The Hindu Business Line : Bay of Bengal most polluted between monsoons: Study

RESEARCHERS at the Max Planck Institute for Chemistry, Germany, have found pollution plumes drifting in from Africa and Southeast Asia pollute Central Indian Ocean in the southern hemisphere during monsoon transition periods.

The most polluted region is the Bay of Bengal, which is influenced by Indian and Southeast Asian outflow during most of the year and China during part of the year. The Bay is more strongly influenced by continental emissions than the Arabian Sea, which was the focus of the Indian Ocean Experiment (INDOEX).

This experiment was an international field campaign during the winter monsoon period in 1999 to study how air pollution affects climate processes over the tropical Indian Ocean. Satellite pictures showed a thick haze - one of the now well-known `Atmospheric Brown Clouds' - that spreads thousands of kilometres south of India during this period.

In the present instance, however, the Max Planck Institute used a combination of satellite observations and computer modelling to study nitrogen oxides pollution over the Indian Ocean. The region is dynamically and chemically active because of the strong tropical sunlight, high humidity and the increasing anthropogenic emissions.

Research led by the institute during the summer of 2001 showed that the same monsoon storms which produce torrential rains also lift insoluble gases like carbon monoxide into the upper troposphere, where they are then transported towards the Mediterranean. The main maritime nitrogen oxide sources are long-range transport of continental emissions, lightning and ships.

New data has shown pronounced, semi-regular plumes of nitrogen oxide pollution extending all the way across the central Indian Ocean, mainly in the middle troposphere, originating from Africa in the west and from Southeast Asia (Indonesia and other countries) in the east.

The results suggest that the Central Indian Ocean is not always "as pristine as found during the winter monsoon", according to Dr Mark Lawrence, leader of the research group. It is especially interesting that during this period, pollution levels are exactly the opposite of the situation encountered during INDOEX.

The research has also taken a more general view of how emissions from the surrounding continental regions influence nitrogen oxide pollution over the Indian Ocean. This work shows that feedbacks in atmospheric chemistry can result in downwind regions being highly insensitive to upwind emissions: that is, a 50 per cent reduction in global emissions of nitrogen oxides - from all sources - might only lead to a 15 per cent reduction in the nitrogen oxide levels over parts of the Indian Ocean - and vice versa for increases in nitrogen oxides emissions.

The results show that the influence of Indian emissions on the Central Indian Ocean is limited to the winter monsoon period only, and is nevertheless quite weak - a 10 per cent change in Indian emissions only leads to a three per cent change in northern Indian Ocean nitrogen oxide levels.

This is in contrast to the strong effect of the Indian outflow on aerosols and long-lived gases like carbon monoxide and is due to the short lifetime of nitrogen oxide (about a day on average).

Generally, the strongest influence on the Central Indian Ocean nitrogen oxide chemistry throughout the year is from Southeast Asia. African emissions are important especially at higher altitudes.