Identification and characterization of carbon quantum dots in PM1 emitted in smoke from domestic wood combustion

We present a methodology for the identification and characterization of carbon quantum dots (CQDs) emitted during wood combustion in a domestic heating appliance. CQDs were isolated in the nanometric fraction of particulate matter (PM₀.₁, aerodynamic diameter < 0.1 μm) of the fume using an Electrical Low-Pressure Impactor (ELPI) during both transient log-burning phases and steady-state pellet combustion. Their presence was confirmed by complementary techniques, including SEM, TEM, Raman spectroscopy, and X-ray Photoelectron Spectroscopy (XPS). The PM₀.₁ fraction contains nanocrystalline graphene/graphite domains with an amorphization degree of 13–17%, arising from sp³-type defects within an sp² matrix and contributions from amorphous carbon, as confirmed by C1s XPS spectra. We show that quantitative Raman analysis revealed that a five-component deconvolution of the D and G bands, together with the observation of sloping baselines (indicating an underlying photoluminescence background), provides robust criteria for identifying CQDs. TEM images further demonstrate that CQDs adopt nano-onion morphologies with diameters near the impactor cut-off, consisting of an amorphous core encased in concentric graphitic shells. The measured interplanar spacing of 3.84 Å closely matches values reported for synthetic nano-onions derived from biomass precursors.