Estimation of the average visibility in central Europe

Visibility has been obtained from spectral extinction coefficients measured with the University of Vienna Telephotometer or size distributions determined with an Aerosol Spectrometer. By measuring the extinction coefficient in different directions, possible influences of local sources could be determined easily. A region, undisturbed by local sources 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 found independent of the location in central Europe, thus this represents the average European “clean” air. Under rare occasions (normally rapid change of air mass) the visibility can be 100–150 km. In towns, the visibility is a factor of approximately 2 lower. In comparison to this the visibility in remote regions of North and South America is larger by a factor of 2–4. Obviously the lower visibility in Europe is caused by its higher population density. Since the majority of visibility 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 visibility measured in central Europe and in Vienna, the difference in visibility inside and outside the town can be explained quantitatively. It thus is confirmed, that the generally low visibility in central Europe is a consequence of the emissions in connection with human activities and the low visibility (compared, e.g. to North or South America) in remote location such as the Alps is caused by the average European pollution.