An Aerosol Extinction-to-Backscatter Ratio Database Derived from the NASA Micro-Pulse Lidar Network: Applications for Space-based Lidar Observations

Backscatter lidar signals are a function of both backscatter and extinction. Hence, these lidar observations alone cannot separate the two quantities. The aerosol extinction-to-backscatter ratio, S, is the key parameter required to accurately retrieve extinction and optical depth from backscatter lidar observations of aerosol layers. S is commonly defined as 4*pi divided by the product of the single scatter albedo and the phase function at 180-degree scattering angle. Values of S for different aerosol types are not well known, and are even more difficult to determine when aerosols become mixed. Models of S are complicated by having to estimate the index of refraction, and particle size and shape (especially for non-spherical aerosols). Direct measurements of S are difficult and only a few instruments exist that are capable of measuring both extinction and backscatter, at or very near 180 degrees. In addition, direct measurements are not conducted under ambient conditions. However, models and direct measurements have begun to form a better understanding of S for key aerosol species. Another method of determining S is to use backscatter lidar in conjunction with co-located sunphotometer measurements of aerosol optical depth (AOD). The independent measurement of AOD is used as a constraint to solve the lidar equation, a process that calculates an extinction profile and also a layer averaged value for S. The resulting extinction profile may be over or under-estimated at a given altitude because S is a layer average, however this calculation of S does not require any assumption of aerosol composition, size, or shape and is done at ambient conditions. Thus, lidar-sunphotometer S values represent an improvement over models and direct measurements in this regard. Here we present a new lidar-sunphotometer S database derived from observations of the NASA Micro-Pulse Lidar Network (MPLNET). MPLNET is a growing worldwide network of eye-safe backscatter lidars co-located with sunphotometers in the NASA Aerosol Robotic Network (AERONET). Values of S for different aerosol species and geographic regions will be presented. A framework for constructing an S look-up table will be shown. Look-up tables of S are needed to calculate aerosol extinction and optical depth from space-based lidar observations in the absence of co-located AOD data. Applications for using the new S look-up table to reprocess aerosol products from NASA's Geoscience Laser Altimeter System (GLAS) will be discussed.