Relationship of visibility, aerosol optical thickness and aerosol size distribution in an ageing air mass over South-West Germany

The ageing process of an air mass over South-West Germany during 5 consecutive days has been examined. In-situ measurements of the visibility, aerosol size distributions, aerosol scattering coefficients and meteorological variables as relative humidity were measured during that situation. Additionally, we measured column aerosol properties, and PM10 mass concentrations of a nearby station of the local environmental protection agency were available. A distinct decrease in the visibility from 45 to 25 km, which was directly measured by two independent methods, a significant increase in PM10, and an increase in the aerosol optical thickness from 0.1 to 0.6 have been detected. The visibility decrease is associated with distinct changes in the aerosol number–size distribution. The number of particles with diameters larger than 300 nm increased considerably during the period, whereas smaller particles did not show such a clear trend. A comparison with meteorological ranges derived from nephelometer scattering coefficients using the Koschmieder formula and a humidity correction is presented. The quality of the nephelometer measurements is verified by a comparison with meteorological ranges calculated from scanning mobility particle sizer (SMPS)/aerodynamic particle sizer (APS) aerosol size distributions applying Mie theory. The calculated meteorological ranges are too high in comparison with the measured visibilities in the beginning, whereas this overestimation decreases at the end of the period when more relatively large particles were abundant. Our results demonstrate the general lack of representativeness of an in-situ measurement for a horizontally averaged parameter as the observed visibility. However, this representativeness was improved when the visibility decreased. In addition, a clear reciprocal behaviour of the visibility and the aerosol optical thickness has been found.