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"Black episode" and "white episode" are terms used for situations when high concentrations of dark or colorless particles, respectively, are transported in over Sweden from distant sources. Particles in black episodes come from the European continent. They contain, ia, ammonium sulfate and nitrate, and are slightly acid. Black episodes are at times accompanied by increased concentrations of sulfur dioxide and nitrogen dioxide. Characteristic of white episodes are small particles consisting mainly of acid ammonium sulfates, probably formed photochemically over the North Sea during dry and sunny weather. Air flowing into Sweden from the north contains low concentration of particles consisting mainly of common salt and neutral ammonium sulfate.
Ambio © 1976 Royal Swedish Academy of Sciences