A Dustfall Episode on the Swedish West Coast, October 1987

Abstract

The author has made an analysis of particles brought down by rain on the Swedish west coast on 29 October 1987. The analyses included a quantitative study of the deposition, and a qualitative one including organic/inorganic relationship of particles, mineralogical composition and grain size distribution. The quantitative part indicated that during this rain the Lysekil region received about 0.6 g/m² of solid particles. The qualitative part showed that the dust consisted of single mineral grains and loose aggregates of mineral grains in addition to a few pollen grains and diatoms. Mineralogically, the dust was dominated by angular quartz grains and flakelets of mica. Median size of the single grains was 2.6 μm. The particles had two size maxima: at 0.9-1.1 μm and at 8–10 μm. Median size of the aggregates was 36 μm and the distribution was unimodal. The largest single quartz grain found was 22 μm in size. The aggregates with sizes ranging from 15 to 110 μm consisted of small mineral grains (0.5-15μm). Cemented together by iron oxides and hydroxides, this gave the dust its red colour. The results of the study, along with meteorological data and similar observations made in Western Germany on 28 October, lead to the conclusion that the material most likely originated in northern Africa. Dust mobilization was reported on 25 and 26 October around Oujda in Morocco as well as in Tlemcem and in the Béchar area in Algeria.