Long-range transport of Saharan dust over Northern Europe: EUCAARI 2008 campaign

The evolution of dust optical properties is illustrated in this paper through a case of long-range transport of Saharan dust over northwestern Europe during the European Integrated Project on Aerosol-Cloud-Climate and Air Quality Interactions (EUCAARI) experimental campaign in 2008. This spread of dust over northwestern Europe is investigated by combining satellite, airborne, ground-based observations and the non- hydrostatic meso-scale model Meso-NH. The total dust amount emitted during the study period is estimated to 185 Tg. The analysis of the removal processes reveals that only 12.5 Tg is lost by dry deposition, and that wet deposition is the main process of dust removal (73 Tg). The observed aerosol optical thickness ranged from 0.1 to 0.5 at the wavelength of 440 nm, with a maximum value close to 1 is found over the Netherlands (51.97 N, 4.93 E). Over that site, the main dust layer is located between 2.5 and 5.2 km above sea level (asl), moreover dust was also present at 0.9 km asl. The nephelometer measurements on board the ATR-42 aircraft revealed a strong wavelength dependence of the scattering coefficient over the Netherlands. The Angstrom exponent is greater than 0.5, whereas usually it approaches zero in presence of Saharan dust. This is due to high precipitation scavenging efficiency for the coarse mode, particularly below 4 km. Our results confirm that atmospheric conditions govern the life cycle of dust microphysical phenomena, providing conditions for transformation processes during transport, and removal of particles from the atmosphere.