Greenhouse gas emission from vehicles in Seoul megacity, South Korea: Molar ratios (N2O:CO2, CH4:CO2) and stable isotopic compositions of N2O (d15N, d18O)

Fossil fuel combustion is one of the largest contributors to anthropogenic greenhouse gas (GHG) emissions, especially in megacities around the world. To characterize the vehicle emissions in Seoul, the megacity of South Korea, we collected air samples from the entry and exit points of three tunnels (Sang-Do, Bong-Cheon, and Gwan-Ak Tunnel) and Seoul National University Campus and measured dry molar mixing ratios of major greenhouse gases spices (CO₂, CH₄, and N₂O). The N₂O:CO₂ emission molar ratio from vehicles is 4.3 ± 0.3 × 10^-5, within a range of 1.8 – 18.7 × 10^-5 previously reported in Germany, Switzerland, Sweden, and the USA. The CH₄:CO₂ emission molar ratio from the Sang-Do tunnel is 50.6 ± 18.0 × 10^-5, which is significantly greater than those observed in Switzerland, the USA, and China (3.5 - 15 ± 4 ×10^–5). In the Bong-Cheon and Gwan-Ak tunnels, however, there was little difference in entry and exit, or rather, the exit was smaller, and it might be related to the ventilation system and vehicle types. We also compared our estimation of the GHG emissions from vehicles with the National Greenhouse Gas Inventory Report of Korea (GIR, 2021) which is based on a bottom-up emission calculation method. With the CO₂ emissions (8108.33 Gg CO₂eq) from the GIR, the N₂O and CH₄ emissions in Seoul are estimated to be (108.08±24.60) Gg CO₂eq and (62.75±18.92) Gg CO₂ eq, respectively. The differences between our observations and inventory imply that the estimation of the non-CO₂ gas (CH₄, N₂O) emission factors should be improved. To characterize the N₂O from vehicles, we analyzed N₂O stable isotopic compositions by an IRMS method. The δ¹⁵N and δ¹⁸O values of N₂O emitted from the vehicles are estimated as -7.1 ± 1.5 ‰ and 41.2 ± 0.2 ‰, respectively. The δ¹⁵N values support the idea that the N₂O is produced through catalytic convertors in vehicles which are attached to reduce the NOx emission at the tailpipe. The newly measured data from Seoul may help us better understand greenhouse gas emissions from vehicles in megacities.