Summary
The atmospheric aerosol scattering coefficientσ s , measured for more than a year more or less continuously in Vienna, Austria, exhibits unexpected patterns of variation. Apart from the usual ones following changes in relative humidity or traffic characteristics,a distinctive pattern is found before a change in air mass.σ s rises by a factor of 1.5 to 2 some hours (usually two or three) before the passage of the front without a corresponding change in emission characteristics or relative humidity and then falls either below or to its previous level. This behaviour ofσ s occurred at all frontal passages during the sampling period at all times of day and of year except when the wind speeds were very high.
An explanation is attempted by examining the mixing heights before a change in airmass since a reduced vertical dispersion due to pre-frontal changes of stability could account for the increase inσ s (and thus the aerosol concentration). It has been found that calculated mixing heights are reduced by nearly the same factor as the value ofσ s is increased before the front. After the front the factors are similar, but then the aerosol concentration depends also on the origin of the air mass.
Zusammenfassung
Der Streukoeffizient des atmosphärischen Aerosols (σ s ) wurde in Wien mehr als ein Jahr lang mehr oder weniger kontinuierlich gemessen. Dabei zeigten sich unerwartete Änderungen. Abgesehen vom üblichen Tagesgang im Zusammenhang mit Verkehr und relativer Feuchte fand sich vor einem Luftmassenwechsel ein charakteristischer zeitlicher Verlauf.σ s steigt einige Stunden (meist zwei oder drei) vor dem Frontdurchgang an, ohne daß sich die relative Feuchte oder die Quellencharakteristik entsprechend ändert, und fällt dann entweder auf oder unter seinen ursprünglichen Wert. Dieses Verhalten trat bei allen Frontdurchgängen zu jeder Tages- und Jahreszeit auf. Die einzigen Ausnahmen waren Fronten mit sehr hohen Windgeschwindigkeiten.
In dieser Arbeit wird versucht, das Verhalten des Streukoeffizienten (und damit der Aerosolkonzentration) durch eine Betrachtung der Mischungshöhen zu erklären, da eine Reduktion der vertikalen Ausbreitung durch Stabilitätsänderungen vor der Front den Anstieg vonσ s bewirken könnte. Eine Berechnung der Mischungshöhen ergab, daß sie vor der Front um fast denselben Faktor abnahmen um denσ s anstieg. Nach der Front waren die Änderungsfaktoren einander ähnlich, obwohl die Aerosolkonzentration auch vom Ursprung der Luftmasse abhing.
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Hitzenberger, R. The behaviour of the atmospheric aerosol scattering coefficient under varying meteorological conditions. Theor Appl Climatol 37, 175–183 (1986). https://doi.org/10.1007/BF00867575
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DOI: https://doi.org/10.1007/BF00867575