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

Abstract

The widespread use of urea-SCR (selective catalytic reduction) aftertreatment to meet stringent heavy-duty vehicle (HDV) NOx 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.