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INDOEX finds air pollution affects climate

Air pollutants have a dramatic impact on climate, according to the preliminary results of the Indian Ocean Experiment (INDOEX), a field experiment to study how air pollution affects climate processes over the tropical Indian Ocean.

INDOEX involves the United Kingdom, France, ...
Air pollutants have a dramatic impact on climate, according to the preliminary results of the Indian Ocean Experiment (INDOEX), a field experiment to study how air pollution affects climate processes over the tropical Indian Ocean.

INDOEX involves the United Kingdom, France, Germany, India, The Netherlands, Sweden, and the USA. An international group of over 150 scientists is conducting the field experiment.

The support of EUMETSAT, the European meteorological satellite organization, took the form of METEOSAT-5, a stand-by satellite, which was re-located to a position at 63°E. The re-location manoeuvres started in January 1998, and took around 130 days. After some mission tests last June, the routine INDOEX mission started in July.

Asia and the Indian subcontinent, which together have a population of over 2 billion people, emit large quantities of pollutants that can be carried to the Indian Ocean during the northern hemisphere winter by monsoon winds from the north-east. INDOEX is investigating how these pollutants are transported through the atmosphere and how they affect the atmospheric composition and solar radiation processes over the ocean. A major objective is to estimate the climate effects of man-made airborne particles.

Preliminary results show that air pollutants dramatically impact this region. Scientists were surprised to find that a dense brownish pollution haze extended from the ocean surface to an altitude of between 1 and 3km. The haze layer covered much of the research area almost constantly during the six-week intensive experiment. The affected area includes most of the northern Indian Ocean, including the Arabian Sea, much of the Bay of Bengal, and the equatorial Indian Ocean to about 5 degrees south of the equator. The haze is caused by high concentrations of small particles, also known as aerosols, with sizes mostly less than a few micrometres in diameter. The haze particles are primarily composed of soot, sulphates, nitrates, organic particles, fly ash and mineral dust.

Because of this pollution, visibility over the open ocean was often under 10km, a range that is typically found near polluted source regions of the United States and Europe.

The haze layer also contains relatively high concentrations of gases, including carbon monoxide, various organic compounds, and sulphur dioxide. The concentrations of these gases are conclusive evidence that the haze layer is caused by pollution. Haze particles scatter solar radiation, a process that reduces the amount of sunlight reaching the ocean surface, thus reducing the amount of solar energy that would otherwise heat the Earth-atmosphere system. In the polluted INDOEX region, the haze particles reduce the solar radiation absorbed by the ocean surface by as much as 10%.

The airborne particles over the northern Indian Ocean are unusually dark because they contain large amounts of soot and other materials from incompletely burned fuels and wastes. Dark aerosols lead to the increased absorption of solar radiation, a process that can affect the dynamic properties of the atmosphere in ways that could alter weather and climate.

The pollution aerosols can affect both visibility and cloud formation because water vapour condenses on the pollution particles. Over the Arabian Sea and perhaps in other regions of the northern Indian Ocean, low-level clouds are typically imbedded in haze so that they are often obscured.

Scientists are trying to estimate to what extent the haze layer is altering cloud properties, such as the concentrations of cloud droplets, the development of rainfall, and cloud brightness.

Changes in cloud brightness are important in climate processes because these can alter the amount of radiation that is reflected back to space.

INDOEX scientists were surprised to find such a dense pollution haze layer derived from sources at least a thousand or more km away. The scientists suggest that the pollution events observed in INDOEX may be symptomatic of large-scale pollution transport that may be occurring in other regions of the Earth. In contrast to the situation over the northern Indian ocean, the lower atmosphere over the southern Indian Ocean remains remarkably clean, thanks to the InterTropical Convergence Zone (ITCZ) which is found around 5-10 degrees south of the Equator at this time of the year. A narrow zone of deep and towering thunderstorms forming over the warmest part of the equatorial ocean, the ITCZ intercepts polluted air masses and removes much of the pollution in rainfall. But the ITCZ clouds can also move substantial amounts of pollutants into the upper atmosphere, where they can be spread over large areas.

INDOEX is the first major international program to study the transport and effects of pollutants between their source and their removal over this remote ocean region. INDOEX will provide a wealth of important and unique data for global climate-modelling purposes to assess the impact on climate. These measurements in the Indian Ocean are also important because they characterize emissions from the rapidly emerging economies in this region. We might expect to see increasing emissions of pollutants over the Indian Ocean and in other parts of the globe, as similar economies continue to grow.

The INDOEX Web site is at:

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