Vertical variation of extinction and atmospheric transmission due to aerosol particles close above the sea surface in the Mediterranean coastal zone

Jacques PiazzolaSerge DespiauUniversity of Toulon-VarUFR Science et TechniquesB.P. 132F-83957 La Garde CedexFranceE-mail: despiau@isitiv.univ-tln.frAbstract. The vertical variation of extinction and backscatter in the low-est part of the atmospheric mixed layer is of considerable interest for theassessment of the performance of electro-optical systems, but is poorlyknown. Our purpose is to estimate the vertical variation of extinction andatmospheric transmission due to aerosol particles near the air-sea inter-face. Atmospheric extinction coefficients in the 3- to 5- and 8- to 12-mmIR bands calculated using vertical aerosol distributions measured in theMediterranean coastal zone are presented. The data reveal a negativeextinction gradient between 1 and 2 to 3 m height during winds of marineorigin lower than 9 m/s, whereas a positive gradient occurs between 1and 2 to 3 m height during high-wind-speed periods (V>9 m/s). A posi-tive extinction gradient is also noted for vertical profiles recorded duringmoderate continental winds. These extinction gradients result in a rela-tive difference in atmospheric transmission between two sample heightsthat reach 60% in both the 3- to 5- and 8- to 12-mm bands for an opticalpath of 10 km during high-wind-speed periods. These observed differ-ences cannot be explained by current models to predict the vertical aero-sol distribution.

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