Abstract. Reactive nitrogen (Nr) species such as particulate ammonium (pNH4+) and nitrate (pNO3-) are critical drivers of air pollution and ecosystem health, yet their transformation in mountain forests remains poorly characterized. We performed a one-week field campaign in a subtropical mountain forest at Xitou, Taiwan, using size-segregated aerosol sampling, stable isotopic techniques, and Bayesian modeling. Functional groups were analyzed by Fourier-transform infrared spectroscopy (FTIR-ATR), and isotopes δ15N and δ18O were measured using gas chromatography-isotope ratio mass spectrometry (GC-IRMS) to quantify pNH4+ source contributions and pNO3- formation pathways. During the sampling week, diurnal patterns of higher daytime particle concentrations were disrupted by a 26 h fog event, which suppressed δ15N-enriched urban plume influx and promoted aqueous-phase uptake of isotopically depleted local gas-phase species. Under clear conditions, size-resolved δ15N-NH4+ exhibited a bell-shaped distribution peaking at the accumulation mode, whereas this gradient flattened during the fog event. Size-resolved δ15N and δ18O signatures of pNO3- revealed two nitrate formation regimes: urban plumes retained O3-driven oxidation signatures with higher δ18O, and rural/local regimes were dominated by RO2-involved processes with greater isotopic depletion and/or biogenic contributions. Bayesian source apportionment constrained by δ15N-NH4+ indicated 50 %–83 % of NH3 emissions originated from combustion-related sources. Concurrently, δ18O source apportionment showed RO2-initiated oxidation dominated daytime pNO3- formation (42 %–95 %) and heterogeneous reactions contributed 6 %–84 % at night. Although based on a short-term campaign capturing a single fog episode, this case study highlights the value of size-resolved isotopic approaches for characterizing reactive nitrogen transport and evolution under contrasting meteorological conditions, providing mechanistic insights into complex environments.