Fine particles (PM2.5) were sampled indoors and outdoors at 40 sampling sites; in ten classrooms in five schools, at ten preschools and 20 non-smoking homes, in three communities in Stockholm, Sweden, during nine 2-week periods. Each sampling site was sampled twice, once during winter and once during spring. The samples were analysed for elemental concentrations using X-ray fluorescence (XRF) spectroscopy. In all locations significantly higher outdoor concentrations were found for elements that are related to long-range transported air masses (S, Ni, Br and Pb), while only Ti was higher indoors in all locations. Similar differences for S, Br and Pb were found in both seasons for homes and schools. In preschools different seasonal patterns were seen for the long-range transported elements S, Br and Pb and the crustal elements Ti, Mn and Fe. The indoor/outdoor ratios for S and Pb suggest an outdoor PM2.5 particle net infiltration of about 0.6 in these buildings. The community located 25 km from the city centre had significantly lower outdoor concentrations of elements of crustal or traffic origin compared with the two central communities, but had similar levels of long-range transported elements. Significant correlations were found between PM2.5 and most elements outdoors (rs = 0.45–0.90). Copper levels were found to correlate well (rs = 0.64–0.91) to the traffic marker NO2 during both winter and spring in all locations. Copper may be a suitable elemental marker for traffic-related aerosols in health studies in areas without other significant outdoor Cu sources.
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