The behaviour of the atmospheric aerosol scattering coefficient under varying meteorological conditions

Abstract

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.