Unexpected Urban Methane Hotspots Captured from Aircraft Observations

Cities are key contributors to climate change as they are major sources of greenhouse gases and air pollutants. We use aircraft measurements of carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO) made during 2 observation days to monitor the urban atmosphere of Seoul, a megacity with large anthropogenic emissions. We estimate that CO2, CH4, and CO concentrations of anthropogenic emissions extracted from background concentrations (i.e., urban enhancements; ΔCO2, ΔCH4, and ΔCO) are higher on average by 50.2, 68.6, and 26.9%, respectively, in the areas within Seoul (17.1 ppm, 85.8 ppb, and 295.3 ppb, respectively) compared to areas in the outer parts of the boundaries of the city (11.4 ppm, 50.9 ppb, and 232.6 ppb, respectively). We also compare the emission ratios of ΔCO:ΔCO2, ΔCH4:ΔCO2, and ΔCH4:ΔCO to detect hotspots and characterize emission sources using the urban enhancements. We find that the urban emission ratios of ΔCH4:ΔCO2 show the highest slopes in the West and East sectors of Seoul (10.33 ± 4.55 and 14.50 ± 1.91, respectively). Moreover, in addition to discovering urban CH4 hotspots around Seoul, we unveil unexpected CH4 emissions and leakage during the supply of LNG from a large commercial multicomplex building. Our study suggests the effectiveness of aircraft measurements and ratio analyses to understand anthropogenic emissions of urban areas which support and improve the efforts of urban greenhouse gas and air pollutant monitoring.