Nitrogen Isotope Fractionation Associated with Ammonia Gas-to-Aerosol Conversion and Implications for Determining...

While the nitrogen isotopic compositions of ammonia (δ¹⁵N-NH₃) can be used to trace the NH₃ origin and fate, subsequent atmospheric processes after emissions alter the δ¹⁵N signatures of initial NH₃ and its aerosol products. In a "lab-in-the-field" study, we determined hourly δ¹⁵N discrimination factors (Δδ¹⁵N_NH4⁺-NH3 = [δ¹⁵N-NH₄⁺ - δ¹⁵N-NH₃]) to reflect the combined nitrogen isotope effects during NH₃ 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 ¹⁵N-depleted aerosol NH₄⁺ (mean_min^max ± 1σ = - 20.2_-25.5^16.1 ± 3.8‰) relative to NH₃ (1.6_-7.2^9.4 ± 6.1‰), yielding a net Δδ¹⁵N_NH4⁺-NH3 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 Δδ¹⁵N_NH4⁺-NH3 (20.4_4.4^31.0 ± 5.2‰). These findings suggest (i) that, in previous studies, the large variability of NH₃ isotopic source signatures may partly be attributed to the differential nitrogen isotope alteration due to kinetic effects and (ii) that the inference of δ¹⁵N-NH₃ signatures based on the measured δ¹⁵N-NH₄⁺ likely leads to inaccurate NH₃ source apportionment results, if only single and not the combined nitrogen isotope effects are considered.