Lidar Measurements Of Aerosol Scattering In The Troposphere And Stratosphere

Abstract

A twocolor Rayleigh/Raman lidar has been developed to study the properties of the middle and lower atmosphere. The LAMP (Lidar Atmospheric Measurements Program) lidar profiler was placed in service at Penn State University during the summer of 1991. The LAMP lidar uses two wavelengths, 532 and 355 MI, in the transmitted beam and up to eight detectors in the receiver. The instrument is arranged in a monostatic configuration, which permits useful measurements in the near field, as well as in the far field. The detector system uses a mechanical shutter to block the high intensity low altitude signal from the high altitude detectors until the beam has reached an altitude of 20 km. The Nd:YAG laser includes a doubling crystal and a mixing crystal to produce a 532 and a 355 nm beam. The low altitude backscatter signals of the visible and ultraviolet beams are detected as analog signals and digitized at 10 MSps to provide 15 meter resolution from the surface to 25 km. The high altitude signals, obtained by photon counting techniques, are separated into 500 nanosecond range bins to provide 75 meter resolution, from 20 to 80 km. The detector also contains two first Stokes vibrational Raman channels to measure the N2 signal at 607 nm and the H,O signal at 660 nm. Measurements of the rotational Raman backscatter provides the possibility to obtain temperature profiles in the presence of clouds and in the boundary layer. The results reveal the continuous presence of a relatively small aerosol particles through the troposphere. These particle sizes are comparable to the wavelength of the light and exhibit a signal, in the vicinity of 5 km, which is typically greater than the molecular backscatter by a factor of 2 at the 532 nm wavelength and by a factor of I O at the 355 nm wavelength. The small aerosol component of the tropospheric backscatter was found to be relatively uniform as a function of latitude over the ocean, from Arctic (7C"N) to Antarctic (65%). III the presence of clouds, the variation in the background small aerosol was remarkably small. The cloud presence does not significantly change the slope or magnitude of the small aerosol component near the cloud layer except for the expected attenuation by the cloud. The magnitude of the ultraviolet extinction due to this small aerosol component is quite significant. The influence of the turbidity due to small aerosol scattering has been investigated to prepare these results for a study of the turbidity contribution to the radiative transfer in the atmosphere. EXPERIMENT BACKGROUND Results from the ARL/PSU LAMP (Lidar Atmospheric Measurements Program) lidar instrument have been examined to determine the aerosol component of the lower atmosphere. The instrument has been used since mid1991 to measure the properties of the atmosphere and is based upon developments of two previous instruments (Philbrick, 1991). The two-color lidar approach is most useful in examining and separating the molecular, aerosol and cloud scattering components. Most of the results have been obtained at the PSU campus but a most significant data set was obtained during the LADIMAS campaign. The LAtitudinal ustribution of Middle Atmosphere Structure (LADIMAS) experiment (Philbrick, et al. 1992) has provided a unique set of measurements which are improving our understanding of the atmosphere. The project included ship-board and rocket range coordinated measurements between 70N to 65s to study the structure, dynamics and chemistry of the atmosphere. Results on dynamical processes, such as gravity waves, tidal components, as well as, the formation of the layers of meteoric ion and neutral species, have been obtained with lidar, digisonde, microwave radiometer, and spectrometers. The cooperative study of the atmosphere was undertaken by researchers from several laboratories, including Penn State University, University BOM, University Wuppertal, Lowell University, and others. Several of the parameters studied have never