Modern Saharan dust collected onshore and offshore the northwest African margin : sources, transport and deposition

Abstract

Hundreds to thousands of tons of mineral dust are emitted from the eartha s deserts to the atmosphere each year. Subsequently, mineral dust can be transported over thousands of kilometres with the prevailing winds in the atmosphere until it is deposited on land or in the ocean. On the one hand, the atmospheric dust transport and deposition has an important impact on human health and the eartha s climate system. On the other hand, the dust emission, transport and deposition is controlled by parameters of climate change such as precipitation, wind speed, temperature and vegetation cover. Last but not least, the sensitivity of mineral dust properties to environmental parameters is used as a proxy to reconstruct the climate of the past which is required to test climate models. The large mineral dust emissions, ita s relation to climate and impact on human health underscores the importance to understand the dust cycle. However, research gaps still exist with respect to modern emission, transport and deposition of mineral dust. The Saharan desert is the worlda s largest source of atmospheric mineral dust and has been studied at least since the 18th century. Up to now, the major potential dust source areas are known and it has been unravelled that precipitation results in a decrease in the particle size of deposited Saharan dust on the NW African margin. Further, it is well-known that the seasonal change in insolation on the African continent results in a seasonal change in the altitude of Saharan dust transport. However, research gaps still exists with respect to the influence of the seasonal change in the dust transport on the resulting particle size, mineralogical composition and provenance of Saharan dust. Therefore, the aim of the present Ph.D. thesis was to investigate whether the seasonal change in atmospheric circulation results in a seasonal variation in particle size, mineralogical composition and provenance of Saharan dust. The strategy was to measure the particle size and mineralogy of modern Saharan dust collected during several years onshore NW Africa in continental dust traps and offshore NW Africa in marine sediment trap moorings. By comparing the particle size and mineralogy data to concurrent meteorological data and satellite images, possible connections between the dust properties and meteorological conditions were inferred. Dust which deposited offshore NW Africa was transported with the trades in winter and in the Saharan air layer (SAL) during summer. The particle size of the deposited dust varied seasonally according to dust-storm events year-round, trade-wind speed in winter and wet deposition from the SAL in summer. A seasonal change in the dust mineralogical composition and provenance was observed which could be related to the seasonal change in the dust transport. Onshore NW Africa, the particle size of deposited dust varied seasonally according to meteorological parameters depending on the location of the sampling site. In the major dust source area at 20 AdegreeN, dust was predominantly transported in the trade wind layer year-round. Therefore, the particle size varied seasonally according to the trade wind speed and the mineralogical composition and dust sources according to the direction of the trade winds. In contrast, the particle size of Saharan dust deposited outside a major dust source area at 15 AdegreeN was controlled by the dust transport distance during spring and by the wet deposition from the SAL during summer.