Precipitation water has been generally sampled for subsequent chemical analysis in one of the following two ways. The first procedure consists of collecting the precipitation in an open rain gauge. In this case the samples also contain a part of the dry deposition. The second is the use of so-called automatic collectors, which are closed in dry periods and open only during rainfall. The second sampling method is obviously better. In spite of this fact, open samplers are frequently used because of their simplicity. The results of precipitation analysis3 are generally expressed in mg 1"1 or ppmm which are numerically equivalent.
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