Theoretical Phase Resolved Ammonia–Ammonium Nitrogen Equilibrium Isotope Exchange Fractionations: Applications for Tracking Atmospheric Ammonia Gas-to-Particle Conversion

Abstract

Nitrogen (N) equilibrium isotope fractionation (15α) involving gaseous, dissolved, and solid phases of ammonia (NH3) and ammonium (NH4+) (e.g., NH3(g)–NH3(aq)–NH4+(aq)–NH4+(s)) represents a fundamental chemical process that has important implications for understanding the environmental dynamics involving NHx (NH3 + NH4+). However, recent literature disagrees with early experimental results from Urey and co-workers, suggesting the need for an update on theoretical estimates. Here, we have calculated theoretical 15α values for NH4+(g)/NH3(g), NH3(aq)/NH3(g), NH4+(aq)/NH3(g), NH4+(aq)/NH3(aq), and NH4+(s)/NH3(g) using HF/6-31G(d) and B3LYP/6-31G(d) levels of theory. Overall, our theoretical calculated values matched experimental data reported by Urey and co-workers, with best agreement obtained at the HF/6-31G(d) level of theory with solvent effect accounted for using water cluster calculations. Our calculated results have important implications for tracing NH3 gas-to-particle phase conversions that may have distinctive isotopic separation factors (Δ15δNH4+/NH3 = δ15 N(NH4+) – δ15 N(NH3)) between N isotopic compositions (δ15 N) of NH4+ and NH3 depending on its conversion mechanism. While further experimental work is necessary to validate our predicted isotope effects over the considered temperature range, this work demonstrates the potential of N isotopic measurements of phase-resolved NHx to better understand its dynamics in the atmosphere.