Chemical data

Four data sets have been used in this study. Aerosol samples for the first set were collected at Sevettijärvi (69$^\circ$35$^\prime$N, 28$^\circ$50$^\prime$E, 130 m above sea level (asl)), in Finnish Lapland, from July 1992 until January 1996 (Figure 1). The sampling device was a virtual impactor which provides two particle size ranges: fine (aerodynamic diameter, AD$<$2.5 $\mu$m) and coarse (AD 2.5-15 $\mu$m). The collection time per sample was 48 hours. The samples were weighed for particulate mass (PM), analysed for black carbon (BC) by a light reflectance technique, for major anions and cations by ion chromatography, and for up to 46 elements by a combination of instrumental neutron activation analysis and particle-induced X-ray emission spectrometry (Virkkula et al., 1999).

Other two data sets were obtained for Birkenes (58$^\circ$23$^\prime$N, 8$^\circ$15$^\prime$E, 190 m asl) and Skreådalen (58$^\circ$49$^\prime$N, 6$^\circ$43$^\prime$E, 465 m asl) in Southern Norway. The samplings at the two sites were conducted in parallel, according to a 2-2-3 day schedule, from January 1991 until March 1996. Gent PM10 stacked filter unit (SFU) samplers were used for separating the size fractions of 2 to 10 $\mu$m AD and of $<$2 $\mu$m AD, respectively. The filters were analysed for PM, BC, and over 40 elements (Maenhaut et al., 2000).

The fourth data set was measured at Sde Boker (30$^\circ$51$^\prime$N, 34$^\circ$47$^\prime$E, 470 m asl), in the Negev Desert of Israel. Sampling and measurement procedures are similar to those applied at the Norwegian sites (Maenhaut et al., 1996). The data set covers the period from January 1995 to August 1998.