[1] Commiss European Communities, Joint Res Ctr, Inst Environm & Sustainabil, I-21020 Ispra, Italy
[2] Aristotle Univ Thessaloniki, Lab Atmospher Phys, GR-54006 Thessaloniki, Greece
[3] Univ Innsbruck, Inst Med Phys, A-6020 Innsbruck, Austria
[4] Univ Grenoble 1, F-38000 Grenoble, France
[5] Swedish Meteorol & Hydrol Inst, S-60176 Norrkoping, Sweden
[6] Finnish Meteorol Inst, FIN-00101 Helsinki, Finland
[7] Norwegian Univ Sci & Technol, Dept Phys, N-7491 Trondheim, Norway
[8] Univ Manchester, Inst Sci & Technol, Dept Phys, Manchester M60 1QD, Lancs, England
[9] Swedish Radiat Protect Inst, Div Environm Monitoring & Dosimetry Nonionizing R, SE-17116 Stockholm, Sweden