Dust deposition from air with anomalous characteristics

A singular event related with the fall of a considerable amount of dust with anomalous characteristics is reported here, as well as the physicochemical characteristics of samples collected at the site in Lisbon—Portugal. The dust fall occurred during a rainy autumnal overnight and the geographic delimitation of the affected area, as well as the physical state in situ of the powder samples, are not compatible with the one that would be expected considering the climatic conditions registered at that time. The collected samples have been investigated, particularly by modern techniques involving nanoparticle analysis. The results prove that the samples have a very heterogeneous and granulated composition whose nanoparticle size distribution is bimodal (centered, respectively, about 90 and 120 nm) and containing some nanowhiskers and nanorods. Taking into account the official meteo data, it is likely that dust must have been transported by air and dried in the place (after rain) by unusual extremely localized-heat wave. However, the back trajectories software simulations point to a geographic source of clouds which soil chemical composition has no correlation with the chemical composition of the sample and where no industrial-relevant source is assigned.