Significant Changes of δ15N-NOx in Diesel Vehicle Emissions with Emission Standards and Cumulative Mileage: Insight into Isotope-Based NOx Source Analysis

Nitrogen isotope (δ¹⁵N) analysis is a robust method for tracing atmospheric NO_x and nitrate. However, the isotopic signatures of NO_x emissions from heavy-duty diesel vehicles (HDDVs) remain poorly characterized. In this study, we conducted on-site tests on 18 on-road HDDVs with different emission standards in Shanghai under the condition of China World Transient Vehicle Cycle for trucks (C-WTVC). The average δ¹⁵N value under C-WTVC (−5.71 ± 2.60‰) was significantly higher than that in idle mode (−13.85 ± 1.97‰), which is attributed to the lower temperatures in the combustion chamber due to the lower engine output in idle mode, favoring the breakage of the lighter ¹⁴N¹⁴N molecules. The δ¹⁵N-NO_x values under C-WTVC were significantly lower than those in driving mode in the United States, suggesting that δ¹⁵N values may be localized in isotope-based NO_x source analysis. NO_x emissions were reduced considerably with the upgrade of emission standards while δ¹⁵N-NO_x levels showed an opposite trend. The NH₃-selective catalytic reduction (SCR) reduced automobile NO_x emissions by more than 50% and significantly enriched ¹⁵N relative to diesel-generated NO_x. Indicated by the correlations between NO_x emissions, δ¹⁵N-NO_x, and cumulative mileage, the SCR de-NO_x efficiency decreased linearly with the increase of cumulative mileage. Our results suggest that the routine maintenance and mandatory inspection of aged SCR systems should be strengthened. The distribution of provincial δ¹⁵N-NO_x values of diesel vehicle emissions was established based on a weighted average model. The contribution of vehicle NO_x emissions in typical megacities in China has been revisited by upgrading the δ¹⁵N-NO_x values of diesel vehicles, demonstrating that the application of regional δ¹⁵N-NO_x values of diesel vehicles can narrow the gap between isotope-based analysis and emission inventory. This study will enrich the NO_x isotopic fingerprint database in China and minimize the uncertainty in isotope-based NO_x source apportionment, helping to formulate air pollution prevention and control strategies.