Measuring isotopic ratios in aerosol particles is a powerful tool for identifying major sources, particularly in separating fossil from non-fossil sources and investigating aerosol formation processes. We measured the radiocarbon, stable carbon, and stable nitrogen isotopic composition of PM_2.5 in Beijing (BJ) and Changdao (CD) in the North China Plain (NCP) from May to mid-June 2016. The mean PM_2.5 concentrations were 48.6 ± 28.2 μg m⁻³ and 71.2 ± 29.0 μg m⁻³ in BJ and CD, respectively, with a high contribution (∼66%) from secondary inorganic aerosol (SIA; NO₃⁻, NH₄⁺, and SO₄²⁻).

The mean δ¹³C of total carbon (TC) and δ¹⁵N of total nitrogen (TN) values differed significantly between the two sites (p-value of <0.001): −25.1 ± 0.3‰ in BJ and −24.5 ± 0.4‰ in CD and 10.6 ± 1.8‰ in BJ and 5.0 ± 3.1‰ in CD, respectively. In BJ, the average δ¹⁵N (NH₄⁺) and δ¹⁵N (NO₃⁻) values were 12.9 ± 2.3‰ and 5.2 ± 3.5‰, respectively. The ionic molar ratios and isotopic ratios suggest that NO₃⁻ in BJ was formed through the phase-equilibrium reaction of NH₄NO₃ under sufficient NH_{3 (g)} conditions, promoted by fossil-derived NH_{3 (g)} transported with southerly winds. In BJ, fossil fuel sources comprised 52 ± 7% of TC and 45 ± 28% of NH₄⁺ on average, estimated from radiocarbon (¹⁴C) analysis and the δ¹⁵N and isotope mixing model, respectively. These multiple-isotopic composition results emphasize that PM_2.5 enhancement is derived from fossil sources, in which vehicle emissions are a key contributor. The impact of the coal source was sporadically noticeable. Under regional influences, the fossil fuel-driven SIA led to the PM_2.5 enhancements. Our findings demonstrate that the multiple-isotope approach is highly advantageous to elucidate the key sources and limiting factors of secondary inorganic PM_2.5 aerosols.