Nitrate (NO₃⁻) is one of the most important inorganic ions in fine particulate (PM_2.5) and drives regional haze formation; however, the NO₃⁻ sources and formation mechanisms in different seasons and regions are still debated. Here, PM_2.5 samples were collected from Kunming and Nanning in southwestern China from September 1, 2017, to February 28, 2018 (spanning warm and cold months). We measured the daily O and N isotopic compositions of NO₃⁻ (δ¹⁸O–NO₃⁻ and δ¹⁵N–NO₃⁻), estimated the δ¹⁸O–HNO₃ values produced by different oxidation pathways, and quantified the NO₃⁻ formation pathways based on the isotope mass-balance equation. Our results showed that the δ¹⁸O–NO₃⁻ values in Kunming (65.3 ± 7.6‰) and Nanning (67.7 ± 10.1‰) are close to the δ¹⁸O–HNO₃ values arising from the OH radical pathway (P_OH, 54.7 ± 1.2‰ to 61.2 ± 1.8‰), suggesting that the δ¹⁸O–NO₃⁻ values are mainly influenced by P_OH, which showed a contribution greater than 74%. Stronger surface solar radiation and higher air temperatures in low-latitude regions and warm months increased the amount of HNO₃ produced by P_OH and reduced the amount of HNO₃ produced by P_N2O5, which produced low δ¹⁸O–NO₃⁻ values. Increased air pollution emissions decreased the contribution from P_OH and increased the contribution from N₂O₅ and NO₃ pathways (P_N2O5+NO3). The δ¹⁵N–NO₃⁻ values of PM_2.5 in Kunming (7.3 ± 2.8‰) were slightly higher than those in Nanning (2.8 ± 2.7‰). The increased NO_x emissions with positive isotopic values led to high δ¹⁵N–NO₃⁻ values in northern China and during cold months. A higher f_NO2 (f_NO2 = NO₂/(NO + NO₂), temperature, and contribution of P_OH produced lower N isotope fractionation between NO_x and δ¹⁵N–NO₃⁻, which was found to further decrease the δ¹⁵N–NO₃^– values in southwestern China and during warm months.