The usefulness of mercury (Hg) isotopes for tracing the sources and pathways of Hg (and its vectors) in atmospheric fine particles (PM 2.5 ) is uncertain. Here, we measured Hg isotopic compositions in 30 potential source materials and 23 PM 2.5 samples collected in four seasons from the megacity Beijing (China) and combined the seasonal variation in both mass-dependent fractionation (represented by the ratio 202 Hg ∕ 198 Hg, δ 202 Hg) and mass-independent fractionation of isotopes with odd and even mass numbers (represented by Δ 199 Hg and Δ 200 Hg, respectively) with geochemical parameters and meteorological data to identify the sources of PM 2.5 -Hg and possible atmospheric particulate Hg transformation. All PM 2.5 samples were highly enriched in Hg and other heavy metals and displayed wide ranges of both δ 202 Hg (−2.18 to 0.51 ‰) and Δ 199 Hg (−0.53 to 0.57 ‰), as well as small positive Δ 200 Hg (0.02 to 0.17 ‰). The results indicated that the seasonal variation in Hg isotopic composition (and elemental concentrations) was likely derived from variable contributions from anthropogenic sources, with continuous input due to industrial activities (e.g., smelting, cement production and coal combustion) in all seasons, whereas coal combustion dominated in winter and biomass burning mainly found in autumn. The more positive Δ 199 Hg of PM 2.5 -Hg in spring and early summer was likely derived from long-range-transported Hg that had undergone extensive photochemical reduction. The study demonstrated that Hg isotopes may be potentially used for tracing the sources of particulate Hg and its vectors in the atmosphere.