Occasionally we get warnings of dust blowing in from the Sahara, often at the height of summer when people with sensitive lungs are warned to stay indoors. this suggests that the dust is an unusual arrival, but it seems this is not the case. This is from yesterday’s i paper by Jacob Adetunji:
At this time of year you may have noticed something in the air. Dust-laden winds from the Sahara cause “seasonal haze episodes” that can result in mud rain. In West Africa, which experiences haze episodes between November and March, the dust-laden trade wind is called the harmattan and has implications for soil fertility, radio communication, visibility at airports, and on livelihoods and health. Dry skin and cracked lips are symptoms.
The dust presents more serious problems for asthmatics and can lee=ad to silicosis, a lung disease caused by a high content of quartz. In some areas the dust blocks the sunlight enough to make temperatures drop significantly.
Two engines have been identified as the sources of the Saharan dust: the Bodele Depression in north-eastern Chad, which is part of the now dried up Lake Megadhad – the most intense dust source in the world – and the Tibesti mountain region in northern Chad. Dust particles come from rocks in the Tibetsi region broken down by weathering, erosion and pulverisation.
It has been estimated that 400m to 700m tons of dust are transported from the Sahara every year. And the north-easterly harmattan carries this as far as Barbados and Miami. As much as 20% reaches the Amazon – and satellite data has been used to calculate that some 27.7m tons are deposited over the Amazon basin, playing a crucial role in fertilising the still when the wind changes to south-westerly in early spring, the dust is carried across the Mediterranean as far as northern Europe and the UK.
Natural mineral dust affects the climate, as well as fertilising aquatic and terrestrial ecosystems. Studies have shown that the dust is rich in phosphorous, the main source of nutrients for the Amazon. But to understand its full impact we also need to know the level of iron oxide minerals – and iron- within it. Iron exists ini 2 oxidation states in minerals: ferrous (Fe2+) and ferric (FE3+). Ferrous iron is soluble and can be absorbed by plants whereas ferric iron is insoluble. their relative abundance has implications for the fertility of the soils on which the dust settles.
The bulk mineralogical composition of the Bodele Depression is quartz. And using a Moessbuer spectrometer… I found that harmattan dust from the Bodele Basin contains d 3 varieties of iron-bearing clay minerals that were all seriously deficient of the more desirable ferrous irons for healthy plant growth and had greater levels of highly-insoluble ferric irons.
The impact of this depends on the type of soil that the dust lands on. One study shows iron was helpful to maize farmers in northern Nigeria while a 2013 study shows that it was a threat to rice in several tropical savannah areas. But a low ferrous content can still help fertilise soils because of it alkaline range – large amounts of Saharan dust on soils can improve more acidic soil whose pH is too low of plant growth, whereas more alkaline soils should benefit from the addition of iron.
The harmattan dust is blown over predominantly rural regions of West Africa, but the Saharan dust in Europe blows over regions of heavy industry and can mix with industrial pollutants. A report on the health effects of the Saharan dust in southern Europe showed that in Barcelona and Madrid, cardiac mortality during Sahara dust days was statistically significantly higher compared with dust-free days. Similar trends were observed in Greece and Italy.
the full article is at the conversation.com