The use of multi-band transmission data collected at Scripps pier in November 2006 for the investigation of aerosol characteristics

The knowledge of the atmospheric aerosol characteristics is of great importance for the range performance of Infrared and Electro-Optical sensor systems. The composition, the concentration and the size distribution of aerosols determine their scattering behavior as function of wavelength and thus their attenuation of light beams. When studies are made on this attenuation, it is considered to be very useful to incorporate the spatial variation of the aerosol characteristics along the measurement path, such as found during a previous campaign, carried out over the San Diego Bay (August 2005 [1]). In a more recent experiment (November 2006) a trial was set-up at Scripps pier of the Institute of Oceanography near La Jolla (US-West Coast). This place is known for the high frequency of occurrence of hazy conditions, not only variable with location, but also in time. TNO did participate in this experiment with two Particle Measurement Systems (PMS), one on the shore and one at the end of the pier, a weather station and the Multi Spectral Radiometer Transmissometer (MSRT), used in previous trials ([1], [2], [3]). It was shown before, that the use of multi-band transmission data allows a more detailed analysis of the aerosol characteristics in the measurement path ([4]). The advantage of the MSRT concerns a larger measurement volume, thus providing a higher signal to noise (S/N) ratio and a shorter response time, compared to the in-situ PMS systems. A similar method, based on the spectral dependence of the scattering coefficient, has been used during the analysis of the data, collected in November 2006. In this paper representative data samples are presented, showing the variability of the transmission in each of the spectral bands. By using the weather data, a separation has been made between extinction by molecules and aerosols. A brief description is given of the retrieval method, just mentioned. The retrieved particle characteristics are compared to the data from the PMS systems, taking into account the fact, that part of the measurement path, having a length of 6.68 km, was grazing the shore-line. In general the particle density, as found by the PMS systems was less than the retrieved value, especially when the wind was from off-shore directions. At certain occasions, sinusoidal variations in the transmission level, with a period of about 5 minutes, were found, probably due to oscillatory motions of the air mass in the measurement path. It was found, that the value of the Junge exponent of the Particle Size Distribution (PSD) was rather frequently of the order of -3, showing that the concentration of bigger particles (> 3 μm) compared to the smaller particles (< 0.5 μm), was greater than the value, found in experiments at other locations. It is investigated how accurate the transmission levels of the IR bands can be predicted by using the retrieved PSD's and the absolute humidity, obtained from the weather data. Due to the low wind speed during the trial (< 4 m/s), it was not possible to find any particular effect from aerosols, potentially created in the surf zone at higher wind speeds