Combined analyses of hygroscopic properties of organic and inorganic components of three representative black carbon samples recovered from pyrolysis

Hygroscopicity of black carbon (BC) aerosols is a key factor determining their climate forcing effect and atmospheric lifetime. However, the compositional dependence of BC hygroscopicity is not well understood. Here, a variety of different compositional components were separated from three representative BC samples recovered from pyrolysis (grass and wheat straw derived BC, household soot), including water extracted fraction of BC (WEBC, 9–21 wt%), residue fraction of BC after water extraction (R-WEBC, 79–91 wt%), water extracted minerals (WEM, 9–18 wt%), alkali extracted organic carbon (OCAE, 1–9 wt%), and elemental carbon (EC, 37–48 wt%). The bulk BC and separated BC components were analyzed in detail by elemental analysis and combined spectroscopic analyses. Their equilibrium hygroscopicity was measured by gravimetric method over a range of relative humidity (RH) levels (10–94%). Compared with the two organic components (OCAE and EC), the inorganic component (WEM) exhibited much stronger water uptake at all RH levels. At 94% RH level, WEM accounted for 16–139% of the overall water uptake by BC, whereas OCAE and EC accounted for only 1–3% and 6–26%, respectively. The XRD analysis of WEBC and WEM from household soot at varying RH levels indicated that the enhanced water uptake by these two components as well as that by bulk BC at high RH levels was due to the deliquescent salts (e.g., KCl, NH4Cl, KNO3, and NaCl). The strong hysteresis loops observed for bulk BC and WEBC could be attributed to the organic-facilitated drastic structural and morphological rearrangement of mineral particles as evidenced by the optical microscope analysis. The diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis reaffirmed the dominant role played by the inorganic component in the hygroscopic behaviors of BC.