In most of the urban atmosphere, the formation of ammonium (NH₄⁺) is mainly controlled by the initial isotope value of ammonia (δ¹⁵N-NH₃ values) and the corresponding ammonia (NH₃) conversion ratio (NH₄⁺/(NH₄⁺ + NH₃)), as N isotope fractionation may occur under an NH₃-sufficient environment. The significant influence of δ¹⁵N-NH₃ values on the formation of NH₄⁺ in the urban atmosphere has been demonstrated by many previous studies; however, exploration of the effect of the NH₃ conversion ratios on NH₄⁺ formation has been limited, especially during pollution episodes. To better understand the NH₃ sources and the effect of the NH₃ conversion ratios on the formation of NH₄⁺ in the urban environment, NH₄⁺ concentrations (0.2 to 13.4 μg·m⁻³, average 3.9 ± 2.8 μg·m⁻³) and their δ¹⁵N values (−4.0‰ to 29.2‰, average 20.6‰ ± 6.1‰) in PM_2.5 were determined in the urban area of Nanning from September 1 to December 31, 2017. Based on the measured δ¹⁵N-NH₄⁺ values and the isotope fractionation that occurred during gas-to-particle partitioning, we estimated that the initial δ¹⁵N-NH₃ values were − 22.8‰ to 3.1‰ (average − 8.8‰ ± 5.5‰). These initial δ¹⁵N-NH₃ values were influenced by agricultural emission sources (−31.0‰ to −4.4‰), fossil fuel-related NH₃ emission sources (−14.6‰ to 10.1‰) and biomass combustion sources (12‰ and 23‰). Source apportionment indicated that these three potential NH₃ emission sources contributed 43%, 33% and 24% to the ambient NH₃, respectively. On clean days, similar trends in initial δ¹⁵N-NH₃ and δ¹⁵N-NH₄⁺ values were observed, and there was an apparent positive linear correlation between these values (R² = 0.70, p < 0.01). This suggested that the δ¹⁵N-NH₄⁺ signatures recorded changes in the NH₃ sources and that NH₄⁺ formation was controlled by the NH₃ sources on clean days. However, during different pollution episodes the variations in δ¹⁵N-NH₄⁺ values were consistent with the NH₄⁺ concentrations and the NH₄⁺/(NH₄⁺ + NH₃) values, and there were negative linear correlation between these values (R² > 0.6, p < 0.01). This suggested that the δ¹⁵N-NH₄⁺ signatures recorded NH₄⁺ evolution processes and that the increased NH₃ conversion ratios was the main reason for the development of NH₄⁺ and PM_2.5 in pollution episodes. We inferred that the decreased air temperature in cold months may stimulate the increased NH₃ conversion ratios, based on the negative correlation of NH₄⁺/(NH₄⁺ + NH₃) values with air temperature (r = −0.63, p < 0.01).