Microscopy and mineralogy of airborne particles collected during severe dust storm episodes in Beijing, China

Abstract

Asia dust storm (ADS) samples were collected during two severe dust storm episodes in Beijing, China. High‐resolution field emission scanning electron microscopy (FESEM) and environmental scanning electron microscopy‐energy dispersive X‐ray detector (ESEM‐EDX) coupled with an image analysis system were used to study the morphology, chemical compositions, and number‐size distributions of ADS particles. FESEM analysis indicated that Beijing ADS samples were mainly composed of irregularly shaped mineral particles. More than 50% of the ADS particles were larger than 1 μm, measured as equivalent spherical diameter (ESD). Image analysis showed that when the dust storm episodes ended in Beijing, the number of anthropogenic particles, such as soot aggregates and coal fly ash, in the samples increased, and the number of mineral particles decreased. Analysis of individual ADS particles showed that about one fifth of all the particles were mineral aggregates, and at least one fourth of the particles contained sulfur. X‐ray diffraction (XRD) was used to quantify the phase and the clay mineral compositions of ADS samples. Phases in the ADS sample collected during the 20 March 2002 dust storm episode included clay minerals, noncrystalline materials, quartz, calcite, plagioclase, potassium feldspar, pyrite, hornblende, and gypsum in descending order. Clay minerals were mainly illite/smectite mixed layers (78%), followed by illite (9%), kaolinite (6%), and chlorite (7%). Particulate matter (PM) less than 10 μm were enriched with clay minerals and deficient with quartz by mass compared with the total suspended particulates collected during an ADS episode. The PM less than 10 μm collected during the two severe dust storm episodes was characterized by the absence of dolomite, high quartz/clay ratio, and dominance of illite/smectite mixed layers in clay minerals.