Identification of dust sources using satellite data

The knowledge on location and activity of mineral dust sources is a necessary prerequisite for an accurate estimate of the multifaceted dust impacts on weather, climate, and ecosystems. Airborne dust particles interact with radiation directly because of their optical characteristics such as scattering and absorption, and indirectly because of their physicochemical properties as dust particles may act as cloud condensation nuclei and thus change cloud characteristics and precipitation rates.

Airborne mineral dust is further affecting our modern way of life. Visibility reductions by dust plumes lead to closures of airports and roads. Suspended in the air we breathe in, cases of respiratory diseases increase during periods of increased dustiness. Due to its dimming effect, dust plumes further reduce the efficiency of solar power plants, but also the reliability of electrical devices in general is restricted during periods of increased dustiness.

Interactions and influences on dust entrainment

The entrainment of mineral dust into the atmosphere is linked to meteorology and surface properties. Wind erosion and mobilization of soil particles can be understood as a threshold problem, whereby high wind velocities are necessary to entrain dust particles into the atmosphere. Thereby, the emission efficiency of dust sources varies between different soil types. Arid and semi-arid areas, where vegetation is sparse or absent, are in particular susceptible for wind erosion, and thus preferable dust sources. 

Uncertainties in knowledge on dust source characteristics with regard to the spatio-temporal distribution, but also with regard to physicochemical composition and optical properties, still contribute to the uncertainty on dust radiative forcing and climate feedbacks. Thus, detailed knowledge on active dust sources and their characteristics is a necessary prerequisite to improve the understanding of the complex processes forming the Earth system.

Investigation of dust sources

The vast extent of the North African desert, its limited accessibility, and its sparse population reduces the feasibility of human exploration of individual dust sources regions significantly. Here, satellite observations provide useful information on a reasonable high spatial and temporal resolution covering entire North Africa. Sensors flying on-board various satellites provide observations at different spatial and temporal resolution, mainly depending on the satellite’s orbit. Satellites flying on Sun-synchronous orbits such as the NASA’s A-Train satellite chain overpass the Equator at a given longitude twice-daily at around 1:30am and 1:30pm and acquire data along the track with a viewing swath width of about 2,300 km for MODIS, and about 2,600 km for OMI. This provides an almost global daily coverage.

Geostationary satellites such as EUMETSAT’s Meteosat Second Generation (MSG) satellite provide observations for a stationary field of view. For MSG, the hemisphere between around 70°W and 70°E, 70°S and 70°N is covered. Due to its stationary field of view, observations can be acquired at relatively high temporal resolutions. E.g., measurements from the Spinning Enhanced Visible and Infra-Red Imager (SEVIRI) instrument are provided at 15-minute temporal resolution on a spatial resolution of 3km × 3km at nadir.

To identify dust sources in arid and semi-arid regions, different approaches can be applied: the back-tracking method, and the threshold-frequency method.