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The presence of atmospheric aerosols along the line of sight of infrared and electro-optical sensors greatly determines the
range performance of these devices. On the one hand the aerosol particles scatter background (including sun) radiance
into the field of view of the sensor, on the other hand they contribute to the atmospheric contrast reduction of the target.
Proper knowledge of aerosol characteristics such as composition, concentration and size distribution is of vital
importance for the prediction of their scattering and extinction characteristics. It is however found to be very difficult to
collect accurate information on the particle size distribution (PSD) of aerosols. One of the reasons is the variation of the
PSD along the path, which is likely to occur in a coastal area such as the San Diego Bay. One way to overcome these
problems is the use of a multi-band transmissometer, as was done in previous measurement campaigns in the Baltic Sea
[1] and in the Persian Gulf area [2]. The TNO seven-band optical/IR transmissometer system, providing path averaged
transmission data for the intervening atmosphere, is operating at wavelengths between 0.4 and 14 μm,. In this spectral
band, scattering in light hazy conditions is dominated by particles with a diameter of less than 4 μm. In order to simulate
the transmission losses by scattering in various spectral bands a special calculation tool has been developed. This tool,
described in this paper, allows detailed investigation of the possibilities of the retrieval of the PSD from multi-band transmission data. The slope in the plots of the transmission versus wavelength is directly related to the slope of the (log-normal) PSD plots (known as Junge exponent). The average transmission in a selected number of spectral bands is
directly correlated to the average particle concentration (known as Junge coefficient). The principle of the methodology
is illustrated with data collected during a measurement campaign, carried out over the San Diego Bay in August 2005. In
this campaign we used six of the seven spectral bands, providing data over a 7.2 km over water path. It is shown that the
retrieval method is very successful and the data correspond well with those, simultaneously collected with in-situ Particle
Measurement Systems (PMS), located on both sides of the path. In addition to the path averaging, another advantage of
the transmissometer PSD's, is the accuracy, being an order of magnitude higher than that of the PMS probes due to the
fact that the measurement volume is more than a million times larger. A detailed analysis is given of the transmission
data, showing peculiar effects were in the 2.3μm band around 19.00 UTC during most of the days. These effects are
especially well illustrated in plots where the transmission in one spectral band is plotted against the transmission in
another band for various times of the day.
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