UCL Discovery

Abstract

Mineral dust in the atmosphere is an important component of the climate system but is poorly quantified. The Bodélé depression of Northern Chad stands out as the world’s greatest source region of mineral dust into the atmosphere. Frequent dust plumes are a distinguishing feature of the region’s climate. There is a need for more detailed information on processes of dust emission/transport and dust optical properties to inform model simulations of this source. During the Bodélé Dust Experiment (BoDEx) in 2005 instrumentation was deployed to measure dust properties and boundary layer meteorology. Observations indicate that dust emission events are triggered when near surface wind speeds exceed 10ms-1, associated with synoptic scale variability in the large-scale atmospheric circulation. Dust emission pulses in phase with the diurnal cycle of near surface winds. Analysis of dust samples shows that the dust consists predominantly of fragments of diatomite sediment. The particle size distribution of this diatomite dust estimated from sun photometer data, using a modified Aeronet retrieval algorithm, indicate a dominant coarse mode (radius centred on 1-2?m) similar to other Saharan dust observations. Single scattering albedo values are high, broadly in line with other Saharan dust even though the diatomite composition of dust from the Bodélé is likely to be unusual. The radiative impact of high dust loadings results in a reduction in surface daytime maximum temperature of around 7°C in the Bodélé region. Using optical and physical properties of dust obtained in the field we estimate the total dust flux emitted from the Bodélé to be 1.18±0.45Tg per day during a substantial dust event. We speculate that the Bodélé depression (~10,800km2) may be responsible for between 6-18% of global dust emission, although the uncertainty in both the Bodélé and global estimates remain high.