ESTIMATION OF THE AVERAGE VISIBILITY IN CENTRAL-EUROPE

Visibility has been obtained from spectral extinction coefficients mea sured with the University of Vienna Telephotometer or size distributio ns determined with an Aerosol Spectrometer. By measuring the extinctio n coefficient in different directions, possible influences of local so urces could be determined easily. A region, undisturbed by local sourc es usually had a variation of extinction coefficient of less than 10% in different directions. Generally good visibility outside population centers in Europe is considered as 40-50 km. These values have been fo und independent of the location in central Europe, thus this represent s the average European ''clean'' air. Under rare occasions (normally r apid change of air mass) the visibility can be 100-150 km. In towns, t he visibility is a factor of approximately 2 lower. In comparison to t his the visibility in remote regions of North and South America is lar ger by a factor of 2-4. Obviously the lower visibility in Europe is ca used by its higher population density. Since the majority of visibilit y reducing particulate emissions come from small sources such as cars or heating, the emissions per unit area can be considered proportional to the population density. Using a simple box model and the visibilit y measured in central Europe and in Vienna, the difference in visibili ty inside and outside the town can be explained quantitatively. It thu s is confirmed, that the generally low visibility in central Europe is a consequence of the emissions in connection with human activities an d the low visibility (compared, e.g. to North or South America) in rem ote location such as the Alps is caused by the average European pollut ion.