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 (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 (Δδ15NNH4+-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 NH3(g) dominated. This resulted in 15N-depleted aerosol NH4+ (meanminmax ± 1σ = - 20.2-25.516.1 ± 3.8‰) relative to NH3 (1.6-7.29.4 ± 6.1‰), yielding a net Δδ15NNH4+-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 NH3(aq)/NH3(g) (25 ± 12%), NH4+(aq)/NH3(aq) (34 ± 13%), and NH4+(aq/s)/NH3(g) (32 ± 15%) jointly explained over 90% of the observed Δδ15NNH4+-NH3 (20.44.431.0 ± 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.