Urban and rural ultrafine (PM0.1) particles in the Helsinki area

In June 1996-June 1997 Berner low-pressure impactors were used at an urban and at a rural site in the Helsinki area for sampling ultrafine particles ( UFP, PM0.1). Ten sample pairs, each pair measured simultaneously, were coll ected in the size range of 0.03-15 mum of particle aerodynamic diameter. Mo re than 40 chemical components were measured. Surprisingly. the average UFP mass concentration was higher at the rural site (520 ng/m(3)) than at the urban site (490 ng/m(3)). The average chemical composition of UFP was simil ar at the two sites. The most abundant of the measured components were sulp hate (32 and 40 ng/m(3) for the urban and rural sites, respectively), ammon ium (22 and 25 ng/m(3)), nitrate (4 and 11 ng/m(3)) and the Ca2+ ion (5 and 7 ng/m(3)). The most important metals at both sites were Ca, Na, Fe, K and Zn with concentrations between 0.7 and 5 ng/m(3). Of the heavy metals, Ni, V, Cu. and Pb were important with average ultrafine concentrations between about 0.1 and 0.2 ng/m(3). Also the organic anions oxalate (urban 2.1 ng/m (3) and rural 1.9 ng/m(3)) and methanesulphonate (1.3 and 1.7 ng/m(3)) cont ributed similarly at both sites. The measured species accounted for only ab out 15-20% of the total ultrafine mass. The fraction that was not measured includes mainly carbonaceous material and water. It was estimated that the amount of water was about 10% (50 ng/m(3)) and that of carbonaceous materia l about 70% (350 ng/m(3)) at both sites, Aitken modes were observed for mos t components with the average mass mean mode diameters being between about 0.06 and 0.12 mum. The average concentrations in the Aitken mode differed c learly from those in the UFP for several components. The average contribution of ultrafine mass to the fine particle mass (PM2.5 ) was about 7% at the urban site and 8.5% at the rural site. At both sites the contribution of ultrafine to fine was especially high for Se, Ag, B, an d Ni (10-20%) and at the rural site also for Co (20%). Ca2+ (16%) and Mo (1 1 %). Enrichment in the ultrafine particles suggests that local sources may exist for these elements. Aitken modes turned out to be useful indicators of local sources for severa l components, The Aitken modes of Ba, Ca, Mg and Sr were similar in several samples, suggesting a common local combustion source for these elements, p ossibly traffic exhaust. Co, Fe, Mo and Ni formed another group of elements often having similar Aitken modes, the likely source being combustion of h eavy fuel oil. (C) 2001 Elsevier Science Ltd. All rights reserved.