J. Lelieveld,^1* P. J. Crutzen,¹² V. Ramanathan,² M. O. Andreae,¹ C. A. M. Brenninkmeijer,¹ T. Campos,³ G. R. Cass,⁴ R. R. Dickerson,⁵ H. Fischer,¹ J. A. de Gouw,⁶ A. Hansel,⁷ A. Jefferson,⁸ D. Kley,⁹ A. T. J. de Laat,⁶ S. Lal,¹⁰ M. G. Lawrence,¹ J. M. Lobert,² O. L. Mayol-Bracero,¹ A. P. Mitra,¹¹ T. Novakov,¹² S. J. Oltmans,⁸ K. A. Prather,¹³ T. Reiner,¹ H. Rodhe,¹⁴ H. A. Scheeren,⁶ D. Sikka,¹⁵ J. Williams¹

The Indian Ocean Experiment (INDOEX) was an international, multiplatform field campaign to measure long-range transport of air pollution from South and Southeast Asia toward the Indian Ocean during the dry monsoon season in January to March 1999. Surprisingly high pollution levels were observed over the entire northern Indian Ocean toward the Intertropical Convergence Zone at about 6°S. We show that agricultural burning and especially biofuel use enhance carbon monoxide concentrations. Fossil fuel combustion and biomass burning cause a high aerosol loading. The growing pollution in this region gives rise to extensive air quality degradation with local, regional, and global implications, including a reduction of the oxidizing power of the atmosphere.

¹ Max-Planck-Institute for Chemistry, Post Office Box 3060, D-55020 Mainz, Germany.
² Center for Atmospheric Sciences, Scripps Institution of Oceanography, La Jolla, CA 92093-4922, USA.
³ National Center for Atmospheric Research, Boulder, CO 80303, USA.
⁴ School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332-0340, USA.
⁵ Department of Meteorology, University of Maryland, College Park, MD 20742, USA.
⁶ Utrecht University, 3584 CC Utrecht, Netherlands.
⁷ University of Innsbruck, A-6020 Innsbruck, Austria.
⁸ Climate Monitoring and Diagnostics Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80303, USA.
⁹ Research Centre Jülich, ICG-2, D-52428, Jülich, Germany.
¹⁰ Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India.
¹¹ National Physical Laboratory, New Delhi 110 012, India.
¹² Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
¹³ University of California, Riverside, CA 92521-0403, USA.
¹⁴ Department of Meteorology, Stockholm University, S-10691 Stockholm, Sweden.
¹⁵ 40 Mausam Vihar, New Delhi 110051, India.
^*   To whom correspondence should be addressed. E-mail: lelieveld{at}mpch-mainz.mpg.de