Isotopic Variability of Ammonia (δ15N-NH3) Slipped from Heavy-Duty Vehicles under Real-World Conditions

The widespread use of urea-SCR (selective catalytic reduction) aftertreatment to meet stringent heavy-duty vehicle (HDV) NO_x emission standards has worldwide led to increased exhaust ammonia (NH₃) slip. A prerequisite for the nitrogen (N) stable isotope (δ¹⁵N)-based source apportionment of ambient NH₃ is that the N isotopic signatures of NH₃ sources be well characterized. However, knowledge of δ¹⁵N values from HDV-emitted NH₃ is still lacking. Using an on-board HDV emission measurement system, tailpipe-emitted gaseous NH₃ from eight HDVs was collected during real-world driving conditions, and δ¹⁵N values were analyzed. We tentatively report an average δ¹⁵N-NH₃ [mean ± 1σ = (−1.74 ± 6.27)‰] that is significantly higher (and hence potentially of diagnostic value) than that of naturally occurring volatile NH₃ sources (less than −26‰) but only slightly lower than that of gasoline vehicles equipped with three-way catalytic converters. The observed variations in measured HDV δ¹⁵N-NH₃ can be largely explained by the differential N-isotope fractionation during NH₃ formation and the extent of ammonia conversion. We found that minor changes in SCR efficiency can cause relatively large fluctuations in δ¹⁵N-NH₃, highlighting the difficulties and/or limitations with regard to attributing a characteristic δ¹⁵N to tailpipe NH₃ in N-isotope balances and source partitioning efforts.