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Towards the quantification of the contribution of fires to the North African dust emissions

Robert Wagner, Kerstin Schepanski
Leibniz Institute for Tropospheric Research, Leipzig, Saxony, Germany

 

Not only dust emissions from barren or unvegetated soil surfaces like deserts or uncultivated croplands are important sources of airborne mineral dust, also during fire events dust is entrained and appears to contribute noteworthy to the dust burden of the atmosphere. The underlying process, which drives dust entrainment during fires, is the so-called pyro-convection. High temperatures in the center of the fires result in an upward motion of the heated air. Subsequently, air flows towards the fire replacing the raising air. The resulting winds are able to mobilize soil and dust particles up to a size of several millimeters, depending of both the size and the strength of the fire. The particles are then mixed externally with the combustion aerosols into the convective updraft and were finally inject into altitudes up to several kilometers where they will be distributed and transported over long distances by the atmospheric circulation.

Several studies have shown that up to 80% of the mass fraction of the emitted particles during natural or prescribed fires is related to soil or dust particles.

Here, we present a first estimate of fire-induced dust emissions for North Africa. MODIS active fire and burned area products were analyzed and compared with MODIS land cover data in order to investigate the spatial-temporal distribution of frequently burned areas with regard to vegetation and land use. Based on these results, first steps towards a conceptual model linking the atmospheric dust concentration with active fire maps will be presented. This will include satellite observations and ground-based measurements of the dust as well as information on the fire activity in North Africa.

Because of the high fraction of prescribed fires in North Africa (mainly during the cultivation season), pyro-convection must be considered as an important contribution to the non-natural dust burden of the atmosphere. As the quantity of dust emitted during fire events is still unclear, pyro-convection is a source of uncertainty in estimates of the aerosol-climate feedback, which requires further investigation.