Impact of the Saharan dust outbreaks on the ambient levels of total suspended particles (TSP) in the marine boundary layer (MBL) of the Subtropical Eastern North Atlantic Ocean

Six years (1998-2003) of measurements of ambient air concentrations of total suspended particulate (TSP) measured at a rural background monitoring station in Tenerife (Canary Islands), the El Rio station (ER, 28 degrees 08'35"N, 16 degrees 39'20"W, 500 m a.s.l.) were studied. African dust outbreaks were objectively identified using a new quantitative tool, called the African Index. This index indicates the percentage of time that an air mass remained over an African region at one of three possible height intervals of the lower troposphere. After identifying these episodes, a study of the background TSP levels at the ER station and of direct and indirect (those which cause vertical deposition of dust) African air mass intrusion impacts was performed. Taking into account both direct and indirect episodes, a total of 322 days of African dust intrusion were objectively identified (a mean of 54 episodes per year) in the period 1998-2003, some of them caused by "transition episodes" or "return African air masses". A subjective method confirmed that 256 of these days were caused by direct impacts of African dust on the ER station. A mean TSP value of 21.6 mu g m(-3) was found at the station during this period. All the episodes occurred when the TSP concentration was > 28.5 mu g m(-3). The TSP background (similar to 14 mu g m(-3)) can be assumed to be representative of the MBL of the Eastern North Atlantic subtropical region. The highest number of dust gravitational settlement (or indirect) episodes occurs in summer, but the highest contribution of these episodes to the TSP levels is in March with a monthly mean TSP contribution of up to 30.5 mu g m(-3). (C) 2007 Elsevier Ltd. All rights reserved.