the latest bottom-up inventory EDGAR v7.0 in the same years. We found significantly lower emissions than EDGAR for the locations that are mainly determined by observed gas-flaring from satellites. Apart from sources of oil/gas production, the emissions from livestock in Saudi Arabia's irrigation zones, which have been reported neither in EDGAR nor other previous studies, are identified and quantified by using our divergence method. Another unexpected finding is that emissions from landfills are fairly stable and strong in some cities like Tehran.