Size-resolved isotope analysis reveals anthropogenic reactive nitrogen transport and transformation in Taiwan mountain forests

Abstract. Reactive nitrogen (Nr) species such as particulate ammonium (pNH₄⁺) and nitrate (pNO₃⁻) cause air pollution and affect ecosystems, yet their transformation processes in mountain forests are not well-characterized. Size-resolved isotope analysis of aerosols could reveal these processes, but is rarely performed due to low particle concentrations. We overcame this limitation by combining size-segregated aerosol sampling at Xitou, Taiwan, with sensitive isotopic techniques and Bayesian modeling. Functional groups were analyzed by Fourier-transform infrared spectroscopy (FTIR-ATR), and isotopes δ¹⁵N and δ¹⁸O were measured by gas chromatography-isotope ratio mass spectrometry (GC-IRMS), enabling quantification of pNH₄⁺ source contributions and pNO₃⁻ formation pathways. Typical diurnal patterns, with higher daytime particle concentrations, were disrupted during a 26-hour fog caused by stagnant atmospheric conditions. During fog, the average δ¹⁵N-NH₄⁺ decreased from 11.75±2.42 ‰ (mean±1σ) during clear periods to 7.75±1.37 ‰, while δ¹⁵N-NO₃⁻ dropped from −2.57±1.80 ‰ to −4.51±1.79 ‰, indicating continued isotopic fractionation under reduced urban influence. Size-resolved isotope results revealed nitrate evolution during transport: urban plumes retained O₃-driven oxidation signatures with isotopic fractionation, whereas mountain-formed nitrate was produced via RO₂-involved processes with greater isotopic fractionation and enhanced biogenic contributions. Bayesian modeling indicated that 50−83 % of NH₃ emissions originated from combustion-related sources, while 42−95 % of pNO₃⁻ formed through RO₂-initiated oxidation during daytime and 6−84 % through heterogeneous reactions at night. These findings emphasize the importance of controlling urban NO_x and combustion-related NH₃ emissions to reduce downwind Nr pollution and demonstrate how size-resolved isotope analysis elucidates aerosol evolution along transport pathways.