Individual particle characteristics of North African dust under different long-range transport scenarios

After urban sources, mineral dust in Madrid is the second biggest contributor to PM 10, making up 40% on average, of total emissions. Approximately, 50% of the days on which the daily limit of 50 mu g m(-3) marked by the European Directive, are ascribable to Saharan outbreaks. The present study has focused on individual particle characterization of North African dust over Madrid by SEM/EDX, since no previous works on this type of characterization have been found in the region. More than 30,000 particles from 6 different samples have been measured to characterize 4 African episodes with very different meteorological scenarios, transport processes and source origins. Different samples from the same episode have also been characterized to evaluate homogeneity of dust characteristics over time. Silicates, mainly composed of clay minerals, are the main component, with abundances ranging from 65 to 85% by particle volume. Chemical cluster distribution of silicates has been linked to the major topsoil mineralogical composition in the origin of the episodes. Aspect Ratio (AR) has been used to compare particle morphology between episodes. AR values from samples taken under the same scenarios are statistically equal. For all the samples and size ranges AR values are found to be in the same order: AR(sulphates) > AR(silicates) > AR(carbonates). Particles not only maintained morphology during the episode. but also chemical composition, since clusters turned out to be very similar in samples taken on the same day and different days. Similarities and differences in particle chemical composition and morphology between the different transport patterns are discussed in detail throughout the paper. (C) 2008 Elsevier Ltd. All rights reserved.