1. Physical Research Laboratory, Ahmedabad 380 009, India
2. Max Planck Institute for Aeronomy, D-3411, Katlenburg-Lindau-3, Germany
3. Lehrstuhl fur Bioc/imatologie and Immissionforschung, Universitiit Miinchen, Hogenbachernstr. 22, D-8050, Freising Weihenstephan, Germany