Raman lidar measurements of the aerosol extinction-to-backscatter ratio over the Southern Great Plains

We derive profiles of the aerosol extinction-to-backscatter ratio, S-a, at 355 nm using aerosol extinction and backscatter profiles measured during 19 98 and 1999 by the operational Raman lidar at the Department of Energy Atmo spheric Radiation Measurement Program Southern Great Plains site in north c entral Oklahoma. Data from this Raman/Rayleigh-Mie lidar, which measures Ra man scattering from nitrogen as well as the combined molecular (Rayleigh) a nd aerosol (Mie) scattering at the laser wavelength, are used to derive aer osol extinction and backscattering independently as a function of altitude. Because this lidar operates at 355 nm, where molecular backscattering is c omparable to aerosol backscattering, S-a retrievals are generally limited t o conditions where aerosol extinction at 355 nm is >0.03 km(-1). The mean v alue of S-a at 355 nm derived for this period was 68 sr with a standard dev iation of 12 sr. S-a was generally about 5-10 sr higher during high aerosol optical thickness (AOT) (>0.3) conditions than during low AOT (<0.1). A si milar increase in S-a was found when the relative humidity increased from 3 0 to 80%. Large (> 15%) variations in the vertical profile of S-a occurred about 30% of the time, which implies that significant variability in the ve rtical distribution of the aerosol size distribution, shape, and/or composi tion often occurs. The Raman lidar measurements of S-a were compared with e stimates of particle size and refractive index derived from an algorithm th at uses ground-based Sun photometer measurements of Sun and sky radiance. F or 17 cases of coincident Raman lidar and Sun and sky radiance measurements , S-a was linearly correlated with the aerosol fine mode effective radius a nd the volume ratio of fine/coarse particles.