[1] Finnish Meteorol Inst, Erik Palmenin Aukio 1,POB 503, FIN-00101 Helsinki, Finland
[2] North West Univ, Chem Resource Beneficiat, Atmospher Chem Res Grp, Potchefstroom, South Africa
[3] Kansas State Univ, Dept Phys, 1228 N 17th St, Manhattan, KS 66506 USA
[4] Univ Colorado, Cooperat Inst Res Environm Sci, 216 UCB, Boulder, CO 80309 USA
[5] Natl Ocean & Atmospher Adm, Phys Sci Lab, 325 Broadway, Boulder, CO 80305 USA
[6] Univ Maryland, Earth Syst Sci Interdisciplinary Ctr, 5825 Univ Res Court,Suite 4001, College Pk, MD 20740 USA