1. Risø National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark
2. Present address: VK1, Agern Allé 11, DK-2970 Hørsholm, Denmark (e-mail: chg@vki.dk)
3. VKI, Agern Allé 11, DK-2970 Hørsholm, Denmark
4. Royal Agricultural University, Agrovej 10, DK-2630 Taastrup, Denmark