Jong, A.N. de
Eijk, A.M.J. van
TNO Defensie en Veiligheid
|Source:||Hammel, S.M.Kohnle, A., Atmospheric Optical Modeling, Measurement, and Simulation II, 15 August 2006, San Diego, CA, USA.|
Physics · Particle size distribution · Scattering · In situ processing · Information retrieval · Infrared transmission · Particle size analysis · Scattering · Solar radiation · Atmospheric transmission · Electro optical sensors · Multi band transmissometers · Atmospheric aerosols
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  and in the Persian Gulf area . 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 (lognormal) 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 2.3 urn 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.