Remote sensing of optical depth of aerosols and clouds related to air traffic

A ground-based scanning lidar system has been developed to investigate aerosols and persistent contrails in air traffic corridors with respect to growth and microphysical and optical properties. Under conditions not suitable for contrail formation, air traffic aerosols can be observed if the prevailing wind allows an accumulation of aerosol plumes in the air traffic corridor. Under situations favorable for contrail formation, contrail growth rates of up to 140 m min(-1) horizontally and up to 18 m min(-1) vertically have been observed. The depolarization of the light backscattered from contrails ranges from 5 to 50% in the early vortex regime, depending on environmental temperature, but is generally found at about 50% in the aging contrail. Aged contrails show average optical depth values in the range 0.05-0.3, whereas optical depths of the background aerosol in the 8-13 km height range of air traffic vary from 0.01 to 0.05 depending on the natural aerosol load. By calibrating CCD-camera images with lidar information the optical depth of extended areas of contrail cover can be determined. This technique to investigate an additional cloud cover caused by air traffic will be used to validate and possibly improve existing algorithms which determine contrails in AVHRR satellite images. (C) 1998 Elsevier Science Ltd. All rights reserved.