Saharan dust outbreak over southeastern Spain as detected by sun photometer

Abstract

Characterization of mineral aerosol properties in southeastern Spain is important due to its proximity to the African continent that permit studying the Saharan dust just entering European continent before its possible mixing with anthropogenic emissions in Europe. Two Saharan dust events that occurred during 30 August–3 September and on 10 September 1998 were observed at an inland location on the outskirts of Granada in southeastern Spain. Detailed analyses including back trajectories and TOMS aerosol index maps showed transport of Saharan dust from the Sahara desert and North Africa to the study area. The dust episodes were associated with marked increases in aerosol optical depth ( δ a) at all wavelengths examined in this study. Thus δ a (500 nm) increased from a value of ∼0.2 corresponding to normal conditions up to 0.6 in the first event and up to 0.55 in the second. At the end of these events, the arrival of air masses from the Atlantic caused a sharp decrease in δ a (500 nm) that reaches values close to those obtained before the event (∼0.2). The Angström exponent α reached a minimum of 0.36 in the first event and 0.37 in the second. The Angström exponent α and aerosol optical depth values during dust events agree well with those obtained under the same kind of events in AERONET and SKYNET sites, as well as with the results obtained in ACE-2 and ACE-Asia experiments. The aerosol size distributions, retrieved from aerosol optical depth using King's method, demonstrated how the large size fraction of aerosol associated with Saharan dust dominated during these events. When Saharan dust was present, the retrieved aerosol size distributions were bimodal with a well-defined mode centred at a radius of 0.6 μm, and showed an evident increase in the large particles mode with radii in the range 0.4–2 μm. The small particle concentration during the two events did not present any marked change, and was similar to those observed on days without Saharan dust. Large particle concentration was higher by a factor of 8 than during normal conditions.