[1] An extraordinary episode of fine particles (diameter mainly <2.5 mm) occurred in Helsinki, southern Finland, at the end of February 1998. The air masses came from the North Atlantic Ocean and passed over France, Germany, and southern Scandinavia. Particles were collected during the episode as well as before and after it. Uncoated particle samples were adhered to an indium substrate and were studied by a scanning electron microscope (SEM) coupled with an energy dispersive X-ray microanalyzer (EDX). A great proportion of the particles behaved differently than aerosols previously studied by microscopic techniques. The particles (size mainly 0.5-1 mm) did not exhibit solid shape. They were ''bubbling'' or ''pulsating'' continually, enlarging in one part and shrinking in another. Some particles were broken down, especially when the beam of the electron microscope was focused on them. EDX analyses showed that the particles contained much carbon together with oxygen, sulfur, and sodium. Ion analyses by ion chromatography revealed high concentrations of sodium, sulfate, nitrate, and ammonium. The particles were identified as marine sea-salt aerosols, which had accumulated anthropogenic emissions and lost chloride during their flow through continental polluted air. The shape fluctuations and the high carbon content observed by SEM/EDX led to the conclusion that the aerosols were enclosed by an organic membrane. Direct insertion probe/mass spectrometry investigations showed remarkable amounts of fragmented aliphatic hydrocarbons, which were considered as an evidence of a lipid membrane on the surface of the particles. The impact of the posited organic film on the properties of sea-salt particles, as well as on Earth's climate, is discussed.

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