CO ₂ and N ₂ O Emissions From Vehicles in Seoul Megacity, South Korea: Insights From Mixing Ratio and Stable Isotope Ratios

Rationale Vehicular traffic is a major source of anthropogenic greenhouse gases in megacities; however, real‐world constraints on vehicle‐derived N 2 O emissions and their isotopic characteristics remain limited. Methods Air samples were collected from three tunnels and urban background sites (a university campus and mountain) in Seoul, South Korea. The CO 2 and N 2 O mixing ratios and isotopic compositions (δ 13 C, δ 18 O of CO 2 ; δ 15 N bulk , δ 18 O, and site preference of N 2 O) were measured by gas chromatography and isotope ratio mass spectrometry, respectively. The vehicle‐emitted isotopic endmembers were derived using Keeling plot analysis. Results The vehicular N 2 O:CO 2 molar ratio was (4.8 ± 1.9) × 10 −2 (ppb:ppm). Scaling this ratio to Seoul's on‐road CO 2 inventory yields an estimated annual N 2 O emission of 163 ± 66 kt CO 2 equivalents, more than twice the current bottom‐up estimate. Vehicle‐emitted CO 2 exhibited isotopically homogeneous fossil fuel combustion δ 13 C signatures (−27.6‰ ± 0.4‰) and δ 18 O values (26.8‰ ± 0.5‰), and vehicular N 2 O was characterized by low δ 15 N bulk values (−6.5‰ ± 2.3‰), δ 18 O values (34.1‰ ± 2.2‰), and near‐zero site preference (0.2‰ ± 7.3‰). Conclusions The integration of tunnel and tailpipe measurements with stable isotope analyses enables the direct quantification of greenhouse gas emissions from vehicles under real‐world conditions. Vehicle‐derived CO 2 exhibits a narrow and well‐constrained δ 13 C signature, which is consistent with gasoline‐ and diesel‐dominated fleets in Seoul, whereas N 2 O isotopes reflect the formation processes associated with high‐temperature combustion and catalytic after treatment. Furthermore, systematically lower δ 18 O values in vehicle‐derived CO 2 and N 2 O relative to the urban background identify a distinct oxygen‐isotope fingerprint of traffic emissions, thereby establishing δ 18 O as a complementary tracer to traffic‐related greenhouse gas quantification.