Mixing and Deposition Processes during Transatlantic Transport of Saharan Dust

Bernd Heinold, Kerstin Schepanski, Max Ulrich, Moritz Haarig, Franziska Rittmeister, Ina Tegen
Leibniz Institute for Tropospheric Research (TROPOS), Leipzig, Germany

Mineral dust from arid and semi-arid regions plays an important environmental role due to its ability to alter the Earth’s energy budget by aerosol-cloud-radiation interactions as well as due to its impact on the biogeochemical cycle and air quality. The Sahara desert is the world’s main dust source contributing at least 50% to the global dust load. Large amounts of dust are carried towards the Caribbean within the Saharan Air Layer (SAL), with maximum transport in late boreal spring and early summer. During long-range transport, the dust particles are transformed by aging and mixing, which may have significant but as yet unquantified effects on the dust impact on radiation, cloud properties, and the biogeochemical processes of ecosystems.

This study focuses on the important role of mixing and deposition processes on the distribution, lifetime, and particle properties of mineral dust. Regional dust modelling is used to investigate the long-range transport of Saharan dust across the Atlantic Ocean towards the Caribbean. More specifically, we address the questions of (1) how the Saharan dust export towards the Caribbean region is influenced by the atmospheric circulation over West Africa and (2) which role the different removal and mixing processes play during long-range transport? In addition to the potential importance of sporadic downward mixing, a particular focus is on the effect of non-sphericity on gravitational settling of dust particles. The emission, transport, dry and wet deposition of Saharan dust as well as the effect of dust radiative forcing are simulated with the regional model COSMO-MUSCAT. The COSMO-MUSCAT simulations are combined with trajectory analysis to study particle aging and dust-cloud interactions. The model results are compared against various standard observations. In the Caribbean and across the tropical Atlantic, field measurements are available for model evaluation from the transatlantic cruise of the research vessel Meteor in May 2013 and from the Saharan Aerosol Long-Range Transport and Aerosol-Cloud Interaction Experiment (SALTRACE) at Barbados Island in June to July 2013.

First results show that as the source activity, dust deposition is driven by the atmospheric circulation patterns over West Africa. Convective mixing controls dry deposition in the tropics and can explain sporadic deposition events in the subtropics. In contrast to previous, mainly global studies, considering dust particle non-sphericity has a significant impact on dry deposition fluxes. Overall, this study provides an improved model-based assessment of the varying contribution of Saharan dust to the aerosol burden across the Atlantic Ocean.