Physical and chemical characteristics of individual mineral particles in an urban fog episode

Field emission scanning electron microscopy (FESEM) and scanning electron microscopy with energy dispersive X-ray detector (SEM-EDX) were used to study the four aerosol samples collected in Beijing air during the fog and non-fog episodes in winter. Size-distribution of mineral particles in fog sample displayed two major peaks at the size range of 0.1-0.3 μm and 1-2.5 μm. EDX analyses indicated that the major chemical compositions varied greatly in the individual mineral particles of the fog and non-fog episodes, especially the sulfur. A total of 9 different mineral categories were classified, namely, Si-rich, Ca-rich, S-rich, Fe-rich, Mg-rich, Al-rich, Ti-rich, K-rich and Cl-rich. About 55% of the Ca-rich in the fog samples comprised of Ca(50% ± 1.2%) and S(37% ± 1.6%) and about 72% of the S-rich comprised of S(44% ± 1.5%) and Ca(33% ± 2%), illustrating that particles with abundant sulfur were also enriched with abundant calcium. It is suggested that the Ca-rich alkali minerals could alleviate acidity of the fog water in Beijing air. The S/Ca mean ratio of mineral particles in the fog sample was 6.11, being 8 times higher than the S/Ca mean ratio of the non-fog samples (0.73). This result showed that sulfuration on the surfaces of aerosol particles was extremely severe, and that the conversion efficiency from SO₂ to sulfates was relatively high.