Nitrogen Isotope Fractionation Associated with Ammonia Gas-to-Aerosol Conversion and Implications for Determining Isotopic Source Signatures and Tracing Atmospheric Ammonium Origins

While the nitrogen isotopic compositions of ammonia (δ 15 N-NH 3 ) can be used to trace the NH 3 origin and fate, subsequent atmospheric processes after emissions alter the δ 15 N signatures of initial NH 3 and its aerosol products. In a “lab-in-the-field” study, we determined hourly δ 15 N discrimination factors (Δδ 15 N NH 4 + -NH 3 = [δ 15 N-NH 4 + – δ 15 N-NH 3 ]) to reflect the combined nitrogen isotope effects during NH 3 gas-to-aerosol conversion. Near a point source, the kinetic isotope effect (KIE) during the rapid neutralization of NH 3(g) dominated. This resulted in 15 N-depleted aerosol NH 4 + (mean min max ± 1σ = – 20.2 –25.5 16.1 ± 3.8‰) relative to NH 3 (1.6 –7.2 9.4 ± 6.1‰), yielding a net Δδ 15 N NH 4 + -NH 3 of −14.7 –18.6 –9.8 ± 3.2‰. Conversely, in aged ambient air, the system swiftly relaxed toward equilibrium. Bayesian modeling revealed that the equilibrium isotope effect (EIE) between NH 3(aq) /NH 3(g) (25 ± 12%), NH 4 + (aq) /NH 3(aq) (34 ± 13%), and NH 4 + (aq/s) /NH 3(g) (32 ± 15%) jointly explained over 90% of the observed Δδ 15 N NH 4 + -NH 3 (20.4 4.4 31.0 ± 5.2‰). These findings suggest (i) that, in previous studies, the large variability of NH 3 isotopic source signatures may partly be attributed to the differential nitrogen isotope alteration due to kinetic effects and (ii) that the inference of δ 15 N-NH 3 signatures based on the measured δ 15 N-NH 4 + likely leads to inaccurate NH 3 source apportionment results, if only single and not the combined nitrogen isotope effects are considered.