Atmospheric aerosol produce a significant influence on the Earth radiative budget, through scattering and absorption of radiation (direct forcing), and by influencing the cloud nucleation processes and the cloud microphysical properties (indirect forcing). Tropospheric aerosols are among the most uncertain elements in the estimation of the planetary radiative budget because of the high variability of their characteristics and the complex phenomena in which they are involved. Some aerosol types, like sulphates (and in particular stratospheric particles after major volcanic eruptions), scatter a significant fraction of the solar radiation, producing an increase of the planetary albedo and a cooling of the lower atmosphere. On the other hand, strongly absorbing particles, like carbonaceous aerosol and dust, in particular conditions of production and distribution, may induce warming effects on the atmosphere.
Mineral or crustal (dust) particles are among the principal constituents of tropospheric aerosols. It is estimated that a fraction ranging between 30 and 50% of the total mineral aerosols are anthropogenic (i.e. produced in soils which have been disturbed by human activity). Continental aridity is indicated as the main cause of increased dust flux from the deserts, that constitute the most relevant source of these particles. Hyper arid regions however (mean annual precipitaion <80 mm) are less efficient dust sources than arid regions. The mobilization of dust in the atmosphere is due to wind erosion in arid regions, and the average size of transported particles depends on wind strength; dust mobilization also depends on the nature of soil and other parameters, and complex mechanisms, as saltation of relatively large dust grains, are involved.
Anthropic activity, through modification of the land use and/or by inducing changes of climate (primarily through changes of precipitation and temperature regime), may contribute to the production of mineral aerosols. The Sahara desert is one of the main sources of mineral aerosols: the dust particles are captured by the wind at the surface, are raised to considerable altitudes in the troposphere by the strong convective regimes that develop over the desert, and may be transported to large distances. Saharan dust is commonly observed over southern Europe, and, at far distances, in the Carribean, and South America. In rare occasions, Saharan dust reaches northern Europe.
A case of desert dust over Lampedusa observed from space.
North and central African regions have suffered severe droughts in the recent past, and an increase of the dust export has been correspondingly observed. The droughts appear to be connected to large scale phenomena occuring in the ocean-atmosphere system, like El Niño and the North Atlantic Oscillation (NAO). A correlated behavior of the dust export from Sahara to the North Atlantic and to the Mediterranean with these phenomena has been also observed. Dust aerosols strongly influence the radiative balance, and affect the solar irradiance reaching sea and land surface. In its turn, a change of the solar irradiance may influence evaporative fluxes and, on a basin-wide scale, its hydrologic budget.
In the Mediterranean dust particles, that are non-hygroscopic and are not expected to interact with clouds, may encounter and mix with different aerosol types. Continental and anthropogenic particles originating from Europe, as well as marine aerosols from the North Atlantic and the Mediterranean itself, are commonly present in the basin. Dust particles coated with sulphate have been recently observed; mixing of the dust with sulphate is believed to occur as a consequence of cloud evaporation processes. In this way, dust particles become hygroscopic, and may influence the cloud formation and properties. Saharan dust constitutes one of the most relevant inputs of trace elements to the Mediterranean, and an important source of nutrients for oceanic microorganisms. Desert aerosols may also affect the precipitation acidity.
Lampedusa is often reached by desert dust from the Sahara, and is an extraordinary site for he study of its propagation, properties, and radiative effects.
Daily evolution of the lidar backscatter ratio (is equal to 1 when no aerosols are present) on three days characterized by different aerosol conditions. The airmass trajectories ending at Lampedusa on the three days (labeled A, B, and C) are indicated in the right graph. The top graph on the left corresponds to a Saharan dust event, as is also confirmed by the trajectory.
At Lampedusa atmospheric aerosols are measured by Multi Filter Rotating Shadowband Radiometer, Cimel CE-318 sun-photometer, and lidar. Observations have shown that the aerosol oprical properties and vertical distribution significantly depend on the origin of the airmasses. Air masses originating from Sahara are generally characterized by large values of the aerosol optical depth, low values of the Ångström exponent, and particles may be found up to about 8 km (also depending on the season).
Evolution of the aerosol optical depth at 415 (red circles) and 868 nm (black circles), bottom graph, and of the Ångström exponent, top graph, during May and June 1999. Values of the optical depth larger than 0.2 at 868 nm are generally associated with transport of Saharan dust.
Desert dust has been observed to produce a reduction of the surface shortwave irradiance as large as 50 W m-2. Several studies, aimed at determining the optical properties and radiative effects of the different types of aerosols that may be observed at Lampedusa, have been conducted, also integrating satellite observations. Estimates of the daily average direct radiative forcing at the top of the atmosphere in the 300-800 nm interval for unit aerosol optical depth at 500 nm range from -3.4 and +8.3 W m-2 for continental/marine aerosols, and from –3.3 and –20 W m-2 for desert dust. At the surface, for the same spectral range (and for unit optical depth at 500 nm) we obtain a direct forcing efficiency between -141 and -114 W m-2 for continental/marine aerosols, and between –76 and –70 W m-2 for desert dust. Due to the much larger optical depth, however, desert dust generally produces a larger radiative impact at the surface.