Inorganic Components Drive the Latent Heat of Hygroscopic Phase Transition in Atmospheric Black Carbon

The hygroscopic phase transition (HPT) latent heat of black carbon (BC) particles can affect the atmospheric energy budget. However, the source-dependent characteristics and underlying mechanisms remain poorly understood. Herein, three representative BCs (Corn Cob BC, Camphor Wood BC, and Coal BC) were systematically analyzed to quantify HPT latent heat and reveal component-specific contributions. By combining component-resolved analysis with differential scanning calorimetry, it was found that Coal BC exhibited the highest HPT latent heat at 97% RH (ΔH = 93.77 J g–1), which was approximately 253 times higher than that at 11% RH (ΔH = 0.37 J g–1). This was primarily driven by its inorganic component, including the water-extractable fraction (WEBC) and water-extractable minerals (WEM). The corresponding spectral shifts of WEBC (85–100 cm–1 blueshift) and WEM (100–105 cm–1 redshift) in O–H stretching bands under 97% RH indicate strong hydrogen-bonding and solvent effects. These inorganic-rich fractions, although accounting for only 10.1–18.0 wt % in Coal BC, controlled water uptake and latent-heat release, highlighting their pivotal role in BC’s nonlinear thermodynamic behavior. This is the first study to quantitatively resolve BC’s HPT latent heat and attribute it to specific components, providing thermodynamic insights for improving the parametrization of BC radiative effects in atmospheric models.