Size Differentiation of Individual Atmospheric Aerosol during Winter in Xi'an, China

Airborne particulate matter (including TSP, PM10, PM2.5, and PM1) were collected at an urban site in Xi'an during winter 2010. Individual particles were analyzed using scanning electron microscopy and energy dispersive X-ray spectrometer (SEM-EDX). The morphologies, size distributions, and relative abundance of aerosol particles in each size were summarized. The monomodal particle size distribution was found in all the samples under different weather conditions, with the peaks located at less than 1.0 mu m. The majority of particles were composed of soot, mineral dust, and tar balls, with minor fly ash particles. Soot aggregates were the predominant species (in numbers), ranging from 56.6% in TSP on a sunny day to 86.3% in PM1 on a cloudy day, with an average of 73.2% in all the samples. The particle mass concentration and chemical composition, including water-soluble inorganic ions, elemental compositions, organic carbon (OC) and elemental carbon (EC) contents of 24-hr integrated PM2.5, were also subject to chemical bulk analysis. Soot was predominantly observed in the PM2.5 samples (from 74.7% to 82.7% in numbers), whereas EC accounted for only a small amount (< 8%) of the PM2.5 mass. Corresponding to the mass concentrations of geological materials (29.2%, 44.5%, and 37.3% on sunny, cloudy, and hazy days), the number concentrations of mineral dust and fly ash particles on the sunny, cloudy, and hazy days were 14.6%, 7.1%, and 7.7%, respectively.