A Langley Extrapolation Technique Applied to Atmospheric Aerosol LIDARs

摘要

The well-known Langley extrapolation technique produces measurements of atmospheric optical depth (AOD) by collecting direct sun irradiance at multiple zenith angles. One common application of this technique is used by sun photometers such as in NASA's AErosol Robotic Network (AERONET). This large, spatially distributed network collects time averaging data from across the globe and applying Beer's Law, produces hourly estimates of AOD. While this technique has produced excellent data, the dependence on direct sun irradiance requires cloudless skies and line-of-sight to the sun. Atmospheric LIDARs, on the other hand, can operate in the presence of clouds and can also produce range-resolved measurements of AOD by applying the same Langley technique. For aerosol LIDARs, this technique requires that the LIDAR be capable of producing high quality waveforms within the atmospheric coherence time and also be capable of taking measurements off zenith. At least two unique angles are required to produce data, although 3+ are recommended. This paper will present an overview of the Langley technique applied with a 1064 nm atmospheric aerosol LIDAR, an overview of the LIDAR hardware and capabilities, sample data collected by the LIDAR, and challenges associated with this technique. It will be shown that while this technique is useful, it requires measurements at all three angles to be made when the atmosphere is reasonably horizontally homogenous. Furthermore, the system optics, alignment, and laser power must be kept constant (keeping the LIDAR's system constant the same for all measurements) for the data to be useful in a Langley analysis.