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The extinction coefficient of the atmospheric aerosol has been determined in remote locations and the following results were obtained.

(a) Europe: on days with excellent (European) visibility the extinction coefficient usually has a value of 0.03 km-1, on average days values around 0.1 km-1 are typical. It is interesting to note, that these values were found both in many location of the Austrain Alps and at a rural site 70 km south of the heavy industrialized Ruhr district in Germany. This indicates that the aerosol found at remote sites in Europe apparently is the average pollution spread over a large area.

(b) North America: spectral extinction coefficients were measured at a rural site north of Denver, Colorado, and at a site in the desert of northern Arizona in the summer of 1979. Usually the extinction coefficients had values between 0.016 and 0.04 km-1. At the rural site in Colorado an influence of the urban plume of Denver could be noticed, since in most cases the values for sight paths towards the town were lowest, whereas in the desert site in Arizona the same extinction coefficient was measured in all directions. A severe smog episode in Los Angeles (more than 700 km to the west) caused an increase of the extinction coefficient by more than a factor of two at this remote location.

(c) South America: in the Cordillera de los Andes near Santiago de Chile extinction coefficients have been determined at elevations above 3000 m. Values increasing from 0.018 km-1 in the morning to 0.15 km-1 in the afternoon have been found. This mainly depends on the height of the haze layer, which usually increases from below 2000 m in the morning to above 4000 m in the afternoon. The low extinction coefficients in the morning still are very close to the background value measured at the Easter Island in the South Pacific.

From this it can be concluded that an unpolluted atmosphere only exists on rare occasions. The usual background at least in Europe is a more or less diluted average pollution.