Investigating high methane emissions from urban areas detected by TROPOMI and their association with untreated wastewater

Abstract

Even though methane concentrations have contributed an estimated 23% of climate forcing, part of the recent increases in the global methane background concentrations remain unexplained. Satellite remote sensing has been used extensively to constrain emission inventories, for example with the TROPOspheric Monitoring Instrument (TROPOMI) which has been measuring methane since November 2017. We have identified enhancements of methane over 61 urban areas around the world and estimate their emissions using a two-dimensional Gaussian model. We show that methane emissions from urban areas may be underestimated by a factor of 3 to 4 in the EDGAR greenhouse gas emission inventory. Scaling our results to the 385 urban areas with more than 2 million inhabitants suggests that they could account for up to 22% of global methane emissions. The emission estimates of the 61 urban areas do not correlate with the total or sectoral EDGAR emission inventory. They do however correlate with estimated rates of untreated wastewater, varying from 33 kg person(-1) year(-1) for cities with zero untreated wastewater to 138 kg person(-1) year(-1) for the cities with the most untreated wastewater. If this relationship were confirmed by higher resolution remote sensing or in situ monitoring, we estimate that reducing discharges of untreated wastewater could reduce global methane emissions by up to 5 to 10% while at the same time yielding significant ecological and human co-benefits.

Satellite-Derived Estimate City-Level Methane Emission Flux from Calgary, Alberta, Canada

“…Comparisons with widely used global inventories of CH4 emissions suggest that inventories are under-estimating CH4 emissions from the Calgary urban area. The mismatch between the BU emissions inventory and the measurement-based estimate from this study is consistent with the conclusions derived from investigations conducted in other urban areas in North America [10,23,25,42]. There is evidence that urban emissions in general (and Calgary specifically) are higher than initially predicted, but the exact source of the discrepancy cannot be reliably determined from these data.…”

“…Using the mass balanced method, we calculated the CH4 emissions rate in Calgary as 215.4±132.8 t/d. Compared to recent aerial-and TROPOMI-based CH4 emission estimates from other North American cities in the literature [10,23,25], Calgary's annual emissions are on relatively low (Figure 4), but comparable with cities that have slightly lower and higher emissions and populations, respectively(e.g., Milwaukee, Charlotte, Kansas City, Washington DC, Baltimore).…”

“…Urban areas, in particular, are a very poorly understood source of CH4 emissions due to the large diversity of sources. In urban areas wastewater treatment facilities and landfills are generally well known to be major emitters [8][9][10][11]. However, urban areas also contain many smaller, widely distributed sources such as leaks in natural gas (NG) distribution infrastructure, small scale agriculture, sewer emissions, natural gas vehicles, and slip from natural gas combustion [8,[12][13][14][15][16].…”

“…The high observational density and large-scale geographic coverage of the TROPOspheric Monitoring Instrument (TROPOMI) on board the Copernicus Sentinel-5 Precursor satellite provides one of the best options for satellite TD measurements. Multiple approaches have been proposed and used to derive urban CH4 emissions based on TROPOMI observations, including transport inversion, two-dimensional Gaussian modeling, and tracer-tracer approaches [10,[23][24][25]. Based on the methane measurements from a multitiered observing framework (including observations from TROPOMI, aircraft-based system, tow-based monitors, and ground-based stations), Cusworth et al used geostatistical inverse modeling and the prior emissions to derive methane emissions from megacity areas [24].…”

“…Based on the methane measurements from a multitiered observing framework (including observations from TROPOMI, aircraft-based system, tow-based monitors, and ground-based stations), Cusworth et al used geostatistical inverse modeling and the prior emissions to derive methane emissions from megacity areas [24]. De Foy et al used a two-dimensional Gaussian model with TROPOMI observations and successfully derived methane emissions from 61 urban areas globally [10]. The tracer-tracer method, which quantifies emissions by scaling the ratios of prior estimates of CH4:CO or CH4:CO2 with atmospheric measurements has been used to derive CH4 emissions from urban centers in North America [23,25].…”