One area of concern is the Sahel, a vast semi-arid area south of the Sahara desert.

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"This is a crucial piece of information about an area that is now hyper arid but in the past must have been a lush river valley, " he explains.

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And arid conditions, especially during the summer, leave the area without much rainfall to rinse the muck out of the air.

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They include attractions as diverse as the Great Barrier Reef Marine Park, the Uluru-Kata Tjuta National Park (which includes Ayers Rock) in the arid red heart of Australia, and the temperate Tasmanian Wilderness World Heritage area.

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