RESUMO
Soot is typically the dominant component of the nonvolatile particles emitted from internal combustion engines. Although soot is primarily composed of carbon, its chemistry, toxicity, and oxidation rates may be strongly influenced by internally mixed inorganic metal compounds (ash). Here, we describe the detailed microstructure of ash internally mixed with soot from four marine engines and one aviation engine. The engines were operated on different fuels and lubrication oils; the fuels included four residual fuels and five distillate fuels such as diesel, natural gas, and Jet A-1. Using annular-dark-field scanning transmission electron microscopy (ADF-STEM), we observed that ash may occur either as distinct nodules on the soot particle (decorated) or as continuous streaks (painted). Both structures may exist within a single particle. Decorated soot was observed for both distillate and residual fuels and contained elements associated with either the fuel (V, Ni, Fe, S) or with the lubrication oil (Zn, Ca, P). Painted soot was observed only for residual-fuel soot, and only contained elements associated with the fuel. Additional composition measurements by inductively coupled plasma mass spectrometry (ICP-MS) of filter samples indicated that the internal mixing trends of ash with soot were consistent with the overall ash-to-carbon ratio of the sampled combustion aerosols. Painted soot may form when molten ash coagulates with or condenses onto soot within engines.