Optimizing Vehicular CO ₂ Emission Factors in Tunnel Environment: Integrating Mobile Measurements and Radiocarbon Tracing

Abstract We developed a novel approach that integrates mobile CO 2 measurements and radiocarbon (Δ 14 C) tracing to optimize vehicle emission factors (EFs) within an urban tunnel environment. High‐resolution mobile measurements revealed the spatial distribution of CO 2 concentrations, showing that CO 2 concentrations do not increase monotonically from the tunnel entrance to the exit. Instead, they remain stable or even decrease initially and then gradually increase. This suggests that the arbitrary selection of sampling locations within tunnels could result in underestimating EFs. Based on CO 2 profiles, we selected three stationary sampling locations for CO 2 measurements and Δ 14 C analysis. The results show that non‐vehicle emissions, such as passenger respiration and infiltrated ambient air, accounted for 6.7%–9.9% of the CO 2 enhancement in the tunnel. Neglecting these sources could result in an overestimation of vehicular CO 2 EFs by up to 11.2%, with an average overestimation of 5.4%. The optimized approach significantly reduced estimation bias and identified notable seasonal and weekday/weekend variations in EFs. These findings highlight the importance of refined sampling strategies and source identification in accurately quantifying vehicle CO 2 emissions and supporting effective emission reduction policies.