The Asian brown cloud is a layer of air pollution that covers parts of South Asia, namely the northern Indian Ocean, India, and Pakistan. Viewed from satellite photos, the cloud appears as a giant brown stain hanging in the air over much of South Asia and the Indian Ocean every year between January and March, possibly also during earlier and later months. The term was coined in reports from the UNEP Indian Ocean Experiment (INDOEX).
In some humidity conditions, it forms haze. It is created by a range of airborne particles and pollutants from combustion (e.g. woodfires, cars, and factories), biomass burning and industrial processes with incomplete burning. The cloud is associated with the winter monsoon (November/December to April) during which there is no rain to wash pollutants from the air.
This pollution layer was observed during the Indian Ocean Experiment (INDOEX) intensive field observation in 1999 and described in the UNEP impact assessment study published 2002. Scientists in India claimed that the Asian Brown cloud is not something specific to Asia. Subsequently, when the United Nations Environment Programme (UNEP) organized a follow-up international project, the subject of study was renamed the Atmospheric Brown Cloud with focus on Asia.
The cloud was also reported by NASA in 2004 and 2007.
Although aerosol particles are generally associated with a global cooling effect, recent studies have shown that they can actually have a warming effect in certain regions such as the Himalayas.
One major impact is on health. The 2002 study indicated nearly two million people die each year in India alone from conditions related to the brown cloud.
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