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In an earlier paper [1], data from our Multi-Band Radiometer Transmissometer (MSRT) were used to compare the ratio
of extinction coefficients in different spectral bands during periods of changing visibility conditions. This ratio is an
indication of the characteristics and origin (eg rural or maritime) of the haze- or fog particles, present in the measurement
path. In this paper we will analyze the VAMPIRA transmission data in more detail by separating the contributions due to
molecular extinction, scattering and (potentially) refraction. In our analysis we take the contribution due to scattering in
order to obtain the characteristics of the Particle Size Distribution (PSD). For this purpose we take the average value and
the slope of the measured transmission level in two neighboring spectral bands. Via a special simulation tool, developed
for Junge-type PSD's, the slope of the PSD (defined: Junge exponent) and its value at a particle diameter of 1 μm (Junge
coefficient) can be determined via a set of retrieval steps. Reference is made to a similar approach [2] where in stead of a
Junge distribution, three contiguous lognormal distributions are taken. The associated procedure for the Junge-type PSD
is explained in detail in this paper and applied to the VAMPIRA transmission data. The versatility of the new retrieval
method is demonstrated, especially when wavelengths around lμm are chosen (a somewhat higher number than the
diameter of the majority of the particles, so that most of the scattering is in the so-called Rayleigh regime). It is obvious,
that the method fails in conditions of dense fog, when the transmission levels (average value and slope) over the 8.6 km
path approach zero. The results are compared with in-situ PSD measurements, carried out simultaneously with a PMS
(Particle Measurement System) probe at the pier near the Suerendorf shore station. In many conditions different results
appear due to the fact that the MSRT system delivers path integrated data, while the PMS probe measures locally in a
small volume. The MSRT data, collected over an overseas path, are more relevant to be used in the data analysis of the
shore based sensor systems [3], measuring simultaneously signal values of distant point targets. The MSRT system has a
higher signal to noise ratio and due to the shorter time constant, rapid fluctuations in particle characteristics are observed,
not measured by the PMS probe. The availability of reliable aerosol characteristics (i.e. Junge exponent and -coefficient)
allows a more precise interpretation of the data from the surveillance systems.
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