Isotopic Variability of Ammonia (delta N-15-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) NOx emission standards has worldwide led to increased exhaust ammonia (NH3) slip. A prerequisite for the nitrogen (N) stable isotope (delta N-15)-based source apportionment of ambient NH3 is that the N isotopic signatures of NH3 sources be well characterized. However, knowledge of delta N-15 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 delta N-15 values were analyzed. We tentatively report an average delta N-15-NH3 [mean +/- 1 sigma = (-1.74 +/- 6.27)parts per thousand] that is significantly higher (and hence potentially of diagnostic value) than that of naturally occurring volatile NH3 sources (less than -26 parts per thousand) but only slightly lower than that of gasoline vehicles equipped with three-way catalytic converters. The observed variations in measured HDV delta N-15-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 PN-NH3, highlighting the difficulties and/or limitations with regard to attributing a characteristic delta N-15 to tailpipe NH3 in N-isotope balances and source partitioning efforts.