J. Lelieveld,^1* H. Berresheim,² S. Borrmann,¹³ P. J. Crutzen,¹⁴ F. J. Dentener,⁵ H. Fischer,¹ J. Feichter,⁶ P. J. Flatau,⁴⁷ J. Heland,⁸ R. Holzinger,¹ R. Korrmann,¹ M. G. Lawrence,¹ Z. Levin,⁹ K. M. Markowicz,⁴¹⁰ N. Mihalopoulos,¹¹ A. Minikin,⁸ V. Ramanathan,⁴ M. de Reus,¹ G. J. Roelofs,¹² H. A. Scheeren,¹² J. Sciare,¹³ H. Schlager,⁸ M. Schultz,⁶ P. Siegmund,¹⁴ B. Steil,¹ E. G. Stephanou,¹¹ P. Stier,⁶ M. Traub,¹ C. Warneke,¹⁵ J. Williams,¹ H. Ziereis⁸

The Mediterranean Intensive Oxidant Study, performed in the summer of 2001, uncovered air pollution layers from the surface to an altitude of 15 kilometers. In the boundary layer, air pollution standards are exceeded throughout the region, caused by West and East European pollution from the north. Aerosol particles also reduce solar radiation penetration to the surface, which can suppress precipitation. In the middle troposphere, Asian and to a lesser extent North American pollution is transported from the west. Additional Asian pollution from the east, transported from the monsoon in the upper troposphere, crosses the Mediterranean tropopause, which pollutes the lower stratosphere at middle latitudes.

¹ Max Planck Institute for Chemistry, Post Office Box 3060, 55020 Mainz, Germany.
² German Weather Service, Meteorological Observatory, 82383 Hohenpeissenberg, Germany.
³ Institute for Atmospheric Physics, University of Mainz, 55099 Mainz, Germany.
⁴ Scripps Institution of Oceanography, University of California, San Diego, CA 92093-0221, USA.
⁵ Joint Research Centre, Environment Institute, TP280, I-21020 Ispra (Va), Italy.
⁶ Max Planck Institute for Meteorology, Bundesstrasse 55, 20146 Hamburg, Germany.
⁷ Naval Research Laboratory, Monterey, CA 93943, USA.
⁸ Institute for Atmospheric Physics, German Aerospace Center (DLR), 82230 Wessling, Germany.
⁹ Department of Geophysics and Planetary Science, Tel Aviv University, Ramat Aviv, 69978 Israel.
¹⁰ Institute of Geophysics, Warsaw University, 02-093 Warsaw, Poland.
¹¹ Environmental Chemical Processes Laboratory, University of Crete, 71409 Heraklion, Greece.
¹² Institute for Marine and Atmospheric Research, Utrecht University, 3584 CC Utrecht, Netherlands.
¹³ Laboratory of Environmental and Climate Science, F-91191 Gif/Yvette Cedex, France.
¹⁴ Royal Netherlands Meteorological Institute, Post Office Box 201, 3730 AE De Bilt, Netherlands.
¹⁵ NOAA Aeronomy Laboratory, 325 Broadway R/AL7, Boulder, CO 80305, USA.
^*   To whom correspondence should be addressed. E-mail: lelieveld{at}mpch-mainz.mpg.de