Particle size and chemical composition of urban aerosols

Abstract

The size distribution of airborne particulates (PM10) has been measured by using Andersen Mark II cascade impactors. The measurements were done at four sites, three of which were in the Leeds area, including one roadside site and the fourth at a rural site. On average 10–20% of the mass of urban PM10 particles were found to be below 0.43 μm and 50% below l.5 μm. Extracted samples were analysed for sulphate, nitrate and chloride using ion chromatography, ion selective electrode to determine ammonium and Gran's titration to determine acidity. The results show that both sulphate and nitrate peak on the 1.1-μm stage. Nitrate is spread over both coarse and fine modes and is depleted in the finest particles <0.65 μm and enhanced in coarser particles >2.1 μm relative to sulphate. The chloride levels, dominated as they are by sea salt chloride, show a much coarser distribution with <50% being in the fine fractions for either urban or rural areas. The ammonium particulates are totally in the fine particle mode in summer but there is a small amount in the coarse mode in winter. The cumulative size distribution confirms that ammonium is the component with the finest size distribution with 50% <1.0 μm and 80% <1.8 μm. The acidity size distribution is close to the sulphate distribution. The magnitudes of H + for all sites were very low implying the aerosols are in general well neutralised but the fine particles are more acidic than coarse particles. Rural aerosols are less acidic than urban ones.