Vehicular exhaust has been identified as an important ammonia (NH₃) emission source in urban areas using the stable isotope tracer technique (δ¹⁵N); however, its δ¹⁵N signature remains poorly constrained due to difficulties in concentrating and speciating NH_x (NH₃ + particulate NH₄⁺). In this study, we deployed two active sampling techniques, MCIs (Multi-nozzle Cascade Impactor sampler; “particle-gas” order) and CCSCs (ChemComb Speciation Cartridge sampler; “gas-particle” order), to collect vehicular exhaust NH_x in a long urban tunnel in Shenyang, China, and quantified the ¹⁵N from the collected NH₃ and NH₄⁺. While no significant difference in tunnel NH₃ concentrations between the two samplers was observed (n = 21), the δ¹⁵N-NH₃ was slightly higher using the MCIs (6.3 ± 1.6‰) compared to the CCSCs (4.8 ± 2.3‰) during the tunnel operation period. This minor absolute difference of 1.5 ± 2.8‰ may derive from denuder-saturation used within the CCSCs that lead to NH_x speciation ambiguity, which also caused significant δ¹⁵N-pNH₄⁺ differences between the two sampling approaches. Our results suggest that both active samplers can well characterize the δ¹⁵N-NH_x in the ambient environment, but the “particle - gas” sampling approach performs better in elevated NH₃ concentration environments. We recommend that the δ¹⁵N value of 6.3‰ is used as the isotope end member for vehicle NH₃ when performing NH₃ source apportionment.