[1] Carnegie Mellon Univ, Ctr Atmospher Particle Studies, Pittsburgh, PA 15213 USA
[2] Stockholm Univ, Dept Appl Environm Sci, S-10691 Stockholm, Sweden
[3] Stockholm Univ, Bert Bolin Ctr Climate Res, S-10691 Stockholm, Sweden
[4] MIT, Dept Civil & Environm Engn, Cambridge, MA 02139 USA